Electron cyclotron resonance heating (ECRH) can drive large current densities through electron cyclotron current drive (ECCD). ECCD is expected to be crucial for high-performance plasmas in future fusion reactors like ITER and DEMO, making the current drive efficiency of ECCD a critical design parameter for future reactors. In present-day devices, good agreement between measured and predicted current drive efficiency has been found. However, to ensure the reliability in future machines, a direct validation of the electron momentum distribution function is needed. As a first step towards this goal, we present in this paper oblique ECE measurements of a low-density plasma in the ASDEX Upgrade tokamak. Two oblique ECE diagnostics are used to allow the simultaneous measurements of electrons streaming co- and counter-directionally with the plasma current. Predictions for the distribution function are computed with the bounce-averaged Fokker-Planck code RELAX (E. Westerhof et al., Rijnhuizen report,1992). To allow direct comparison with the measurements, synthetic radiation temperatures are computed with the code ECRad (S. Denk et al., Computer Physics Communications, p. 107175, 2020). Good agreement is found if radial transport occurring predominantly at low electron energies is included. We demonstrate that the oblique ECE diagnostics measure the electron distribution function directly at the ECRH deposition site in phase space. Furthermore, they are sensitive to the abundance of pitch-angle scattered electrons that reduce the ECCD efficiency. Limitations and uncertainties of the measurements and the modeling are discussed.

}, doi = {10.1088/1361-6587/abc1bd}, author = {Denk, S. S. and Fischer, R. and Westerhof, E. and Luda di Cortemiglia, E. and Hobirk, J. and Maj, O. and Nielsen, S. K. and Poli, E. and Rasmussen, J. and Stejner, M. and Stober, J. and Stroth, U. and Suttrop, W. and Willensdorfer, M.} } @article {4158, title = {Inline ECE measurements for NTM control on ASDEX Upgrade}, journal = {Nuclear Fusion}, volume = {59}, year = {2019}, pages = {016013}, abstract = {The successful use of a tokamak for generating fusion power requires an active control of magnetic instabilities, such as neoclassical tearing modes (NTMs). Commonly, the NTM location is determined using electron cyclotron emission (ECE) and this is used to apply electron cyclotron heating (ECH) on the NTM location. In this paper, an inline ECE set-up at ASDEX Upgrade is presented in which ECE is measured and ECH is applied via the same path. First results are presented and a means to interpret the measurement data is given. Amplitude and phase with respect to a reference magnetic signal are calculated. Based on the amplitude and phase, the time of mode crossing is determined and shown to compare well with real-time estimates of the mode crossing time. The ECH launcher and [formula]; flux surface geometries at ASDEX Upgrade, which are optimized for current drive by a beam path that is tangential to the flux surface near deposition, make it difficult to identify the mode crossing without inline ECE launcher movement. Therefore, NTM control based on inline ECE requires launcher movement to determine and maintain a reliable estimate of the NTM location.

}, doi = {10.1088/1741-4326/aaeea9}, author = {van den Brand, H. and Bongers, W. A. and Stober, J. and Kasparek, W. and Wagner, D. and Doelman, N. J. and Klop, M. A. and Giannone, L. and Reich, M. and Westerhof, E. and de Baar, M.} } @article {3969, title = {TORBEAM 2.0, a paraxial beam tracing code for electron-cyclotron beams in fusion plasmas for extended physics applications}, journal = {Computer Physics Communications}, volume = {225}, year = {2018}, month = {04/2018}, pages = {36-46}, abstract = {The paraxial WKB code TORBEAM (Poli, 2001) is widely used for the description of electron-cyclotron waves in fusion plasmas, retaining diffraction effects through the solution of a set of ordinary differential equations. With respect to its original form, the code has undergone significant transformations and extensions, in terms of both the physical model and the spectrum of applications. The code has been rewritten in Fortran 90 and transformed into a library, which can be called from within different (not necessarily Fortran-based) workflows. The models for both absorption and current drive have been extended, including e.g. fully-relativistic calculation of the absorption coefficient, momentum conservation in electron{\textendash}electron collisions and the contribution of more than one harmonic to current drive. The code can be run also for reflectometry applications, with relativistic corrections for the electron mass. Formulas that provide the coupling between the reflected beam and the receiver have been developed. Accelerated versions of the code are available, with the reduced physics goal of inferring the location of maximum absorption (including or not the total driven current) for a given setting of the launcher mirrors. Optionally, plasma volumes within given flux surfaces and corresponding values of minimum and maximum magnetic field can be provided externally to speed up the calculation of full driven-current profiles. These can be employed in real-time control algorithms or for fast data analysis.

}, keywords = {Electron cyclotron waves, Magnetic confinement, Paraxial beam tracing, Plasma physics, Wave-plasma interactions}, doi = {10.1016/j.cpc.2017.12.018}, author = {Poli, E. and Bock, A. and Lochbrunner, M. and Maj, O. and Reich, M. and Snicker, A. and Stegmeir, A. and Volpe, F. and Bertelli, N. and Westerhof, E. and Bilato, R. and Conway, G. D. and Farina, D. and Felici, F. and Figini, L. and Fischer, R. and Galperti, C. and Happel, T. and Lin-Liu, Y. R. and Marushchenko, N. B. and U. Mszanowski and Poli, F. M. and Stober, J. and Zille, R. and Peeters, A. G. and Pereverzev, G. V.} } @article {4102, title = {Analysis of electron cyclotron emission with extended electron cyclotron forward modeling}, journal = {Plasma Physics and Controlled Fusion}, volume = {60}, year = {2018}, pages = {105010}, doi = {10.1088/1361-6587/aadb2f}, author = {Denk, S. S. and Fischer, R. and Smith, H. M. and Helander, P. and Maj, O. and Poli, E. and Stober, J. and Stroth, U. and Suttrop, W. and Westerhof, E. and Willensdorfer, M.} } @article {4108, title = {Separation of transport in slow and fast time-scales using modulated heat pulse experiments (hysteresis in flux explained)}, journal = {Nuclear Fusion}, volume = {58}, year = {2018}, pages = {106042}, abstract = {Old and recent experiments show that there is a direct response to the heating power of transport observed in modulated ECH experiments both in tokamaks and stellarators. This is most apparent for modulated experiments in the Large Helical Device (LHD) and in Wendelstein 7 advanced stellarator (W7-AS). In this paper we show that: 1) This power dependence can be reproduced by linear models and as such hysteresis (in flux) has no relationship to hysteresis as defined in the literature; 2) Observations of "hysteresis" (in flux) and a direct response to power can be perfectly reproduced by introducing an error in the estimated deposition profile as long as the errors redistribute the heat over a large radius; 3) Non-local models depending directly on the heating power can also explain the experimentally observed Lissajous curves (hysteresis); 4) How non-locality and deposition errors can be recognized in experiments and how they affect estimates of transport coefficients; 5) That non-linear-non-local transport models offer a path in discerning deposition errors from non-local fast transport components otherwise experimentally indistinguishable. To show all this, transport needs to be analyzed by separating the transport in a slow (diffusive) time-scale and a fast (heating/non-local) time-scale, which can only be done in the presence of perturbations. (DOI dataset, OA: 10.4121/uuid:5fcf4247-da0e-4119-adcd-fc90b85b7f03)

}, doi = {10.1088/1741-4326/aadc17}, author = {van Berkel, M. and Vandersteen, G. and Zwart, H. J. and Hogeweij, G. M. D. and Citrin, J. and Westerhof, E. and Peumans, D. and M. R. de Baar} } @article {3818, title = {Overview of progress in European medium sized tokamaks towards an integrated plasma-edge/wall solution}, journal = {Nuclear Fusion}, volume = {57}, year = {2017}, pages = {102014}, abstract = {Integrating the plasma core performance with an edge and scrape-off layer (SOL) that leads to tolerable heat and particle loads on the wall is a major challenge. The new European medium size tokamak task force (EU-MST) coordinates research on ASDEX Upgrade (AUG), MAST and TCV. This multi-machine approach within EU-MST, covering a wide parameter range, is instrumental to progress in the field, as ITER and DEMO core/pedestal and SOL parameters are not achievable simultaneously in present day devices. A two prong approach is adopted. On the one hand, scenarios with tolerable transient heat and particle loads, including active edge localised mode (ELM) control are developed. On the other hand, divertor solutions including advanced magnetic configurations are studied. Considerable progress has been made on both approaches, in particular in the fields of: ELM control with resonant magnetic perturbations (RMP), small ELM regimes, detachment onset and control, as well as filamentary scrape-off-layer transport. For example full ELM suppression has now been achieved on AUG at low collisionality with n = 2 RMP maintaining good confinement H H(98,y2) =approx 0.95. Advances have been made with respect to detachment onset and control. Studies in advanced divertor configurations (Snowflake, Super-X and X-point target divertor) shed new light on SOL physics. Cross field filamentary transport has been characterised in a wide parameter regime on AUG, MAST and TCV progressing the theoretical and experimental understanding crucial for predicting first wall loads in ITER and DEMO. Conditions in the SOL also play a crucial role for ELM stability and access to small ELM regimes.}, doi = {10.1088/1741-4326/aa6084}, author = {Meyer, H. and Eich, T. and Citrin, J. and Classen, I. and Hogeweij, D. and Jaulmes, F. and Kappatou, A. and van den Brand, H. and Vanovac, B. and Vijvers, W. A. J. and Westerhof, E. and et al.} } @article {3430, title = {A model-based, multichannel, real-time capable sawtooth crash detector}, journal = {Plasma Physics and Controlled Fusion}, volume = {58}, year = {2016}, pages = {075002}, abstract = {Control of the time between sawtooth crashes, necessary for ITER and DEMO, requires real-time detection of the moment of the sawtooth crash. In this paper, estimation of sawtooth crash times is demonstrated using the model-based interacting multiple model (IMM) estimator, based on simplified models for the sawtooth crash. In contrast to previous detectors, this detector uses the spatial extent of the sawtooth crash as detection characteristic. The IMM estimator is tuned and applied to multiple ECE channels at once. A model for the sawtooth crash is introduced, which is used in the IMM algorithm. The IMM algorithm is applied to seven datasets from the ASDEX Upgrade tokamak. Five crash models with different mixing radii are used. All sawtooth crashes that have been identified beforehand by visual inspection of the data, are detected by the algorithm. A few additional detections are made, which upon closer inspection are seen to be sawtooth crashes, which show a partial reconnection. A closer inspection of the detected normal crashes shows that about 42\% are not well fitted by any of the full reconnection models and show some characteristics of a partial reconnection. In some case, the measurement time is during the sawtooth crashes, which also results in an incorrect estimate of the mixing radius. For data provided at a sampling rate of 1 kHz, the run time of the IMM estimator is below 1 ms, thereby fulfilling real-time requirements.

}, doi = {10.1088/0741-3335/58/7/075002}, author = {van den Brand, H. and M. R. de Baar and van Berkel, M. and Blanken, T. C. and Felici, F. and Westerhof, E. and Willensdorfer, M. and ASDEX Upgrade Team and EUROfusion MST1 Team} } @article {3432, title = {New insights into the generalized Rutherford equation for nonlinear neoclassical tearing mode growth from 2D reduced MHD simulations}, journal = {Nuclear Fusion}, volume = {56}, year = {2016}, pages = {036016}, doi = {10.1088/0029-5515/56/3/036016}, author = {Westerhof, E. and de Blank, H. J. and Pratt, J.} } @article {3572, title = {Numerical and experimental study of the redistribution of energetic and impurity ions by sawteeth in ASDEX Upgrade}, journal = {Nuclear Fusion}, volume = {56}, year = {2016}, pages = {112012}, abstract = {In the non-linear phase of a sawtooth, the complete reconnection of field lines around the q = 1 flux surface often occurs resulting in a radial displacement of the plasma core. A complete time-dependent electromagnetic model of this type of reconnection has been developed and implemented in the EBdyna_go code. This contribution aims at studying the behaviour of ions, both impurity and fast particles, in the pattern of reconnecting field lines during sawtoothing plasma experiments in the ASDEX Upgrade tokamak by using the newly developed numerical framework. Simulations of full reconnection with tungsten impurity that include the centrifugal force are achieved and recover the soft x-ray measurements. Based on this full-reconnection description of the sawtooth, a simple tool dedicated to estimate the duration of the reconnection is introduced. This work then studies the redistribution of fast ions during several experimentally observed sawteeth. In some cases of sawteeth at ASDEX Upgrade, full reconnection is not always observed or expected so the code gives an upper estimate of the actual experimental redistribution. The results of detailed simulations of the crashes are compared with measurements from various diagnostics such as collective Thomson scattering and fast-ion D-alpha (FIDA) spectroscopy, including FIDA tomography. A convincing qualitative agreement is found in different parts of velocity space.

}, doi = {10.1088/0029-5515/56/11/112012}, author = {Jaulmes, F. and Geiger, B. and Odstrcil, T. and Weiland, M. and Salewski, M. and Jacobsen, A. S. and Rasmussen, J. and Stejner, M. and Nielsen, S. K. and Westerhof, E. and EUROfusion MST1 Team and ASDEX Upgrade Team} } @article {3609, title = {Molecular dynamics simulations of ballistic He penetration into W fuzz}, journal = {Nuclear Fusion}, volume = {56}, year = {2016}, pages = {126015}, abstract = {Results are presented of large-scale Molecular Dynamics simulations of low-energy He bombardment of W nanorods, or so-called {\textquoteleft}fuzz{\textquoteright} structures. The goal of these simulations is to see if ballistic He penetration through W fuzz offers a more realistic scenario for how He moves through fuzz layers than He diffusion through fuzz nanorods. Instead of trying to grow a fuzz layer starting from a flat piece of bulk W, a new approach of creating a fully formed fuzz structure 0.43 {\textmu} m thick out of ellipsoidal pieces of W is employed. Lack of detailed experimental knowledge of the 3D structure of fuzz is dealt with by simulating He bombardment on five different structures of 15 vol\% W and determining the variation in He penetration for each case. The results show that by far the most important factor determining He penetration is the amount of open channels through which He ions can travel unimpeded. For a more or less even W density distribution He penetration into fuzz falls off exponentially with distance and can thus be described by a {\textquoteleft}half depth{\textquoteright}. In a 15 vol\% fuzz structure, the half depth can reach 0.18 {\textmu} m. In the far sparser fuzz structures that were recently reported, the half depth might be 1 {\textmu} m or more. This means that ballistic He penetration offers a more likely scenario than He diffusion through nanorods for how He moves through fuzz and may provide an adequate explanation for how He penetrates through the thickest fuzz layers reported so far. Furthermore, the exponential decrease in penetration with depth would follow a logarithmic dependence on fluence which is compatible with experiments. A comparison of these results and molecular dynamics calculations carried out in the recoil interaction approximation shows that results for W fuzz are qualitatively very different from conventional stopping power calculations on W with a similarly low but homogeneous density distribution.

}, doi = {10.1088/0029-5515/56/12/126015}, author = {Klaver, T. P. C. and Nordlund, K. and Morgan, T. W. and Westerhof, E. and Thijsse, B. J. and M. C. M. van de Sanden} } @article {3639, title = {Early evolution of electron cyclotron driven current during suppression of tearing modes in a circular tokamak}, journal = {Physics of Plasmas}, volume = {23}, year = {2016}, pages = {102507}, doi = {10.1063/1.4964785}, url = {http://www.euro-fusionscipub.org/wp-content/uploads/WP15ERPR16_16130_submitted.pdf}, author = {Pratt, J. and Huijsmans, G. T. A. and Westerhof, E.} } @article {2768, title = {Closure of the single fluid magnetohydrodynamic equations in presence of electron cyclotron current drive}, journal = {EPJ Web of Conferences}, volume = {87}, year = {2015}, pages = {01005}, doi = {10.1051/epjconf/20158701005}, author = {Westerhof, E. and Pratt, J. and Ayten, B.} } @article {2652, title = {Closure of the single fluid magnetohydrodynamic equations in presence of electron cyclotron current drive}, journal = {Physics of Plasmas}, volume = {21}, number = {10}, year = {2014}, pages = {102516}, abstract = {In the presence of electron cyclotron current drive (ECCD), the Ohm{\textquoteright}s law of single fluid magnetohydrodynamics is modified as E + v {\texttimes} B = η(J {\textendash} J EC). This paper presents a new closure relation for the EC driven current density appearing in this modified Ohm{\textquoteright}s law. The new relation faithfully represents the nonlocal character of the EC driven current and its main origin in the Fisch-Boozer effect. The closure relation is validated on both an analytical solution of an approximated Fokker-Planck equation as well as on full bounce-averaged, quasi-linear Fokker-Planck code simulations of ECCD inside rotating magnetic islands. The new model contains the model put forward by Giruzzi et al. [Nucl. Fusion 39, 107 (1999)] in one of its limits.

}, keywords = {Fokker-Planck equation, plasma magnetohydrodynamics, plasma radiofrequency heating, plasma simulation}, doi = {10.1063/1.4900871}, author = {Westerhof, E. and Pratt, J.} } @article {2659, title = {The European Integrated Tokamak Modelling (ITM) effort: achievements and first physics results}, journal = {Nuclear Fusion}, volume = {54}, year = {2014}, pages = {043018}, abstract = {A selection of achievements and first physics results are presented of the European Integrated Tokamak Modelling Task Force (EFDA ITM-TF) simulation framework, which aims to provide a standardized platform and an integrated modelling suite of validated numerical codes for the simulation and prediction of a complete plasma discharge of an arbitrary tokamak. The framework developed by the ITM-TF, based on a generic data structure including both simulated and experimental data, allows for the development of sophisticated integrated simulations (workflows) for physics application. The equilibrium reconstruction and linear magnetohydrodynamic (MHD) stability simulation chain was applied, in particular, to the analysis of the edge MHD stability of ASDEX Upgrade type-I ELMy H-mode discharges and ITER hybrid scenario, demonstrating the stabilizing effect of an increased Shafranov shift on edge modes. Interpretive simulations of a JET hybrid discharge were performed with two electromagnetic turbulence codes within ITM infrastructure showing the signature of trapped-electron assisted ITG turbulence. A successful benchmark among five EC beam/ray-tracing codes was performed in the ITM framework for an ITER inductive scenario for different launching conditions from the equatorial and upper launcher, showing good agreement of the computed absorbed power and driven current. Selected achievements and scientific workflow applications targeting key modelling topics and physics problems are also presented, showing the current status of the ITM-TF modelling suite.

}, doi = {10.1088/0029-5515/54/4/043018}, author = {G.L. Falchetto and Coster, D. and Coelho, R. and Scott, B. D. and Figini, L. and Kalupin, D. and Nardon, E. and Nowak, S. and L.L. Alves and Artaud, J. F. and Basiuk, V. and Jo{\~a}o P.S. Bizarro and C. Boulbe and Dinklage, A. and Farina, D. and B. Faugeras and Ferreira, J. and Figueiredo, A. and Huynh, P. and Imbeaux, F. and Ivanova-Stanik, I. and Jonsson, T. and H.-J. Klingshirn and Konz, C. and Kus, A. and Marushchenko, N. B. and Pereverzev, G. and M. Owsiak and Poli, E. and Peysson, Y. and R. Reimer and Signoret, J. and Sauter, O. and Stankiewicz, R. and Strand, P. and Voitsekhovitch, I. and Westerhof, E. and T. Zok and Zwingmann, W. and ITM-TF contributors and ASDEX Upgrade Team and JET-EFDA Contributors} } @article {2671, title = {Non-linear effects in electron cyclotron current drive applied for the stabilization of neoclassical tearing modes}, journal = {Nuclear Fusion}, volume = {54}, year = {2014}, pages = {073001}, abstract = {Due to the smallness of the volumes associated with the flux surfaces around the O-point of a magnetic island, the electron cyclotron power density applied inside the island for the stabilization of neoclassical tearing modes (NTMs) can exceed the threshold for non-linear effects as derived previously by Harvey et al (1989 Phys. Rev. Lett. 62 [http://dx.doi.org/10.1103/PhysRevLett.62.426] 426 ). We study the non-linear electron cyclotron current drive (ECCD) efficiency through bounce-averaged, quasi-linear Fokker{\textendash}Planck calculations in the magnetic geometry as created by the islands. The calculations are performed for the parameters of a typical NTM stabilization experiment on ASDEX Upgrade. A particular feature of these experiments is that the rays of the EC wave beam propagate tangential to the flux surfaces in the power deposition region. The calculations show significant non-linear effects on the ECCD efficiency, when the ECCD power is increased from its experimental value of 1\ MW to a larger value of 4\ MW. The nonlinear effects are largest in the case of locked islands or when the magnetic island rotation period is longer than the collisional time scale. The non-linear effects result in an overall reduction of the current drive efficiency for this case with absorption of the EC power on the low-field side of the electron cyclotron resonance layer. As a consequence of the non-linear effects, also the stabilizing effect of the ECCD on the island is reduced from linear expectations.

}, doi = {10.1088/0029-5515/54/7/073001}, url = {http://stacks.iop.org/0029-5515/54/i=7/a=073001}, author = {Ayten, B. and Westerhof, E. and ASDEX Upgrade Team} } @article {2714, title = {Redistribution of fast ions during sawtooth reconnection}, journal = {Nuclear Fusion}, volume = {54}, year = {2014}, pages = {104013}, abstract = {In a tokamak-based fusion power plant, possible scenarios may include regulated sawtooth oscillations to remove thermalized helium from the core of the plasma. During a sawtooth crash, the helium ash and other impurities trapped in the core are driven by the instability to an outer region. However, in a fusion plasma, high energy ions will represent a significant population. We thus study the behaviour of these energetic particles during a sawtooth. This paper presents the modelling of the redistribution of fast ions during a sawtooth reconnection event in a tokamak plasma. Along the lines of the model for the evolution of the flux surfaces during a sawtooth collapse described in Ya.I. Kolesnichenko and Yu.V. Yakovenko 1996 Nucl. Fusion 36 159, we have built a time-dependent electromagnetic model of a sawtooth reconnection. The trajectories of the ions are described by a complete gyro-orbit integration. The fast particles were evolved from specific initial parameters (given energy and uniform spread in pitch) or distributed initially according to a slowing-down distribution created by fusion reactions. Our modelling is used to understand the main equilibrium parameters driving the motions during the collapse and to determine the evolution of the distribution function of energetic ions when different geometries of reconnection are considered.

}, doi = {10.1088/0029-5515/54/10/104013}, author = {Jaulmes, F. and Westerhof, E. and de Blank, H. J.} } @article {2471, title = {Evaluating neoclassical tearing mode detection with ECE for control on ITER}, journal = {Nuclear Fusion}, volume = {53}, number = {1}, year = {2013}, note = {Times Cited: 1}, month = {Jan}, pages = {013005}, abstract = {Neoclassical tearing mode (NTM) control on ITER requires detection of the mode location to be accurate and with low latency. This paper presents a systematic way to evaluate mode detection algorithms for ITER using numerical simulations of electron cyclotron emission (ECE), taking into account the radial asymmetry in the temperature perturbation by a rotating magnetic island. Simulated ECE is detected using a synthetic radiometer, in the ITER equatorial port plug, and processed by two detection algorithms for the 2/1 and 3/2 NTMs for a burning H-mode ITER plasma. One of the algorithms also incorporates simulated Mirnov data. The video bandwidth is set at 2 kHz. This allows for intermediate frequency bandwidths of B-IF = 400 MHz and BIF = 300 MHz for the two algorithms, respectively. The intermediate frequency bandwidth provides a trade-off between radial accuracy ( low bandwidth) and low noise/latency ( large bandwidth). 2/1 and 3/2 NTMs, seeded with widths up to 9 and 11 cm, are detectable with the required accuracy within 250 ms. With appropriate settings for the radiometer, the NTM detection using ECE is accurate and with low latency. The algorithm that incorporates both ECE and Mirnov data showed the lowest detection latencies.

}, isbn = {0029-5515}, doi = {10.1088/0029-5515/53/1/013005}, author = {van den Brand, H. and M.R. de Baar and Cardozo, N. J. L. and Westerhof, E.} } @article {2484, title = {Summary of EC-17: the 17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating (Deurne, The Netherlands, 7-10 May 2012)}, journal = {Nuclear Fusion}, volume = {53}, year = {2013}, month = {Feb}, pages = {027002}, abstract = {An overview is given of the papers presented at the 17th Joint Workshop on Electron Cyclotron Emission (ECE) and Electron Cyclotron Resonance Heating (ECRH). The meeting covered all aspects of the research field ranging from theory to enabling technologies. From the workshop, advanced control by electron cyclotron heating and current drive is emerging as probably the main application of ECRH in fusion devices. Large progress is reported from various experiments on real-time control applications. At the same time ECE is developing into a multi-dimensional plasma diagnostic taking advantage of new technological developments. The resulting multi-dimensional ECE data reveal exciting new details of the complicated plasma dynamics in fusion devices.

}, isbn = {0029-5515}, doi = {10.1088/0029-5515/53/2/027002}, author = {Westerhof, E. and Austin, M. E. and Kubo, S. and Lin-Liu, Y. R. and Plaum, B.} } @article {2513, title = {Experimental characterization of anomalous strong scattering of mm-waves in TEXTOR plasmas with rotating islands}, journal = {Plasma Physics and Controlled Fusion}, volume = {55}, number = {11}, year = {2013}, pages = {115003}, abstract = {Anomalous scattering of high power millimetre waves from gyrotrons at 140 and 110\ GHz is investigated for plasma with rotating islands at TEXTOR. The magnetic field and plasma density influence the spectral content of the scattered waves and their power levels significantly. Anomalous strong scattering occurs in two density regimes, one at low densities and one at high densities, that also depend on the magnetic field. The two regimes are separated by a quiescent regime without anomalous scattering. Investigations suggest that scattering in the high-density regime is generated at the low-field side intersection of the gyrotron beam and the island position. The transition from the quiescent regime to the high-density regime occurs when the gyrotron frequency is twice the upper hybrid frequency at this position. There is some evidence that the scattering in the low-density regime is generated near the plasma centre. Under this assumption all the observed scattering is generated when the gyrotron frequency is near or below twice the upper hybrid frequency.

}, doi = {10.1088/0741-3335/55/11/115003}, author = {Nielsen, S.K. and Salewski, M. and Westerhof, E. and Bongers, W. and Korsholm, S. B. and Leipold, F. and Oosterbeek, J. W. and Moseev, D. and Stejner, M. and TEXTOR team} } @article {2257, title = {Consequences of plasma rotation for neoclassical tearing mode suppression by electron cyclotron current drive}, journal = {Physics of Plasmas}, volume = {19}, number = {9}, year = {2012}, note = {ISI Document Delivery No.: 014UNTimes Cited: 0Cited Reference Count: 43}, month = {Sep}, pages = {092506}, type = {Article}, abstract = {In the generalized Rutherford equation describing the nonlinear evolution of the width of the magnetic island associated with a neoclassical tearing mode, the effect of localized current drive is represented by a term Delta(CD){\textquoteright}. We investigate oscillations in Delta(CD){\textquoteright} originating from the rotation of the island through the electron cyclotron power deposition region and their dependence on the collisional time scale on which the driven current is generated, the rotation period, the island size, and the power deposition width. Furthermore, their consequences for the island growth or the stabilization are analyzed. This work shows that the net result of the oscillations in Delta(CD){\textquoteright} is a slight increase in the stabilizing effect of electron cyclotron current drive and consequently, a reduction in the minimum power requirement to fully suppress an island. [http://dx.doi.org/10.1063/1.4751873]}, keywords = {ASDEX UPGRADE, DISCHARGES, ITER, RF CURRENT DRIVE, STABILIZATION, TOKAMAK, WAVES}, isbn = {1070-664X}, doi = {10.1063/1.4751873}, author = {Ayten, B. and Westerhof, E.} } @article {2261, title = {Integrated modelling of island growth, stabilization and mode locking: consequences for NTM control on ITER}, journal = {Plasma Physics and Controlled Fusion}, volume = {54}, number = {9}, year = {2012}, note = {ISI Document Delivery No.: 995WUTimes Cited: 0Cited Reference Count: 4216th Workshop on Magnetohydrodynamic (MHD) Stability Control - Optimizing and Understanding the Role of Coils for Mode ControlNOV 20-22, 2011San Diego, CAColumbia Univ, Princeton Plasma Phys Lab, Univ Wisconsin-Madison}, month = {Sep}, pages = {094003}, type = {Article; Proceedings Paper}, abstract = {Full suppression of neoclassical tearing modes (NTMs) using electron cyclotron current drive (ECCD) should be reached before mode locking (stop of rotation) makes suppression impossible. For an ITER scenario 2 plasma, the similar time scales for locking and island growth necessitate the combined modelling of the growth of the mode and its slow down due to wall induced drag. Using such a model, the maximum allowed latency between the seeding of the mode and the start of ECCD deposition and maximum deviation in the radial position are determined. The maximum allowed latency is determined for two limiting models for island growth; the polarization model with W-marg = 2 cm, representing the worst case, and the transport model with W-marg = 6 cm, representing the best case. NTMs with seed island widths up to 9.5 cm and 12 cm for the 2/1 and the 3/2 NTM, respectively, are suppressible. The maximum allowed latency is 1.05 s and 2.95 s for the 2/1 and 3/2 NTM, respectively, for the worst case model. Radial misalignment should not exceed 7-10 mm for the 2/1 NTM and 5-16 mm for the 3/2 NTM depending on the model for island growth. As long as the alignment suffices, it does not reduce the maximum allowed latency. Mode locking has serious implications for any real-time NTM control system on ITER that aims to suppress NTMs by ECCD.}, keywords = {BEAM, CURRENT DRIVE, ELECTRON-CYCLOTRON WAVES, INSTABILITIES, NEOCLASSICAL TEARING MODES, PERTURBATIONS, TOKAMAK PLASMAS}, isbn = {0741-3335}, doi = {10.1088/0741-3335/54/9/094003}, author = {van den Brand, H. and M.R. de Baar and Cardozo, N. J. L. and Westerhof, E.} } @article {2293, title = {Nonlinear control for stabilization of small neoclassical tearing modes in ITER}, journal = {Nuclear Fusion}, volume = {52}, number = {6}, year = {2012}, note = {ISI Document Delivery No.: 956LNTimes Cited: 0Cited Reference Count: 28}, month = {Jun}, pages = {063007}, type = {Article}, abstract = {In this paper, the feasibility of feedback stabilization of neoclassical tearing modes at small island sizes, corresponding to otherwise unstable island sizes in ITER scenario 2, is demonstrated. The islands are stabilized by application of electron cyclotron resonance heating and current drive in a regime where the application of current drive in open loop normally results in a complete suppression of the island. By applying current drive in closed loop with feedback of real-time measurements of the island width, complete suppression is avoided and the island is stabilized at a specific reduced size. In contrast to complete suppression, control of islands at a specific size will allow the manipulation of a plasma{\textquoteright}s current density profile in hybrid scenarios. Three conceptual (non-)linear feedback controllers with varying complexity, performance, robustness and required model knowledge are introduced. Simulations show the theoretical feasibility of small island stabilization at a specific reduced width. The controllers are applied to the generalized Rutherford equation, which governs the island evolution subject to electron cyclotron current drive. A strategy for the gradual implementation of the controllers is suggested. Stabilization of small islands by feedback control will allow the use of system identification to extend the model knowledge on the evolution of small islands, and in addition will extend the operational regime.}, keywords = {CURRENT DRIVE, ELECTRON-CYCLOTRON WAVES, TOKAMAK}, isbn = {0029-5515}, doi = {10.1088/0029-5515/52/6/063007}, author = {Hennen, B.A. and Lauret, M. and Hommen, G. and Heemels, Wpmh and M.R. de Baar and Westerhof, E.} } @article {2310, title = {Robust sawtooth period control based on adaptive online optimization}, journal = {Nuclear Fusion}, volume = {52}, number = {7}, year = {2012}, note = {ISI Document Delivery No.: 970BXTimes Cited: 1Cited Reference Count: 35SI}, month = {Jul}, pages = {074006}, type = {Article}, abstract = {The systematic design of a robust adaptive control strategy for the sawtooth period using electron cyclotron current drive (ECCD) is presented. Recent developments in extremum seeking control (ESC) are employed to derive an optimized controller structure and offer practical tuning guidelines for its parameters. In this technique a cost function in terms of the desired sawtooth period is optimized online by changing the ECCD deposition location based on online estimations of the gradient of the cost function. The controller design does not require a detailed model of the sawtooth instability. Therefore, the proposed ESC is widely applicable to any sawtoothing plasma or plasma simulation and is inherently robust against uncertainties or plasma variations. Moreover, it can handle a broad class of disturbances. This is demonstrated by time-domain simulations, which show successful tracking of time-varying sawtooth period references throughout the whole operating space, even in the presence of variations in plasma parameters, disturbances and slow launcher mirror dynamics. Due to its simplicity and robustness the proposed ESC is a valuable sawtooth control candidate for any experimental tokamak plasma, and may even be applicable to other fusion-related control problems.}, keywords = {CONFINEMENT, DISCHARGES, EXTREMUM SEEKING CONTROL, PERFORMANCE IMPROVEMENT, STABILITY, SYSTEMS}, isbn = {0029-5515}, doi = {10.1088/0029-5515/52/7/074006 }, author = {Bolder, J. J. and Witvoet, G. and M.R. de Baar and van de Wouw, N. and Haring, M. A. M. and Westerhof, E. and Doelman, N. J. and Steinbuch, M.} } @article {2311, title = {Sawtooth period control strategies and designs for improved performance}, journal = {Nuclear Fusion}, volume = {52}, number = {7}, year = {2012}, note = {ISI Document Delivery No.: 970BXTimes Cited: 1Cited Reference Count: 33SI}, month = {Jul}, pages = {074005}, type = {Article}, abstract = {The sawtooth instability is associated with the triggering of neo-classical tearing modes, core fuelling, alpha-confinement and the exhaust of thermal helium. Sawtooth control is therefore important for optimal reactor performance in ELMy H-modes. Control schemes for the sawtooth period have been published in the literature, but the systematic design of high-performance controllers (yielding accurate and fast convergent responses) has not been addressed. In this work, three control strategies for high-performance sawtooth control are presented using electron cyclotron current drive (ECCD). Both degrees of freedom of the ECCD actuator will be explored and combined with advanced controller designs. First, the ECCD deposition location is used as a control variable, for which a gain-scheduled feedback controller and static feedforward control is derived. Second, the use of the driven current as a control variable is explored, and a simple controller is designed based on the identified dynamics. In the third approach both control variables are joined in an overall controller design, which enables the combination of high-performance control of the sawtooth period and control of the gyrotron power. Time-domain simulations with a combined Kadomtsev-Porcelli sawtooth model show that each strategy obtains a better closed-loop performance than standard linear feedback techniques on merely the deposition location.}, keywords = {CONFINEMENT, CURRENT DRIVE, DISCHARGES, JET}, isbn = {0029-5515}, doi = {10.1088/0029-5515/52/7/074005}, author = {Witvoet, G. and Steinbuch, M. and M.R. de Baar and Doelman, N. J. and Westerhof, E.} } @article {2313, title = {Systematic design and simulation of a tearing mode suppression feedback control system for the TEXTOR tokamak}, journal = {Nuclear Fusion}, volume = {52}, number = {7}, year = {2012}, note = {ISI Document Delivery No.: 970BXTimes Cited: 0Cited Reference Count: 23SI}, month = {Jul}, pages = {074009 }, type = {Article}, abstract = {Suppression of tearing modes is essential for the operation of tokamaks. This paper describes the design and simulation of a tearing mode suppression feedback control system for the TEXTOR tokamak. The two main control tasks of this feedback control system are the radial alignment of electron cyclotron resonance heating and current drive (ECRH/ECCD) with a tearing mode and the stabilization of a mode at a specific width. In order to simulate these control tasks, the time evolution of a tearing mode subject to suppression by ECRH/ECCD and destabilization by a magnetic perturbation field is modelled using the generalized Rutherford equation. The model includes an equilibrium model and an ECRH/ECCD launcher model. The dynamics and static equilibria of this model are analysed. The model is linearized and based on the linearized model, linear feedback controllers are designed and simulated, demonstrating both alignment and width control of tearing modes in TEXTOR.}, keywords = {DYNAMIC ERGODIC DIVERTOR, STABILIZATION}, isbn = {0029-5515}, doi = {10.1088/0029-5515/52/7/074009 }, author = {Hennen, B.A. and Westerhof, E. and Nuij, Pwjm and M.R. de Baar and Steinbuch, M.} } @article {2330, title = {Electron cyclotron waves}, journal = {Fusion Science and Technology}, volume = {61}, year = {2012}, month = {Feb}, pages = {304-311}, type = {Article}, abstract = {This lecture gives an overview of heating and current drive with electron cyclotron waves. We present the main theoretical aspects of wave propagation, wave absorption, and non-inductive current drive, as well as important technical aspects for the application of high power electron cyclotron waves, and the major achievements in their experimental application.}, keywords = {ABSORPTION, ASDEX UPGRADE, COMPLETE SUPPRESSION, CURRENT-DRIVE EXPERIMENTS, DIII-D, DISCHARGES, FUSION PLASMAS, SAWTOOTH PERIOD, TCV, TEARING MODE STABILIZATION, TOKAMAK}, isbn = {1536-1055}, url = {http://www.ans.org/pubs/journals/fst/a_13517}, author = {Westerhof, E.} } @article {2337, title = {Non-inductive current drive}, journal = {Fusion Science and Technology}, volume = {61}, year = {2012}, month = {Feb}, pages = {312-319}, type = {Article}, abstract = {This lecture addresses the various ways of non-inductive current generation. In particular, the topics covered include the bootstrap current, RF current drive, neutral beam current drive, alternative methods, and possible synergies between different ways of non-inductive current generation.}, keywords = {BOOTSTRAP CURRENT, COAXIAL HELICITY, CURRENT, DIII-D, ELECTRON-CYCLOTRON WAVES, FUTURE DEVICES, GENERATION, HYBRID CURRENT DRIVE, INJECTION, NEUTRAL INJECTION, STEADY-STATE, SYNCHROTRON-RADIATION}, isbn = {1536-1055}, url = {http://www.ans.org/pubs/journals/fst/a_13518}, author = {Westerhof, E.} } @conference {2380, title = {Benchmarking of electron cyclotron heating and current drive codes on ITER scenarios within the European Integrated Tokamak Modelling framework}, booktitle = {17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating (EC-17)}, volume = {32}, year = {2012}, month = {2012/05/07}, pages = {01011}, publisher = {EDP Sciences, EPJ Web of Conferences }, organization = {EDP Sciences, EPJ Web of Conferences }, address = {Deurne, The Netherlands}, doi = {10.1051/epjconf/20123201011}, author = {Figini, L. and Decker, J. and Farina, D. and Marushchenko, N. B. and Peysson, Y. and Poli, E. and Westerhof, E.} } @conference {2381, title = {Dynamical modelling of neoclassical tearing mode suppression by ECCD}, booktitle = {17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating (EC-17)}, volume = {32}, year = {2012}, month = {2012/05/07}, pages = {01010}, publisher = {EDP Sciences, EPJ Web of Conferences }, organization = {EDP Sciences, EPJ Web of Conferences }, address = {Deurne, The Netherlands}, doi = {10.1051/epjconf/20123201010}, author = {Ayten, B. and Westerhof, E.} } @conference {2382, title = {ECE for NTM control on ITER}, booktitle = {17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating (EC-17)}, volume = {32}, year = {2012}, month = {2012/05/07}, pages = {03004}, publisher = {EDP Sciences, EPJ Web of Conferences }, organization = {EDP Sciences, EPJ Web of Conferences }, address = {Deurne, The Netherlands}, doi = {10.1051/epjconf/20123203004}, author = {van den Brand, H. and M.R. de Baar and N.J. Lopes Cardozo and Westerhof, E.} } @conference {2383, title = {Toward 3D MHD modeling of neoclassical tearing mode suppression by ECCD}, booktitle = {17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating (EC-17)}, volume = {32}, year = {2012}, month = {2012/05/07}, pages = {01014}, publisher = {EDP Sciences, EPJ Web of Conferences }, organization = {EDP Sciences, EPJ Web of Conferences }, address = {Deurne, The Netherlands}, doi = {10.1051/epjconf/20123201010}, author = {Pratt, J. and Westerhof, E.} } @article {2393, title = {Modification of the collective Thomson scattering radiometer in the search for parametric decay on TEXTOR}, journal = {Review of Scientific Instruments}, volume = {83}, year = {2012}, pages = {113508}, publisher = {AIP}, keywords = {electromagnetic wave scattering, millimetre waves, plasma diagnostics, radiometers}, doi = {10.1063/1.4768668}, url = {http://link.aip.org/link/?RSI/83/113508/1}, author = {Nielsen, S.K. and Salewski, M. and Bongers, W. A. and Korsholm, S. B. and Leipold, F. and Meo, F. and Michelsen, P. and Moseev, D. and Oosterbeek, J. W. and Stejner, M. and Westerhof, E.} } @conference {2404, title = {Commissioning of inline ECE system within waveguide based ECRH transmission systems on ASDEX upgrade}, booktitle = {EPJ Web of Conferences}, volume = {32}, year = {2012}, pages = {03006}, doi = {10.1051/epjconf/20123203006}, author = {Bongers, W. A. and Kasparek, W. and Doelman, N. J. and Van den Braber, R. and van den Brand, H. and Meo, F. and M.R. de Baar and Amerongen, F.J. and Donne, A. J. H. and Elzendoorn, B. S. Q. and Erckmann, V. and Goede, A. P. H. and Giannone, L. and Gr{\"u}nwald, G. and Hollmann, F. and Kaas, G. and Krijger, B. and Michel, G. and Lubyako, L. and Monaco, F. and Noke, F. and Petelin, M. and Plaum, B. and Purps, F. and ten Pierik, J. G. W. and Schuller, C. and Slob, J.W. and Stober, J. K. and Sch{\"u}tz, H. and Wagner, D. and Westerhof, E. and Ronden, D. M. S.} } @article {2407, title = {Coupling the beam tracing code TORBEAM and the Fokker-Planck solver RELAX for fast electrons}, journal = {Journal of Physics: Conference Series}, volume = {401}, number = {1}, year = {2012}, pages = {012013}, abstract = {In this paper the interface between the beam tracing code TORBEAM [Poli, Peeters and Pereverzev, Comp. Phys. Comm. 136, 90 (2001)] and the quasi-linear Fokker-Planck solver RELAX [Westerhof, Peeters and Schippers, Rijnhuizen Report No. RR 92-211 CA, 1992] is presented together with preliminary testing results for electron cyclotron waves in ITER plasmas and their effects on the electron distribution function. The resulting numerical package allows us to account for diffraction effects in the construction of the quasi-linear wave-particle diffusion operator. The coupling of the paraxial-WKB code TORBEAM to the ray-based code RELAX requires a reinterpretation of the paraxial wave field in terms of extended rays, which are addressed in details.}, doi = {10.1088/1742-6596/401/1/012013}, url = {http://stacks.iop.org/1742-6596/401/i=1/a=012013}, author = {Maj, O. and Poli, E. and Westerhof, E.} } @conference {2428, title = {A new mechanism for sawtooth period control}, booktitle = {EPJ Web of Conferences}, volume = {32}, year = {2012}, pages = {02008}, doi = {10.1051/epjconf/20123202008}, author = {Lauret, M. and Felici, F. and Witvoet, G. and Goodman, T. and Vandersteen, G. and Westerhof, E. and Steinbuch, M. and Sauter, O. and M.R. de Baar} } @article {2062, title = {Comparison of measured and simulated fast ion velocity distributions in the TEXTOR tokamak}, journal = {Plasma Physics and Controlled Fusion}, volume = {53}, number = {10}, year = {2011}, note = {ISI Document Delivery No.: 823VVTimes Cited: 0Cited Reference Count: 40}, month = {Oct}, pages = {105004}, type = {Article}, abstract = {Here we demonstrate a comprehensive comparison of collective Thomson scattering (CTS) measurements with steady-state Monte Carlo simulations performed with the ASCOT and VENUS codes. The measurements were taken at a location on the magnetic axis as well as at an off-axis location, using two projection directions at each location. The simulations agree with the measurements on-axis, but for the off-axis geometries discrepancies are observed for both projection directions. For the near perpendicular projection direction with respect to the magnetic field, the discrepancies between measurement and simulations can be explained by uncertainty in plasma parameters. However, the discrepancies between measurement and simulations for the more parallel projection direction cannot be explained solely by uncertainties in plasma parameters. Here anomalous fast ion transport is a possible explanation for the discrepancy.

}, keywords = {collective Thomson scattering, JET, PLASMAS, TRANSPORT}, isbn = {0741-3335}, doi = {10.1088/0741-3335/53/10/105004}, author = {Moseev, D. and Meo, F. and Korsholm, S. B. and Koskela, T. and Albergante, M. and Asunta, O. and Bindslev, H. and Burger, A. and Furtula, V. and Kantor, M. Y. and Leipold, F. and Michelsen, P. K. and Nielsen, S.K. and Salewski, M. and Schmitz, O. and Stejner, M. and Westerhof, E.} } @article {2083, title = {Numerical demonstration of injection locking of the sawtooth period by means of modulated EC current drive}, journal = {Nuclear Fusion}, volume = {51}, year = {2011}, month = {Oct}, pages = {103043}, type = {Article}, abstract = {In this paper the sawtooth period behaviour under periodic forcing by electron cyclotron waves is investigated. The deposition location is kept constant while the gyrotron power is modulated with a certain period and duty cycle. Extensive simulations on a representative dynamic sawtooth model show that when this modulation is properly chosen, the sawtooth period quickly synchronizes to the same period and remains locked at this value. It is shown that the range of modulation periods and duty cycles over which sawtooth period locking occurs, depends on the deposition location, but is particularly large for depositions near the q = 1 surface. The simulation results reveal a novel approach to control the sawtooth period in open loop, based on injection locking, which is a well-known technique to control limit cycles of non-linear dynamic oscillators. The locking and convergence results are therefore used in a simple open-loop locking controller design, with which accurate sawtooth period tracking to any desired value is indeed demonstrated. Injection locking appears to let the sawtooth period converge to the modulation period quickly, partly because it does not suffer from slow EC mirror launcher dynamics. Moreover, simulations show that the method has a relatively large robustness against general uncertainties and disturbances. Hence, injection locking is expected to outperform conventional sawtooth control methods using a variable deposition location and constant gyrotron power. Finally, the recent result with sawtooth pacing is shown to be a special case of the general locking effect.}, keywords = {ASDEX UPGRADE, DISCHARGES, FREQUENCY CONTROL, JET, OSCILLATIONS, SAWTEETH, STABILIZATION, TOKAMAK}, isbn = {0029-5515}, doi = {10.1088/0029-5515/51/10/103043}, author = {Witvoet, G. and Lauret, M. and M.R. de Baar and Westerhof, E. and Steinbuch, M.} } @article {2088, title = {Requirements on localized current drive for the suppression of neoclassical tearing modes}, journal = {Nuclear Fusion}, volume = {51}, year = {2011}, month = {Oct}, pages = {15}, type = {Article}, abstract = {A heuristic criterion for the full suppression of an NTM was formulated as eta(NTM) = j(CD,max)/j(BS) >= 1.2 (Zohm et al 2005 J. Phys. Conf. Ser. 25 234), where j(CD,max) is the maximum in the driven current density profile applied to stabilize the mode and j(BS) is the local bootstrap current density. In this work we subject this criterion to a systematic theoretical analysis on the basis of the generalized Rutherford equation. Taking into account only the effect of j(CD) inside the island, a new criterion for full suppression by a minimum applied total current is obtained in the form of a maximum allowed value for the width of the driven current, w(dep), combined with a required minimum for the total driven current in the form of w(dep)eta(NTM), where both limits depend on the marginal and saturated island sizes. These requirements can be relaxed when additional effects are taken into account, such as a change in the stability parameter Delta{\textquoteright} from the current driven outside the island, power modulation, the accompanying heating inside the island or when the current drive is applied preemptively. When applied to ITER scenario 2, the requirement for full suppression of either the 3/2 or 2/1 NTM becomes w(dep) less than or similar to 5 cm and w(dep)eta(NTM) greater than or similar to 5 cm in agreement with (Sauter et al 2010 Plasma Phys. Control. Fusion 52 025002). Optimization of the ITER ECRH Upper Port Launcher design towards minimum required power for full NTM suppression requires an increase in the toroidal injection angle of the lower steering mirror of several degrees compared with its present design value, while for the upper steering mirror the present design value is close to the optimum.}, keywords = {ASDEX, CYCLOTRON CURRENT DRIVE, DIII-D, DISCHARGES, ECCD, ITER, JT-60U TOKAMAK, PERTURBATIONS, STABILIZATION, TOKAMAK PLASMAS, UPGRADE}, isbn = {0029-5515}, doi = {10.1088/0029-5515/51/10/103007}, author = {Bertelli, N. and De Lazzari, D. and Westerhof, E.} } @article {2105, title = {Wave Beam Propagation Through Density Fluctuations}, journal = {IEEE Transactions on Plasma Science}, volume = {39}, year = {2011}, month = {Nov}, pages = {3012-3013}, type = {Article}, abstract = {Perturbations induced by edge density fluctuations on electron cyclotron wave beams propagating in fusion plasmas are studied by means of a quasi-optical code. The effects of such fluctuations are illustrated here by showing the beam propagation in the case of single harmonic perturbations to the wave phase front.}, keywords = {PLASMAS, TOKAMAKS, wave propagation}, isbn = {0093-3813}, doi = {10.1109/TPS.2011.2158666}, author = {Balakin, A. A. and Bertelli, N. and Westerhof, E.} } @article {2135, title = {The role of asymmetries in the growth and suppression of neoclassical tearing modes}, journal = {Plasma Physics and Controlled Fusion}, volume = {53}, year = {2011}, month = {Mar}, pages = {17}, type = {Article}, abstract = {The evolution of neoclassical tearing modes (NTMs) is usually described by the generalized Rutherford equation for a symmetric magnetic island. Despite the success of this representation, various experiments have found the evidence of asymmetries in the island geometry. A generalization of the model suggests that a number of effects, such as a quasi-linear correction of the constant-psi approximation, a shear flow or a temperature gradient across the island, might be responsible for the deformation of the island geometry. In addition, it is noted that the symmetry is broken in the radial direction also by a finite third order derivative in the equilibrium helical flux function. This paper addresses the role of these asymmetries in the growth and suppression of NTMs in a slab geometry, with particular attention to the implications for the local current drive (ECCD) and resonant heating (ECRH) terms. The stabilizing contributions provided by electron cyclotron waves to NTMs are found to be largely unaffected by these perturbations. These results correct and extend some of the conclusions presented in Lazzaro and Nowak (2009 Plasma Phys. Control. Fusion 51 035005).}, keywords = {ASDEX UPGRADE, CURRENT DRIVE, density, DIII-D, DISCHARGES, JT-60U, LONG-PULSE, STABILIZATION, TOKAMAK, VISCOSITY}, isbn = {0741-3335}, doi = {10.1088/0741-3335/53/3/035020}, author = {De Lazzari, D. and Westerhof, E.} } @article {2139, title = {Comment on {\textquoteright}The role of the RF induced electric field in the current drive by EC waves in the presence of magnetic islands{\textquoteright}}, journal = {Nuclear Fusion}, volume = {51}, number = {6}, year = {2011}, note = {ISI Document Delivery No.: 766MWTimes Cited: 0Cited Reference Count: 11}, month = {Jun}, pages = {068001}, type = {Editorial Material}, abstract = {It is shown that the equations solved in the paper (Rosa and Ziebell 2010 Nucl. Fusion 50 115009) do not describe the consistent evolution of the parallel electric field and the electron cyclotron driven current density in a geometry with a magnetic island. Therefore, do the {\textquoteright}effects{\textquoteright} of the induced parallel electric field on the electron cyclotron driven current as claimed in this paper have no bearing on the stabilizing effect of electron cyclotron current drive on neoclassical tearing modes.

}, keywords = {DIII-D, STABILIZATION, TEARING MODES}, isbn = {0029-5515}, doi = {10.1088/0029-5515/51/6/068001}, author = {Westerhof, E.} } @article {2141, title = {Dynamics of fast ions during sawtooth oscillations in the TEXTOR tokamak measured by collective Thomson scattering}, journal = {Nuclear Fusion}, volume = {51}, number = {6}, year = {2011}, note = {ISI Document Delivery No.: 766MWTimes Cited: 0Cited Reference Count: 39}, month = {Jun}, pages = {063014}, type = {Article}, abstract = {Experimental investigations of sawteeth interaction with fast ions measured by collective Thomson scattering on TEXTOR are presented. Time-resolved measurements of localized 1D fast-ion distribution functions allow us to study fast-ion dynamics during several sawtooth cycles. Sawtooth oscillations interact strongly with the fast-ion population in a wide range of plasma parameters. Part of the ion phase space density oscillates out of phase with the sawtooth oscillation during hydrogen neutral beam injection (NBI). These oscillations most likely originate from fast hydrogen ions with energies close to the full injection energy. At lower energies passing fast ions in the plasma centre are strongly redistributed at the time of sawtooth collapse but no redistribution of trapped fast ions is observed. The redistribution of fast ions from deuterium NBI in the plasma centre is found to vary throughout velocity space. The reduction is most pronounced for passing ions. We find no evidence of inverted sawteeth outside the sawtooth inversion surface in the fast-ion distribution function.

}, keywords = {BEHAVIOR, CRASHES, DISRUPTIONS, FLUCTUATIONS, FUSION PLASMAS, JET, REDISTRIBUTION, STABILIZATION, TEMPERATURE, WAVES}, isbn = {0029-5515}, doi = {10.1088/0029-5515/51/6/063014}, author = {Nielsen, S.K. and Salewski, M. and Bindslev, H. and Burger, A. and Furtula, V. and Kantor, M. and Korsholm, S. B. and Koslowski, H. R. and Kramer-Flecken, A. and Leipold, F. and Meo, F. and Michelsen, P. K. and Moseev, D. and Oosterbeek, J. W. and Stejner, M. and Westerhof, E.} } @article {2146, title = {Intermediate frequency band digitized high dynamic range radiometer system for plasma diagnostics and real-time Tokamak control}, journal = {Review of Scientific Instruments}, volume = {82}, year = {2011}, month = {Jun}, pages = {15}, type = {Article}, abstract = {An intermediate frequency (IF) band digitizing radiometer system in the 100-200 GHz frequency range has been developed for Tokamak diagnostics and control, and other fields of research which require a high flexibility in frequency resolution combined with a large bandwidth and the retrieval of the full wave information of the mm-wave signals under investigation. The system is based on directly digitizing the IF band after down conversion. The enabling technology consists of a fast multi-giga sample analog to digital converter that has recently become available. Field programmable gate arrays (FPGA) are implemented to accomplish versatile real-time data analysis. A prototype system has been developed and tested and its performance has been compared with conventional electron cyclotron emission (ECE) spectrometer systems. On the TEXTOR Tokamak a proof of principle shows that ECE, together with high power injected and scattered radiation, becomes amenable to measurement by this device. In particular, its capability to measure the phase of coherent signals in the spectrum offers important advantages in diagnostics and control. One case developed in detail employs the FPGA in real-time fast Fourier transform (FFT) and additional signal processing. The major benefit of such a FFT-based system is the real-time trade-off that can be made between frequency and time resolution. For ECE diagnostics this corresponds to a flexible spatial resolution in the plasma, with potential application in smart sensing of plasma instabilities such as the neoclassical tearing mode (NTM) and sawtooth instabilities. The flexible resolution would allow for the measurement of the full mode content of plasma instabilities contained within the system bandwidth. (C) 2011 American Institute of Physics.}, keywords = {ALGORITHM, CYCLOTRON CURRENT DRIVE, DISCHARGES, ECCD, ECE, FOURIER-TRANSFORM, TEARING MODE STABILIZATION}, isbn = {0034-6748}, doi = {10.1063/1.3594101}, author = {Bongers, W. A. and van Beveren, V. and Thoen, D. J. and Nuij, Pjwm and M.R. de Baar and Donne, A. J. H. and Westerhof, E. and Goede, A. P. H. and Krijger, B. and van den Berg, M. A. and Kantor, M. and M. F. Graswinckel and Hennen, B.A. and Sch{\"u}ller, F. C.} } @article {2161, title = {Systematic design of a sawtooth period feedback controller using a Kadomtsev-Porcelli sawtooth model}, journal = {Nuclear Fusion}, volume = {51}, year = {2011}, month = {Jul}, pages = {073024}, type = {Article}, abstract = {A systematic methodology for structured design of feedback controllers for the sawtooth period is presented, based on dedicated identification of the sawtooth dynamics. Therefore, a combined Kadomtsev-Porcelli model of a sawtoothing plasma actuated by an electron cyclotron current drive system has been set-up. This is used to derive the linearized input-output relations (transfer functions) from the varying deposition location of the electron cyclotron waves (ECW) to the sawtooth period. These transfer functions are derived around a large collection of operating points. Assessment of these control-relevant transfer functions shows that a sawtooth period controller requires an integral (I) action to guarantee closed-loop stability with zero steady-state error. Additional proportional-integral (PI) action can be applied to further increase the closed-loop performance. The parameters of both the I and PII controllers have been optimized in terms of stability, performance and robustness. Moreover, the effect of the mechanical ECW launcher on the closed-loop performance is studied for realistic cases. It is shown that the launcher dynamics seriously affects the achievable closed-loop performance in present-day experiments.}, keywords = {CONFINEMENT, CURRENT DRIVE, DISCHARGES, JET}, isbn = {0029-5515}, doi = {10.1088/0029-5515/51/7/073024}, author = {Witvoet, G. and M.R. de Baar and Westerhof, E. and Steinbuch, M. and Doelman, N. J.} } @article {2228, title = {Modelling of tearing mode suppression experiments in TEXTOR based on the generalized Rutherford equation}, journal = {Nuclear Fusion}, volume = {51}, year = {2011}, month = {Apr}, pages = {043007}, type = {Article}, abstract = {Modelling of the experiments on TEXTOR on tearing mode suppression by electron cyclotron resonance heating and current drive based on the generalized Rutherford equation (GRE) is presented. The comparison between the model and the experimental data provides a satisfactory agreement taking into account the experimental uncertainties. Both the model and the experimental observations confirm that in TEXTOR heating is the dominant suppression mechanism above that of current drive. As a conclusion, these experiments provide a positive benchmark for the stabilizing term in the GRE arising from the localized heating.}, keywords = {CURRENT DRIVE, DIII-D, DISCHARGES, ELECTRON-CYCLOTRON WAVES, ITER, LOCAL CURRENT-DENSITY, STABILIZATION, TOKAMAK}, isbn = {0029-5515}, doi = {10.1088/0029-5515/51/4/043007}, author = {Ayten, B. and De Lazzari, D. and M.R. de Baar and Hennen, B.A. and Westerhof, E.} } @article {1923, title = {Real-time control of tearing modes using a line-of-sight electron cyclotron emission diagnostic}, journal = {Plasma Physics and Controlled Fusion}, volume = {52}, number = {10}, year = {2010}, note = {ISI Document Delivery No.: 670AFTimes Cited: 0Cited Reference Count: 24}, month = {Oct}, pages = {20}, type = {Article}, abstract = {The stability and performance of tokamak plasmas are limited by instabilities such as neoclassical tearing modes. This paper reports on an experimental proof of principle of a feedback control approach for real-time, autonomous suppression and stabilization of tearing modes in a tokamak. The system combines an electron cyclotron emission diagnostic for sensing of the tearing modes in the same sight line with a steerable electron cyclotron resonance heating and current drive (ECRH/ECCD) antenna. A methodology for fast detection of q = m/n = 2/1 tearing modes and retrieval of their location, rotation frequency and phase is presented. Set-points to establish alignment of the ECRH/ECCD deposition location with the centre of the tearing mode are generated in real time and forwarded in closed loop to the steerable launcher and as a modulation pulse train to the gyrotron. Experimental results demonstrate the capability of the control system to track externally perturbed tearing modes in real time.}, keywords = {DISCHARGES, LIMITS, STABILIZATION, SYSTEM, TEXTOR, TOKAMAK}, isbn = {0741-3335}, url = {