Plasma surface interactions - Operations

Aims and scientific program

 

The PSI-Operations group is responsible for construction and operation of the new linear plasma device Magnum-PSI. This device will operate in steady-state with a 3-T superconducting magnet with a 10-cm diameter plasma beam. For hydrogen plasmas, flux densities of ~10²⁴ m^-2s^-1 should be reachable at low electron temperatures (<10 eV). Scientific operation of the device will start mid 2011. This includes operation of several techniques for plasma and surface diagnostics.

The research program of the group involves technical studies relevant for construction and operation of the device and for plasma diagnostics. In addition, hydrogenic retention in refractory metals exposed to high-flux plasmas is studied; this research line will include experiments with novel plasma-facing materials.

 

Personnel

 

Function	Name	E-mail
Group Leader	P.A. Zeijlmans van Emmichoven	P [dot] A [dot] Zeijlmans [te] differ [dot] nl
Research Engineer	H.J. van der Meiden	H [dot] J [dot] vanderMeiden [te] differ [dot] nl
Research Technicians	P.H.M. Smeets	P [dot] H [dot] M [dot] Smeets [te] differ [dot] nl
	H.J.N. van Eck	H [dot] J [dot] N [dot] vanEck [te] differ [dot] nl
	S. Brons	S [dot] Brons [te] differ [dot] nl
	P.R. Prins	P [dot] R [dot] Prins [te] differ [dot] nl
	O.G. Kruyt	O [dot] G [dot] Kruyt [te] differ [dot] nl
	J. Scholten	J [dot] Scholten [te] differ [dot] nl
	R.S. Al	R [dot] S [dot] Al [te] differ [dot] nl
	A.R. Lof	A [dot] R [dot] Lof [te] differ [dot] nl
PhD Student	M.H.J. ’t Hoen	Hoen [te] differ [dot] nl

 

Magnum PSI

 

Magnum-PSI will be a world-wide unique facility for plasma-surface interaction research relevant for ITER and reactors beyond ITER. Magnum-PSI will provide a steady-state high flux plasma with up to 1024 ions m-2 s-1 at a temperature in the eV range, a large beam area of up to 80 cm2 and a continuous magnetic field of 3 T. The power density on the target can be as high as 5-10 MW m-2 under continuous operation and up to 2 GW m-2 in pulsed operation. Magnum-PSI will be the only device so far to enter the strongly coupled regime, in which molecules and dust particles that come off the surface are trapped and remain part of the plasma-surface interaction system. Scientific operation of Magnum-PSI is scheduled to start mid 2011.

 

D3 overview of Magnum

 

Present status of Magnum-PSI

Basic construction of Magnum-PSI has been finished in 2009. The complete vacuum system with its most critical components is now available. Targets can be transported through the system as well as rotated in any given orientation. Plasmas can be made on a routinely basis. The present focus of the group is on the implementation of diagnostics and on reaching all specified parameters of the experiment. In the following, an overview of the major components of Magnum-PSI is given.

Vacuum system

The main vacuum system, where plasma production and exposure takes place, is constructed in modules and placed on a rail-system for easy access. The system is pumped by two turbo molecular pumps with a pump capacity of 4.4 m3/h leading to a base pressure of 10-4 Pa. The plasma source running at full power requires a gas inlet of ~50 Pa∙m3/s. Since at the target the pressure has to be reduced to <1 Pa, a differentially pumped system was chosen. The system consists of 3 vacuum vessels separately pumped by 3 roots pumps (accumulated pump-speed of ~55000 m3/h) (hyper link).

 

Main vacuum vessel

 

Part of the vacuum system is the TEAC (Target Exchange and Analysis Chamber). The target, which is attached to a 5-m long target manipulator [hyper link], can be withdrawn from the main vacuum system to the TEAC. The chamber will be equipped with a variety of surface analysis techniques [hyper link]. The TEAC has its own turbo molecular pump and can be disconnected from the rest of the experiment by a double gate valve system. With the valves closed, a vacuum of low 10-6 Pa can be reached, and the target can be removed or exchanged without venting the rest of the system.

Magnet

The superconducting magnet consists of 5 solenoids (NbTi cooled by liquid He) wound on a single stainless steel former. It has a warm bore of 1300 mm and an axial length of 2450 mm. The magnet produces a homogeneous magnetic field of up to 3 T and covers the main vacuum system. The magnetic energy stored in the magnet at a field of 3 T is 23 MJ. The magnet has 16 radial access ports of 190 mm diameter to allow for diagnostic access during plasma exposure. The magnet is now being assembled and tested at the manufacturer site. 

 

3T superconducting magnet 

 

Target manipulator

The target is attached to the 5-m long target manipulator allowing transport of the target from the main vacuum system to the TEAC. The manipulator can be rotated by ±120◦ and allows tilting of the target against the magnetic field in a range of ±90◦ (with 0◦ being perpendicular to the magnetic field). This rotation and tilting is particularly challenging due to the water cooling requirements of 50 l/min making the design of a rotating water feed-through necessary.

 

Target and Target manipulator

 

Plasma source

The plasma source presently used in Magnum-PSI is a cascaded arc source powered by a 45 kW power source [hyper link]. With this source a relatively small (1-2 cm) diameter high flux plasma beam can be produced. For large beam dimensions (~10 cm diameter), a 270 kW source will be needed (5-10 MW m-2 power density on target). For that reason four power supplies, developed by the Eindhoven University of Technology, have been installed. In addition, a scheme for a pulsed plasma source has been developed to simulate transient heat loads with duration of 0.5–1ms as they occur during so-called ELM instabilities in the tokamak (up to 2 GW m-2 power density on target).

 

 Source powersupplies and source manipulator

 

Currently, the ‘Low Temperature and Plasma Physics Heating’ group [Hyper link] concentrates on broadening of the plasma beam diameter to the specified 10 cm and on prolonging the lifetime of the source. 

RF Heating

Experiments on PILOT-PSI show that with a 45 kW plasma source, plasma electron temperatures up to 5 eV can be achieved. In order to reach electron temperatures up to 10 eV, additional heating by means of electromagnetic wave power in the radio frequency (RF) range is planned. Two RF heating methods are proposed: lower hybrid (LH) heating and ion cyclotron resonance (ICR) heating. Both methods are presently under study and will be tested on PILOT-PSI. 

Cooling system

An important aspect in the design of Magnum-PSI was thermal effects originating in the excess heat and gas flow from the plasma source and radiation from the target. A total of approximately 500 kW of power and cooling capacity was foreseen. This is based on a total of 50 kW on the target (25 kW from a 270 kW plasma source and 25 kW from a 50 kW RF-heating source). The remaining cooling capacity is mainly needed for cooling the vacuum pump systems and power supplies. Most of the dissipated power is removed by 4 separate water cooling systems: high pressure circuit for cooling of components receiving hig heat flux; low pressure circuit for cooling of vacuum vessel walls; cold water circuit for cooling of vacuum pumps and power supplies; emergency cooling circuit for cooling of heat shield inside superconducting magnet and for cryocoolers.

Diagnostics

The main vacuum system and the TEAC will be equipped with a suite of diagnostics to characterize the plasma, exposure conditions and the target. In the TEAC, characterization of the target can be carried out under better vacuum conditions than in the main vacuum system.
Basic operation diagnostics comprise of calorimetry (source and target), temperature and pressure measurements, residual gas analysis, current and voltage measurements (source and target) and monitoring (with cameras). For characterization of the plasma, the main diagnostics is Thomson Scattering delivering very accurately electron density and temperature profiles. Thomson Scattering has been implemented both near the source and near the target via 2 dedicated laser beam lines linked to the vacuum system. Information on the purity of the plasma is obtained via Optical Emission Spectroscopy. During plasma exposure, the target will be investigated by a fast Infrared camera, a multi-wavelength pyrometer, laser induced desorption spectroscopy (LIDS), laser induced ablation spectroscopy (LIAS). In the TEAC, the target will be characterized by laser induced desorption/ablation (LID/LIA). These techniques will be made operational before scientific operation of Magnum-PSI will start. More diagnostics will be implemented in the future.

 

Argon plasma beam with electron density graph 

 

Control and data acquisition

The control and monitoring of the basic installation is realized with PLC based systems. From the Control-room the operator can control Magnum via graphical user interfaces. The control and data acquisition interfaces are being developed mainly in-house together with the DIFFER software engineering department [Hyper link]. A selection of 150 recorded signals is stored to a central database. The stored data (vacuum pressures, voltages, gas flows, temperatures, etc.) can be analyzed later with a dedicated viewer or with analyses software. Currently, the control and data acquisition system is being extended to include the diagnostic measurement systems. An accurate timing and fast data acquisition will be build to support these diagnostics.

 

                             Graph of two recorded vacuum pressures.

 

Research Highlights

 

Modeling and experiments on differential pumping in linear plasma generators operating at high gas flows 

We have used neutral gas simulations and done experiments to show that differential pumping can be used effectively in linear plasma generators operating at high gas flows. The neutral gas dynamics of the linear plasma generator Magnum-PSI has been modeled with the DSMC code developed by Bird¹. This code was chosen because Magnum-PSI will operate in the transitional gas flow regime, with local Knudsen numbers well above 0.1. An efficient way to reach low pressures with large gas flows is differential pumping, where the vacuum vessel is divided by skimmers into separate chambers that are individually pumped. In a two stage differentially pumped system, the optimum shape and position of the first skimmer has been determined. For a good performance of the skimmer, it was found that the tip of the skimmer should be inside the low density region of the expansion since the neutral density increases in the shock region. Therefore the skimmer should be able to penetrate the shock with a minimum influence on the flow. The optimum position thereby depends on the operating conditions of the source (e.g. atomic mass number and the gas flow). The simulation results agree with experimental data obtained on the linear plasma devices Pilot-PSI and PLEXIS. The angle between the skimmer and the gas flow must be kept shallow enough as to not interfere with the expansion, but a skimmer that is too shallow will form a flow restriction for the plasma beam. The optimum inner angle of the skimmer was found to be around 53 degrees. It is shown that differential pumping works in large linear plasma generators operating in the transitional regime. In Magnum-PSI the distance between the source and the skimmer can be varied. This makes it possible to place the skimmer before the shock position in different operating conditions (e.g. gas flow, atomic mass number, background pressure). In the Magnum-PSI operating conditions, a factor 4 pressure reduction in the case of H₂ can be achieved with a two stage differential pumping scheme. This factor increases for heavier gasses (e.g. D₂ and Ar). In Magnum-PSI a 3 stage differentially pumped vacuum system will be used to keep the neutral pressure in the target chamber below 1 Pa, the limit set by the ITER relevance of PSI studies.

 

¹) A. Bird, Molecular gas dynamics and the direct simulation of gas flows (Clarendon, Oxford, 1994).

 

 

Fraction of particles which travel through a 10 cm diameter sampling area with its center on axis, as a function of the distance from the source (lines) and fraction of particles crossing the opening of a 10 cm diameter skimmer at different positions (open symbols). A lower fraction indicates a higher skimmer performance.

 

Pressure plot of the DSMC calculation where 40 slm D2 gas expands in a three stage differentially pumped vacuum system. Some flow lines are shown for clarity. Neutral pressure below 1 Pa in the target chamber is reached.

 

Hydrogenic retention in tungsten exposed to ITER divertor relevant plasma flux densities

To better understand the effect of high-flux plasmas on refractory metals, tungsten (W) targets were exposed to high density, low temperature deuterium plasmas in Pilot-PSI. This investigation measured the amount of plasma-implanted deuterium that was trapped and retained in the tungsten target for a range of plasma exposure times (4 – 160 s). The plasma conditions were similar to what is expected in the ITER divertor (n_e ~ 10²⁰ m^-3, T_e ~ 2 eV, heat load ~ MW∙m^-2) and the W target surface temperature was ~1600 K at the center of the target and decreased to ~1000 K at the edges. Deuterium retention was measured locally in the first 3 µm of the surface by nuclear reaction analysis (NRA). A 2-D NRA scan of the surface revealed significantly higher retention at the cooler edges (6 mm off center, T_w ~ 1000 K) of the target as compared to the center of the target. This indicated that surface temperature was playing a dominant role in determining hydrogenic retention properties as compared to plasma flux density or plasma fluence. Thermal desorption spectroscopy (TDS) measured the global retained deuterium inventory in the exposed W targets. TDS analysis showed very low retained fractions (10^-5-10^-7 D_retained/D_incident) and overall D inventory (D_retained = 0.5-1.5 x 10¹⁶ D). TDS also revealed that polishing the surface of the target enhances retention by a factor of ~2 while annealing the target at 1300 K for 30 minutes has little effect on hydrogenic retention under these conditions. Both TDS analysis and NRA showed no clear dependence of retained D on incident plasma fluence possibly indicating the absence of plasma-driven trap production under these exposure conditions. These results indicate that when operating at surface temperatures of 1000-1600 K, the W strikepoints of the ITER divertor will not retain significant amounts of deuterium (or tritium) due to the bombardment of the surface by the high flux of low energy plasma hydrogenic ions.

Read more in: G.M Wright et al., J. Nuc. Mat., accepted for publication

 

The left graph shows the results from the 2-D NRA scan of the W target exposed to D plasma for a total of 80 s (~10²² D total fluence). This clearly shows a minimum in retention at the hot center of the target and the highest retention at the cooler edges. The low retention seen at some of the 8 mm off center locations may be a shadowing effect from the target clamping ring. The right graphs show a) the total retained fluence as a function of incident fluence integrated across the entire exposed surface, and b) the retained fraction as a function of total incident fluence. The steep decrease and then flattening of the retained fraction in b) may indicate saturation in the W target.

 

Annual Report

 

Read about the 'Plasma Surface Interactions' group in the Annual Report 2009 (page 43 - 54)

 

Recent publications

 

‘Pre-design of magnum-PSI: A new plasma-wall interaction experiment’, H.J.N van Eck et al., Fusion Engineering and Design 82 (2007) 1878

 

‘Hydrogenic retention in tungsten exposed to ITER divertor relevant plasma flux densities’, G.M. Wright et al., Journal of Nuclear Materials 390–391 (2009) 610

 

'Modeling and experiments on differential pumping in linear plasma generators operating at high gas flows', H.J.N. van Eck et al., Journal of Applied Physics 105 (2009) 063307

 

‘Carbon film growth and hydrogenic retention of tungsten exposed to carbon-seeded high density deuterium plasmas’, G.M. Wright et al., Journal of Nuclear Materials 396 (2010) 176

 

‘Hydrogenic retention of high-Z refractory metals exposed to ITER divertor-relevant plasma conditions’, G.M.Wright et al., Nucl. Fusion 50 (2010) 055004 (9pp)

 

 ‘Hydrogenic retention in irradiated tungsten exposed to high-flux plasma’, G.M.Wright et al., Nucl. Fusion 50 (2010) 075006 (8pp)

 

'Collective Thomson scattering for ion temperature and velocity measurements on Magnum-PSI: a feasibility study', H.J. van der Meiden, Plasma Physics and Controlled Fusion, 52, 045009, March 2010