Thomson Scattering
Double-pulse Thomson scattering
In March 2000, a double-pulse multi-point Thomson scattering system has been installed to measure the electron temperature and density profile at 120 spatial points along a full vertical chord through the plasma. The principle of the diagnostic is that the photons in the laser beam are scattered by the electrons in the plasma. The number of scattered photons from any volume in the plasma is proportional to the local electron density in that volume. Due to the thermal motion of the electrons in the plasma, the scattering spectrum is Doppler broadened: its width is proportional to the square root of the local electron temperature.
The ruby laser (l = 694.3 nm) was mostly operated in a mode with two 10 - 12 J laser pulses, with a time separation of 200 - 500 ms. The laser beam enters and exits the plasma through about three meter long vacuum tubes, with baffles to reduce stray light from the windows. The laser chord is imaged by means of a six-element lens onto a fiber-optic dissector. Fibers guide the scattered light to a Littrow spectrometer, which is equipped with two separate intensified CCD cameras to record the two laser pulses. The resemblance of the detector to a TV camera has led to the name TV Thomson scattering. Figure 1 gives an overview of the detector optics (fiber bundle, spectrometer and cameras).
Figure 1: The collected scattered light is relayed to a Littrow spectrometer giving a 2D image ((l,z). This image is projected onto the high-sensitive cathode of a TV-like detector, capable to record 12-bit images of 512´384 pixels with a rate of 10,900 frames per second.
The double-pulse Thomson scattering diagnostic has become a routine instrument and has given input to many physics programs. Electron temperature and density profiles have been measured in a large range of different experiments. High-density RI-mode profiles turned out to be relatively broad and smooth. Very clear MHD islands have been seen with a density peaking inside the islands, confirming earlier work with a prototype four-channel pulsed radar reflectometer.
Multi-pulse Thomson scattering
Although the double-pulse Thomson-scattering system yields electron temperature and density profiles with unprecedented spatial resolution, it has a drawback that it can only give at most two snapshots during a plasma discharge. For this reason the double-pulse system has recently been upgraded to a multi-pulse TV Thomson-scattering system. The goal of the new system is to measure with the same high spatial resolution as the double-pulse system on a time scale of 10 kHz during three bursts of 5 – 10 ms. The system consist of three main parts:
- an intra-cavity multi-pulse laser,
- a coherent fiber optic bundle to relay the collected light to the spectrometer,
- and a Littrow spectrometer equipped with an ultra fast detector.
A schematic lay out of this system is shown in Figure 1 (fiber bundle, spectrometer and cameras) and Figure 2 (the intra-cavity laser).
Figure 2: Schematic overview of the TEXTOR TV-Thomson scattering system with an intra-cavity ruby laser producing three bursts of ~ 50 pulses with ~15 J/pulse, at a rep. rate of ~10 kHz.
For this system the Ioffe Institute in St. Petersburg has developed a so-called intra-cavity laser. This name indicates that the plasma volume is included in the laser cavity and, hence, the beam travels up and down through the scattering volume. In this way high probing energies can be obtained with only a single ruby rod of 19 × 200 mm2 (0.03% Cr+). At a pumping power of 5 MW during 10 ms, a train of up to 50 pulses with 15 J each can be produced at a maximum frequency of 10 kHz. A Pockels’ cell is used as active Q-switch to control the burst of laser pulses. Operation of the laser in a three or even four burst mode is in preparation.
After relay and spectral analysis, the scattered light is detected with a complex detector. It consists of a 25 mm Generation III image intensifier with ~ 50% quantum efficiency, a stack of three proximity focused image intensifiers, a tandem lens system and two fast CMOS cameras for recording. Each camera can sample images with 512 × 384 pixels of 22 mm at a frame rate of 10,900 frames/s and a 12 bit dynamic range. One camera is used to record the Thomson scattering light, while the other one is required to sample the plasma light between subsequent laser pulses. This additional recording is needed because of the rather long laser pulse duration of 1.5 – 2.0 ms, which is 20 times longer than for the previous double-pulse TS system with a laser pulse width of 20 ns. The sensitivity of the CMOS camera of ~ 50 photons/count and the light losses of the tandem coupling lens make it necessary to apply additional light amplification. Therefore, a stack of three proximity-focused image intensifiers is used to pile up the photon gain by a factor of 200. Fast sampling with a TV like detector sets special requirements to the image-to-image cross talk. To keep this as small as possible, fast P46 phosphors are used for the output screens of the image intensifiers. The image-to-image cross talk of the CMOS cameras is ~0.02 when a bias light level is given to the camera (i.e. each image has a ~ 2% contribution from the previous image). A fast programmable burst generator (home made) is used to control the timing of the laser and the detector.
Temperature and density profiles along the full plasma diameter of 900 mm can be sampled at 120 points with a spatial resolution of 7.5 mm (see Figure 3). A separate system for edge observations between z = 340 and 500 mm with a resolution of 1.7 mm has been installed recently. Along the full chord the expected statistical errors on Te and ne for 50 eV < Te <2 keV are ~ 8 and 4%, respectively, at ne ~ 2.5 × 1019 m-3. These data were obtained from simulations in which the plasma light is about 25% of the TS light level, as was extrapolated from experiments with the double pulse system. Also included was the effect of the different spectral widths due to forward and backward scattering of the down and up going beams.
Figure 3: Electron temperature profile from Thomson scattering, clearly showing a shoulder from the magnetic island of the locked m/n = 2/1 mode (the profile is obtained by averaging over nine subsequent Thomson spectra). The z-coordinate roughly corresponds to the minor radius of the plasma. Since, however, the line of sight is shifted by Dr/a = 0.2 away from the plasma center, as sketched on the right hand side, the profile is asymmetric: For negative z it goes through the O-point of the island, whereas for positive z only the X-point is seen.
Multi-pulse multi Pass Thomson scattering
The latest modification of the Thomson system is the multi-pass option. By including an second spherical mirror in the beam path and tilting this slightly, a single laser pulse will travel through the plasma several times, effectively increasing the probing energy. (at this moment the setup is designed for 12 passes, which indeed can be reached routinely now. Combining this option with the multi-pulse option, allows to have an effective probing energy of up to 3 kJ within the 8 ms of Thomson laser train, as depicted in Figure 4. With this high energy detailed structures in the plasma can be observed. A typical example of rotating islands is shown n Figure 5. Please note the accuracy down to a few eV only.
Figure 4: Effective probing energy of the Thomson scattering laser in the different stages of the development, ging from a conventional double pulse system, to a multi-pulse double pass system to a 12 pass system, which is from 2009 in routine operation at TEXTOR.
Figure 5: Typical result of the multi pass mulit-pulse Thomson scattering system: observation of temperature changes due to rotating m=2 islands. The contour plot shows only the deviation from the averaged temperature profile. The resolution is better than 10 eV, on a central temperature exceeding 1 keV. (The black line depicts the oscillation as observed with an ECE channel)