1. HSX Thomson Scattering Experiment K. Zhai, F.S.B. Anderson, and D.T. Anderson HSX Plasma Laboratory University of Wisconsin-Madison, The HSX Thomson Scattering system is based upon the design of GA divertor TS system and is constructed in collaboration with the MST group. The system is capable of 10-point profile measurement and double pulse operation that can provide measurement of rapid change of plasma parameters. Large collection optics are used in the system to get enough scattered photons from the HSX plasma with a typical density of 1  10 12 /cm 3. Ten identical fiber bundles with a transmission rate of 0.6 couple the collected photons to ten polychromators that disperse the collected light. Four wavelength channels in each of the ten polychromators are optimized for temperature measurement range from 10eV-2keV. A dedicated CAMAC system is used to record the data. The initial test results of the system will be presented. *Work supported by US DoE under grant DE-FG02-93ER54222

2. Incoherent Thomson Scattering Plasma electrons get accelerated in laser field and then emit electromagnetic radiations. -Shape of the scattered laser light spectrum: electron temperature -Amplitude of the scattered light: density Small cross section, requiring a powerful monochromatic light source.

3. TS spectrum for an isotropic relativistic Maxwellian electron velocity distribution (Plasma Physics, Vol. 14, pp. 783 to 791 ) Theoretical relativistic scattering spectrum for a plasma diagnosed with YAG (1060nm) with a scattering angle of 90 degree in which, A is a constant and Theoretical TS Spectrum

4. Schematic Diagram of the HSX Thomson Scattering System Nd:YAG laserBeam transportationHSX vesselBeam Dump Fiber Bundles Collection Optics Polychromators Avalanche Photodiodes Amplifier Data AcquisitionControl System Basic features: 10 points (20 cm radial range), double pulse (40-100us)

5. Interdependent Subsystems Laser system Beam transportation and stray light control Collection optics of the scattered light Spectrum dispersion and detection system Signal handling and data acquisition Control system

6. A commercial YAG laser is used as the scattering source. 10ns and 1J output pulse at the fundamental wavelength of 1.06  m Located on optical table in clean room Double pulse operation Laser System YAG Laser trigger F=3.05m focus lens CCD frame grab camera 6.2m 3.05m Ceramic disc attenuator

7. Laser focus spot viewed on a ceramic disc with a CCD camera and video capture card. Laser spot size relative to the distance with the focus (cm) Laser Focus Spot Using a 3m Focus Lens Position relative to focus (cm) Spot waist (mm)

8. Beam Transportation and Stray Light Control Beam is guided by three laser mirrors and is focused to the HSX vessel with an f=3.05m focus lens. A 1/2 waveplate is used to adjust the beam polarization. Entrance and exit tubes are specially designed with baffles to control the stray light. Entrance and exit windows are Brewster angle orientated fused silica windows.

9. HSX TS Beam Transportation YAG Laser HW plate mirror focusing lens Brewster window Collection optics Plasma region dump 120cm 388cm 305cm 297cm Total length from laser exit to focal point: 925cm Lens focus length: 305cm

10. Entrance Tube and Exit Tube Specially designed baffles prevent the stray light reflected from the entrance tube wall from passing into the vessel directly. And the critical aperture will prevent the stray light originating from the entrance window from getting into the vessel. Fused silica windows are oriented at Brewster angle to the incident laser. baffle critical aperture

11. Layout of the collection optics with respect to plasma region Observation vacuum window Collection lens Laser beam Image plane of fiber bundle surface Gate valve Collection Optics Collection solid angle: (2.9-3.1)  10 -2 Scattered photons: N s =(2.4-2.6)×10 4 Spectrum width:  =17-246nm

12. System Aperture:Entrance Pupil Diameter Effective Focal Length: 16.70 cm (in image space) Back Focal Length: 4.11 cm Working F/#: 2.05 Image Space NA: 0.237 Object Space NA: 0.11 Paraxial Magnification: -0.459 Entrance Pupil Diameter:10 cm Entrance Pupil Position: 4.66 cm Exit Pupil Diameter: 20.11cm Exit Pupil Position: -40.03cm Primary Wave: 1064 nm Angular Magnification: 0.49 Optical Properties of the Collection Lens

13. Layout of the Collection Lens and Its Coupling to Fiber Bundles 40cm 45cm 11.56cm 19.01cm Two-layer doublet: BK7 and SF1 Diameter:10cm

14. Fiber Optics fiber length:7m single fiber NA:0.24-0.25

15. Fiber Transmission Test Expanded laser beam Cylindrical lens rectangular fiber end circular fiber end Viewing lens detector Cylindrical lens linear focus the beam into fiber bundle at a given NA=0.24 Viewing lens collect the transmitted lights at a given NA=0.25

16. Fiber Bundle Transmission Test Result Ten fiber bundles corresponding to ten radial channels. Total transmission The transmission of ten fiber bundles is within the range from 0.625-0.585, comparing the ideal transmission of 0.63.

17. Coupling to Fiber Bundles Length unit: cm Each square corresponds to an individual fiber bundle’s rectangular surface of 0.8mm*7mm Laser beam image on the fiber surface length unit:mm

18. Ten identical polychromators designed and manufactured by GA. Four wavelength channels in each polychromator optimized for the measurement of the electron temperature range from 10eV to 2keV. Silicon avalanche photodiode detector ( EG&G C30956E ) and amplifier provided by GA are attached to the polychromators. Output from the amplifier range from 0.0 to –1.0 volt. Spectrum Dispersion and Detection System

19. Polychromator Calibration The spectral calibration determines the response of the detection system to a radiation source of constant spectral emissivity. The result of spectral calibration will be used to build a look-up-table for electron temperature measurement. Spectral calibration of each channels in a polychromator can be measured separately in the lab. Absolute calibration of the absolute sensitivity of the complete detection system can only be performed in site on HSX machine to get electron density measurement.

20. Experimental Setup for Spectral Calibration Tungsten lamp monochromator Reference detector Fiber optics polychromator Absolutely calibrated detector DATA acquisition Computer control

21. Wavelength (nm) Spectral response function and TS scattering spectrum for different electron temperature Measured Polychromator (SN:39024-124) Spectral Response Function Together with the Scattering Form Factor S (Te,  =90°)

22. Electron Temperature (eV) Ratio of signals in different spectral channels Conversion Function Based on the Ratios of Signals in Different Spectral Channels

23. A computer controlled CAMAC system dedicated for HSX Thomson scattering experiment. –A GPIB crate controller from KINETICS SYSTEM is used to communicate between the CAMAC crate and the computer. –The signal is recorded by gating Leroy Model 2250 charge integrating digitizer. These digitizers have a sensitivity of 0.5pC/count, with a range of 512 counts. –LabView program ready. System synchronized with HSX timing with a NI6602 timing card and a DG535 digital pulse generator from Stanford Research Systems. –LabView program ready. Signal Handling DATA System and Control system

24. Overview of the Thomson Scattering Timing Sequence During HSX Discharge Plasma time Laser emission Scattering light Digitizer gating 05001000 ~800-850Time:s Stray light background HSX master triggerTS timingLaser trigger Digital delay Gating digitizer Personal computer LabVIEW control

25. Acquired Signal During a Thomson Scattering Experiment Type of signal Sample realization Sampling time Stray light 1 ~300-400ms, before plasma breakdown Scattering light 1 ~800-850ms, during plasma discharge Pedestal subtraction Fluctuation(noise) 5 After plasma

26. Weighted average of electron Temperature and its error: Computation of the Weighted Average Electron Temperature,where Error analysis 1.Uncertainty of the scattered signal in spectral channel j, 2.Upper and lower limits of spectral channel signal ratio, 3.Electron temperature range associated with the ratio limits

27. Flow Chart of Thomson Scattering Data Processing Program Get raw data Separate raw data: Get stray light Get scattering light Get pedestal subtraction Subtract pedestal Subtract stray light Background pedestal analysis compute standard error of pedestal, stray light, and scattering Build ratios Compute error in channel ratio based on deviation of stray light, scattering light, and background noise Get Te for different channel ratios from look-up table Compute final Te from weighted average Computer error from weighted average END

28. 10-point, double-pulse Thomson scattering system optimized for the measurement of HSX plasma parameters of electron temperature of 10eV-2keV and electron density of 10 12 /cm 3 or higher. While all the subsystem function properly, successful operation also require precise alignment of the whole system, the input optics and collection optics. One point measurement of system is expected to operate in this year. Summary of the System and its Operation Schedule

29. Copies 1 2 3 4 5 6 7 8 9 10 11 12