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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

Dec 30, 2015

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  • HSX Thomson Scattering ExperimentK. Zhai, F.S.B. Anderson, and D.T. AndersonHSX Plasma LaboratoryUniversity 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 11012/cm3. 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

  • 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: densitySmall cross section, requiring a powerful monochromatic light source.

  • 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 degreein which, A is a constant andTheoretical TS Spectrum

  • Schematic Diagram of the HSX Thomson Scattering System Basic features: 10 points (20 cm radial range), double pulse (40-100us)

  • 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

  • A commercial YAG laser is used as the scattering source.10ns and 1J output pulse at the fundamental wavelength of 1.06mLocated on optical table in clean roomDouble pulse operation

    Laser System

  • 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 LensPosition relative to focus (cm)Spot waist (mm)

  • Beam Transportation and Stray Light ControlBeam 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.

  • HSX TS Beam TransportationYAG LaserHW platemirrorfocusing lensBrewster windowCollection opticsPlasma regiondump120cm120cm388cm305cm297cmTotal length from laser exit to focal point: 925cmLens focus length: 305cm

  • Entrance Tube and Exit TubeSpecially 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.bafflecritical aperture

  • Layout of the collection optics with respect to plasma regionObservation vacuum windowCollection lensLaser beamImage planeof fiber bundlesurfaceGate valveCollection OpticsCollection solid angle: (2.9-3.1) 10-2Scattered photons: Ns=(2.4-2.6)104Spectrum width: =17-246nm

  • System Aperture:Entrance Pupil DiameterEffective Focal Length: 16.70 cm (in image space)Back Focal Length: 4.11 cmWorking F/#: 2.05Image Space NA: 0.237Object Space NA: 0.11Paraxial Magnification: -0.459Entrance Pupil Diameter:10 cmEntrance Pupil Position: 4.66 cmExit Pupil Diameter: 20.11cmExit Pupil Position: -40.03cmPrimary Wave: 1064 nmAngular Magnification: 0.49Optical Properties of the Collection Lens

  • Layout of the Collection Lens and Its Coupling to Fiber BundlesTwo-layer doublet: BK7 and SF1Diameter:10cm

  • Fiber Opticsfiber length:7msingle fiber NA:0.24-0.25

  • Fiber Transmission TestExpanded laser beamCylindrical lensrectangularfiber endcircular fiber endViewing lensdetectorCylindrical lens linear focus the beam into fiber bundle at a given NA=0.24Viewing lens collect the transmitted lights at a given NA=0.25

  • Fiber Bundle Transmission Test ResultTen fiber bundles corresponding to ten radial channels. Total transmissionThe transmission of ten fiber bundles is within the range from 0.625-0.585, comparing the ideal transmission of 0.63.

  • Coupling to Fiber Bundles Length unit: cmEach square corresponds to an individual fiber bundles rectangular surface of 0.8mm*7mmLaser beam image on the fiber surface length unit:mm

  • 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

  • Polychromator CalibrationThe 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.

  • Experimental Setup for Spectral Calibration Tungsten lampmonochromatorReference detectorFiber opticsAbsolutely calibrated detectorDATA acquisitionComputer control

  • Spectral response function andTS scattering spectrum for different electron temperatureWavelength (nm)Measured Polychromator (SN:39024-124) Spectral Response Function Together with the Scattering Form Factor S (Te, =90)

  • Electron Temperature (eV)Ratio of signals in different spectral channelsConversion Function Based on the Ratios of Signals in Different Spectral Channels

  • 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

  • Overview of the Thomson Scattering Timing Sequence During HSX Discharge Plasma timeHSX master triggerTS timingLaser triggerDigital delayGating digitizerPersonal computer LabVIEW control

  • Acquired Signal During a Thomson Scattering Experiment

    Type of signalSample realizationSampling timeStray light1~300-400ms, before plasma breakdownScattering light1~800-850ms, during plasma dischargePedestal subtractionFluctuation(noise)5After plasma

  • Computation of the Weighted Average Electron TemperatureWeighted average of electron Temperature and its error:,where Error analysisUncertainty of the scattered signal in spectral channel j,Upper and lower limits of spectral channel signal ratio,Electron temperature range associated with the ratio limits

  • Flow Chart of Thomson Scattering Data Processing ProgramGet raw dataSeparate raw data:Get stray lightGet scattering lightGet pedestal subtractionSubtract pedestalSubtract stray lightBackground pedestal analysiscompute standard error of pedestal, stray light, and scatteringBuild ratiosCompute error in channel ratio based on deviation of stray light, scattering light, and background noiseGet Te for different channel ratios from look-up tableCompute final Te from weighted averageComputer error from weighted averageEND

  • 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 1012/cm3 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

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