Profile Measurement of HSX Plasma Using Thomson Scattering K. Zhai, F.S.B. Anderson, J. Canik, K. Likin, K. J. Willis, D.T. Anderson, HSX Plasma Laboratory, U. of Wisconsin, Madison 1. Abstract3. The HSX TS System Calibration At the HSX plasma laboratory, a 10 channel Thomson scattering system has been established and is now operational. The system has been absolutely calibrated for density measurement using Raman scattering with nitrogen gas. It is found that for the QHS configuration the electron temperature gradually decreases when we increase the density at a fixed ECRH power and that the whole temperature profile increases with the heating power at a fixed plasma density. The central temperature increases from about 500eV to 950eV while the launched heating power increases from 37KW to 150KW for the QHS configuration plasma with a density of 1.5  cm -3. At the present density and heating power, the difference between the QHS and Mirror mode is not pronounced. Detailed results will be presented at the conference. *Work supported by US DoE under grant DE-FG02-93ER54222 The image spot is less than 100 microns for all the ten channels. Transmission efficiency of the fiber bundle is around 60%. At plasma density of /cm 3, the collection lens collects ~20000 photons for each of the ten fibers, which then couple the photons to the ten polychromators through fiber bundles. 3.a Spectrum Dispersion Calibration 4. Experiment Results 4.b QHS Mode vs Mirror Mode 4.e ECH Off-axis Heating Ten-Channel HSX TS system for electron temperature and density measurement has been calibrated and is now operational for daily routine service. Peaked electron temperature and density profile has been observed for QHS mode. Temperature increases with ECH heating power at fixed density and decreases as density increase at fixed heating power. Off-axis ECH heating affects plasma profiles for both QHS and Mirror mode Signal in different spectral channels indicates the existence of the superthermal electrons. 2.The HSX Device and HSX TS System Major Radius 1.2 m Average Plasma Minor Radius 0.15 m Plasma Volume ~.44 m 3 Rotational Transform Axis1.05 Edge 1.12 Number of Coils/period 12 Magnetic Field Strength (max) 1.25 T Magnet Pulse Length (full field) 0.2 s Auxiliary Coils (total) 48 Heating source28GHz 200 kW HSX (the Helically Symmetric eXperiment) is a new concept in toroidal stellarators that is operational at the HSX Plasma Laboratory of the University of Wisconsin - Madison. It is the only device in the world that will have a magnetic field structure that has been termed Quasi-Helically Symmetric (QHS) 2.a HSX Parameters 2.b HSX TS System Introduction The Thomson scattering system installed on Helical Symmetry Experiment (HSX) is a polychromator-type system, which covers the complete plasma cross section, providing a 10-point plasma parameter profile measurement during a single plasma shot. The system consists of a commercial 1J-10nsYAG laser, 10 polychromators from GA, the specially designed collection optics, a CAMAC data acquisition unit, and a controlling computer. 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. 3.b Absolute Signal Calibration – Raman Scattering Calibration stable over time. This shows that the whole temperature profile increase with the heating power at the fixed density of 1.5  /cm 3. The central temperature increases from about 500eV to 950eV while the heating power increases form 37KW to 150KW. QHS mode, 50 kW ECRH heating power This result shows that the temperature gradually decreases when we increase the density at a fixed ECRH power. Ratio of signals in different spectral channels Electron Temperature (eV) Since the stray light contribution to the first wavelength channel makes the Rayleigh scattering troublesome, we use rotational Raman scattering for density calibration. Nitrogen gas is chosen For safety reason and the second spectral channel extending from 1050 to 1060 nm is used for the calibration, which cover the most Raman scattering power. The TS signal from spectral channel i, after allowance for channel sensitivity, will be For the Raman scattering, the scattered light corresponding to the rotational transition J>J-2 is given by in which Electron density is then given by Raman signal from channel i after allowance for channel sensitivity Principles of Raman Scattering Wavelength (nm) Spectral response function and TS scattering spectrum for different electron temperature. Raman Scattering Calibration Results Most Raman scattering power collected in second spectral channel Raman scattering signal changes linearly with gas pressure. The calibration factor is defined by the slope of the fitted line TS results compared with interferometer results 4.a Plasma Density Scan and ECH Power Scan for QHS Mode Electron temperature is a little higher in QHS mode than Mirror mode. Density profile is more peaked in QHS mode and flat in Mirror mode. TS measurement gives similar density profile compared with the inverted density profile from interferometer measurement. 4.c ECH Heating Power Modulation Energy confinement time from diamagnetic flux measurement: ~0.8 ms Electron temperature drops during ECH modulation of 1 ms. 4.d Superthermal Electron Tail Mirror mode * Electron temperature increases at heating location. * Density profile changes to a center-peaked shape compared with the flat shape of the normal central heating mirror mode. QHS mode * Electron temperature peaks at the heating location and decreases dramatically at center compared with central heating. * Shape of plasma density profile doesn’t change with off-axis heating. Channel 4 have unusual high signal level compared with channel 2 and 3. Fitting the Thomson Scattering signal with bi-Maxwellian electron distribution f total =(1-p) f(T b )+p f (T tail ), we can separate the superthermal electrons’ contribution to the scattering signal. Significant tail in center region at ECH central heating. Tail temperature around 5-8 KeV Increasing ECH heating power increases the tail fraction. Integrated stored energy compared with the diamagnetic flux measurement. 5. Summary Superthermal electron at different ECH heating power and location at QHS mode of 1.5  /cm 3. 70KW 40KW 40KW off-axis Spectral Response (a.u.) Ratio Spatial channel number