1) For equivalent ECRH power, off-axis heating results in lower stored energy and lower core temperature 2) Plasma flow is significantly reduced with off-axis.

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1) For equivalent ECRH power, off-axis heating results in lower stored energy and lower core temperature 2) Plasma flow is significantly reduced with off-axis heating. Doppler shift cannot account for change in frequency or amplitude of density fluctuations 3) Increased core density fluctuations with off-axis heating ; may act to degrade energy confinement Provides measure of density gradient fluctuations Core Density Fluctuation Measurements by Interferometry in the HSX Stellarator Abstract C. Deng, D.L. Brower, University of California, Los Angeles, D.T. Anderson, F.S.B. Anderson, A. Briesemeister, K. Likin, J. Lore, J. Schmitt, J.N. Talmadge, R. Wilcox, K. Zhai, University of Wisconsin-Madison HSX – Quasi-Helically Symmetry Stellarator Interferometry System 53 rd Annual Meeting of the Division of Plasma Physics, November 14-18, 2011, Salt Lake City, Utah Coherent and Broad Band Fluctuation Measured by Far-Forward Scattering major radius: 1.2 m minor radius: 0.15 m magnetic field: 1 T; ECRH: 28 GHz, <150 kW pulse length: < 50 ms QHS HSX Provides Access to Configurations With and Without Symmetry QHS: helical axis of symmetry in |B| Red  |B|  1.0 T Blue  |B|<1.0 T Mirror Flux Surface and Chord Positions Mirror: quasi-helical symmetry broken by adding a mirror field Coherent mode in QHS Plasma Results From Three Techniques Density Fluctuations Increase with off-axis Heating (QHS plasma, Bt=1T in CCW) Plasma Parameter Time Traces Plasma Parameters Coherent mode observed in Mirror Plasma for B T =1 T (ccw) Mode observed in density, not in magnetic signals Density window for mode is narrow: ( )* cm -3 Decrease in ECE emission suggests decrease in energetic electron confinement mode amplitude is higher on high field side  phase shift observed across magnetic axis (m is odd) mode not observed on external magnetic coils Coherent mode in biased QHS plasma with V bias =+300V 1. Interferometry, Differential Interferometry and Far-forward Scattering are used to measure density fluctuations in HSX - line-integrated measurements, some spatial information available by comparing chords - differential interferometry is used to obtain core localized measurements 2. Both coherent modes and broadband fluctuations are observed 3. For QHS plasma - Significant changes (amplitude and frequency) of fluctuations are observed when heating location changes from on-axis heating to HFS heating - Density fluctuation amplitude and frequency decrease with of electron temperature - core density fluctuations increase and confinement degrades with off-axis heating - coherent mode observed on density and magnetic when heating slightly off-axis 4. Mirror plasma: - Coherent mode observed on density fluctuation signal only, - mode not sensitive to ECRH location Future work will focus on identification of fluctuations Measurement Techniques 1. Interferometry (phase measurement) 2. Far-Forward Collective scattering (a) collective scattering within the divergence of the beam (b) amplitude measurement 3. Differential Inteferometry The multichannel interferometer system on the HSX stellarator is optimized to measure electron density fluctuations by utilizing both phase and amplitude techniques Information on core and edge fluctuations can be realized by comparing chords at different locations or by use of the differential interferometry approach. Both coherent modes and broadband density fluctuations with frequency up to 250 kHz are measured. For quasi-helically symmetric plasmas with B T =1.0 T, significant changes (both amplitude and frequency) in the turbulent density fluctuation spectrum are observed when heating location changes from on-axis to high field side. Density fluctuation amplitude and frequency decrease with increasing of ECRH power (Te). Changes in fluctuations will be compared with measurements of plasma flow (by CHERS) as well as electron density and temperature profile modifications. When HSX is operated without quasi-helical symmetry at B T =1T and n e = 4x10 12 cm -3, a coherent electrostatic mode at 28 kHz is observed. Fluctuation sensitivity to changes of heating location and ECRH power were not observed for these plasmas. QHS: helical bands of constant |B| Mirror: helical bands are broken Localized Density Fluctuation Gradient Measurement (1) For density gradient and gradient fluctuations [differential interferometry] (2) For density fluctuations (m=1) [standard interferometry] Z Plasma Conditions: B T =1T, Counter Clockwise, QHS Main Field Current I B =10891A n e =4x10 12 cm -3, P ECRH =50kW Scattering measurements R-R o =1.0 cm 1. Broadband fluctuations 2. Coherent mode at 16 kHz (stronger on neg. side) 3. Coherent mode observed on magnetic coils Interferometer and Far-Forward Scattering Show Similar Results Far-Forward ScatteringInterferometry Density Fluctuations from Differential Interferometry Measurement Directly measures phase difference between adjacent chords [R-Ro: +1 and -0.5 cm]: Coherence between two chords Lower noise level and core localization for differential interferometer measurement on axis heating: I B =11000A off axis heating: I B =10600A Core localized measurement of Density gradient fluctuations shows large increase in amplitude for off-axis heating Significant change in density fluctuations with heating location for central chords No change in density fluctuations with heating location for edge chord chords Coherence between two adjacent central chords Doppler shift (reduced) cannot account for higher frequency density fluctuations with off-axis ECRH Electron Temperature and Density Profiles T e (Thomson Scattering)n e (Interferometer) Very peaked Te profile flat or slightly hollow density profile (dTe/dr driven diffusion) negative density gradient reduction of density fluctuations Coherent mode in Mirror Plasma Coherent mode (density fluctuation) is observed for slightly inboard heating Magnetic fluctuations observed on some Mirnov coils ECE signals reduced when coherent mode present No change in n e (r) B T =1T in CCW direction Frequency Spectrum of Density Fluctuations Density fluctuations Density gradient fluctuations No changes for broad band fluctuations coherent mode was observed for slightly inboard heating Amplitude and Phase of the Coherent Mode Density mode amplitude peaked near plasma edge Density shows  phase shift across magnetic axis; m is odd Weak Magnetic fluctuations, external coils suggest m=1 Differences in coherent mode for Mirror and QHS configurations: 1.Mirror: density fluctuation only; QHS: both density and magnetic fluctuation 2.Mirror: sensitive to equilibrium density; QHS is not 3.Mirror: sensitive to direction of B field [cw or ccw]; QHS is not 4.Mirror: not sensitive to I B (heating location); QHS case is very sensitive to I B Coherent mode observed in Biased QHS Plasma, and while broad band fluctuations did not change Mode observed in both density, and magnetic signals Amplitude and Phase Plasma flow reduced for Off-Axis Heating Case Input ECH power are similar, but significantly lower stored energy for off-axis heated plasma. Measured density fluctuations are of larger amplitude and at higher frequency for off-axis heating. Interferometer System: 1. 9 chords 2. 1 MHz bandwidth cm chord spacing, width 4.k  < 2 cm -1 Density fluctuations can be measured from information of: (i)Amplitude (ii)Phase (iii)Differential Amplitude Phase Summary and Future Plans Frequency Spectrum No Significant Change in Density Profile Frequency SpectrumDensity Profile