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OS2010

Radial transport of high-energy ions due to low-frequency fluctuations in the GAMMA 10 tandem mirror. M. Ichimura, Y. Yamaguchi, R. Ikezoe, Y. Imai, T. Murakami, T. Iwai, T. Yokoyama, Y. Ugajin, T. Sato and T. Imai Budker Institute of Nuclear Physics,

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OS2010

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  1. Radial transport of high-energy ions due to low-frequency fluctuations in the GAMMA 10 tandem mirror M. Ichimura, Y. Yamaguchi, R. Ikezoe, Y. Imai, T. Murakami, T. Iwai, T. Yokoyama, Y. Ugajin, T. Sato and T. Imai Budker Institute of Nuclear Physics, OS2010, July 5-9, 2010, Novosibirsk, Russia Contents 1.Motivation of the research 2. Diffusion near the cyclotron resonance layer 3. Pitch angle scattering due to AIC-modes 4. Radial transport due to low-frequency fluctuations 5. Summary 1 1 OS2010 OS2010

  2. Motivation of the research Saturation and/or reduction of the density and temperature (pressure) have been observed in high-power ICRF heating experimentson GAMMA 10 Transport induced by wavesare possible candidates Interactions * with ICRF waves Turning point diffusion near the cyclotron resonance layer * with high-frequency fluctuations in the ion cyclotron frequency range Alfvén Ion Cyclotron modes (AIC-mode) are spontaneously excited due to strong temperature anisotropy Pitch angle scattering due to AIC-modes * with Low-frequency fluctuations in kHz range Drift-type fluctuations and Flute-type fluctuations have been identified Radial transport of high-energy ions due to low-frequency waves 2 2 OS2010 OS2010

  3. Diffusion due to existence of the resonance layer in GAMMA 10(Nuclear Fusion 1988) GAMMA 10 plasmas are sustained by formation of high b plasmas in the anchor cell. (Formation of plasmas does not depend on the magnetic field strength in the central cell and depends on that in the anchor cell.) Operation window is indicated that the cyclotron resonance layer exists within closed mod-B surface region Operation Window (a) – (b) When the resonance layer exists on mod-B surface with open configuration, ions can diffuse along mod-B surface 3 3 OS2010 OS2010

  4. Diffusion due to existence of the resonance layer in Phaedrus-B(Phys. Fluids B 1992) Diffusion is enhanced when the cyclotron layer exists in thermal barrier cell without resonance layer with resonance layer 4 4 OS2010 OS2010

  5. Phaedrus-B Enhanced end loss current with the resonance layer Radial profile of the density and potential indicate the enhanced radial diffusion when the resonance layer exist in the thermal barrier cell 5 OS2010

  6. GAMMA 10 device and ICRF systems /26 OS2010

  7. GAMMA 10 Device GAMMA 10 is an axisymmetrized tandem mirror with minimum-B anchors ICRF: Plasma Production Ion Heating MHD Stabilization ECH: Potential Formation Electron Heating East plug/barrier cell West plug/barrier cell Central cell East anchor cell West anchor cell ICRF ECH 3 2 1 0 ECH B (T) Potential Z 7 7 OS2010 OS2010

  8. Magnetic Field Line and Antenna Configuration ICRF powers are injected only into the central cell RF1 System 9.9, 10.3 MHz RF2 System 6.36 MHz Double Half Turn antenna Nagoya Type III antenna Single layer Faraday shield 8 8 OS2010 OS2010

  9. Schematic Drawing and Photograph of RF Antenna System TypeIII Antenna DHT Antenna Plasma プラズマ Diamag. Loop Gas Box 9 9 OS2010 OS2010

  10. Typical Plasma Parameters n ~ 2 x 1012 cm-3, Ti > 5 keV Peak ion temperature reached more than 10 keV with the slow Alfvén waveheating. Plasmas with a strong temperature anisotropy more than 10 have been formed Location of cyclotron resonance layers of RF1 and RF2 Temporal evolution of plasma parameters 10 10 OS2010 OS2010

  11. Excitation AIC-modes due to strong anisotropy Alfvén ion cyclotron (AIC) modes are excited due to a strong temperature anisotropy. The modes excited in the central cell of GAMMA 10 have several discrete peaks. The frequency of the AIC mode is just below the ion cyclotron frequency. The spatial mode structures of each discrete peak in radial and azimuthal directions are confirmed to be the same structure. The AIC modes are excited as eigenmodes in the axial direction. Temporal evolution of the AIC modes 11 11 OS2010 OS2010

  12. Diagnostics 12 OS2010

  13. Photograph of Central Limiter and Probes Electrostatic probe array : ESP (Ion saturation current and density fluctuation) Electrostatic probes are also set in the axial direction Segment limiter D = f 0,36 m (Floated and divided into 8 sections in azimuthal direction) (Floating potential and azimuthal structure of fluctuations) Magnetic probe array :MP (RF wave measurement ) 13 13 OS2010 OS2010

  14. central cell High Energy-ion Detector : ccHED Schematic drawing of ccHED Locations of ccHED and eeHED eeHED:at the east end (to measure the axial transport) ccHED : at the central cell midplane (to measure the radial transport) The ccHED has a co-axial geometry. The ccHED is inserted perpendicularly to the magnetic field line and is positioned just outside of the limiter radius. By rotating the inner and the outer pipes together, a pitch angle distribution of hot ions can be measured. When a pin-hole aperture of which diameter is 0.2mm on the outer pipe is used, the resolution of the pitch angle becomes ±3 degrees. When apertures are arranged in the electron diamagnetic direction, no signals are detected and when the aperture covered with an aluminum foil, no signals are detected. These imply the discrimination of protons from electrons, neutrals and UV is possible. 14 OS2010

  15. Measurement of pitch angle scattering of high-energy ions due to AIC-modes 15 OS2010

  16. Pitch angle scattering due to AIC-mode In GAMMA 10, Alfvén Ion Cyclotron (AIC) modes are spontaneously excited due to a strong temperature anisotropy. When the amplitude of the AIC modes becomes strong, high-energy ions trapped in the central cell (ccHED) are scattered to the end (eeHED). ccHED signals on the different pitch angles. Behavior of high-energy ions with small pitch angles (60 and 75 degrees) is the same as the behavior of un-trapped ions. The enhancement of the pitch angle scattering from perpendicular to parallel directions is suggested. 16 OS2010

  17. Observation of low-frequency fluctuations related to AIC-modes AIC-mode has several discrete peaks Fluctuations with beat frequencies between each peak of AIC-modes are observed in the central cell RF2 6.36 MHz AIC-modes Floating potential of cc-limiter Low frequency waves Temporal evolution of the frequency spectrum of the magnetic probe signal Drift-type fluctuations In the central cell, two types of fluctuations are observed in lower-frequency region (drift-type and flute-type) 17 OS2010

  18. Pitch angle scattering due to low frequency fluctuations Limiter Floating Potential Electrostatic Probe eeHED: high energy ion to the end Drift-type fluctuations Pitch angle scattering due to low-frequency waves related to AIC-modes is clearly observed at the east end No end-loss ions due to drift-type fluctuations 18 OS2010 OS2010

  19. Measurement of radial transport of high-energy ions due to low-frequency fluctuations 19 OS2010

  20. central cell High Energy-ion Detector : ccHED Interpretation of ccHED signal When an aperture of the minimum size is used, pile-up signals are still obtained near the plasma edge. When ccHED is set at the location of r = 25 cm, discrete signals are obtained. (Limiter radius is 18 cm and Larmor radius of 10 keV hydrogen is about 1.4 cm.) Signal of 5.5MeV a-particle from 241Am 1v/d 1msec/d These discrete peaks are burst-like escaping High-energy ions (several hundreds particles). This burst frequency corresponds to that of the drift-type fluctuation. 20 20 OS2010 OS2010

  21. Raw signals of ESP and ccHED As indicated in the power spectrum of ESP and ccHED signals, fluctuations with same frequency are clearly observed. 21 21 OS2010 OS2010

  22. Pitch angle dependence of the phase difference between signals of density and high-energy ions To determine the relation between fluctuations in density and high-energy ion signals, the pitch angle dependence of the phase differences between both signals is evaluated. 22 22 OS2010 OS2010

  23. Pitch angles corresponds to the turning points of high-energy ions in the central cell Turning points of ions which have pitch angles of 85, 75 and 65 degrees at the location of ccHED are indicated below. (pitch angle of 75 deg.) 23 OS2010

  24. Mode structure of the fluctuation Axial direction Axial wave number is estimated from phase differences between probes located at z = 0.33 m and 1.20 m. Two probes are set at the location of z = 0.33 m and have azimuthal angles of +22.5 and -22.5 degrees different from the probe at z = 1.2 m, respectively. Axial wave number is determined by values obtained from both probes. Azimuthal direction Rotation in the direction of electron diamagnetic drift 24 24 OS2010 OS2010

  25. Radial transport near the turning points Assume the phase differences at the midplane (pitch angle of 90 degree) is zero High- energy ions interact with low-frequency fluctuations mainly near their turning points ccHED OS2010

  26. Summary • Three types of wave-particle interactions are observed in GAMMA 10. • Turning point diffusion near the cyclotron resonance layer is suggested in minimum-B configuration on the anchor cell. • 2. Pitch angle scattering of high-energy ions due to AIC-modes and low-frequency waves which have differential frequencies between discrete peaks of AIC-modes. • 3. Radial transport of high-energy ions due to drift-type fluctuations near their turning points in the confining mirror field. 26 26 OS2010 OS2010

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