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
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2. Motivation of the research
Saturation and/or reduction of the density and temperature (pressure) have been observed in high-power ICRF heating experiments on GAMMA 10
Transport induced by waves are 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
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3. Diffusion due to existence of the resonance layer
in GAMMA (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
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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
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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
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6. GAMMA 10 device and ICRF systems
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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
ECH B
(T)
Potential Z 7 7
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8. Magnetic Field Line and Antenna Configuration
ICRF powers are injected only
into the central cell
RF1 System
9.9, MHz
RF2 System
6.36 MHz
Double Half
Turn antenna
Nagoya Type III
antenna
Single layer
Faraday shield 8 8
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9. Schematic Drawing and Photograph of RF Antenna System
TypeIII Antenna
DHT Antenna
Plasma
プラズマ
Diamag. Loop
Gas Box 9 9
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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 wave heating.
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
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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
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12. Diagnostics 12
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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
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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
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15. Measurement of pitch angle scattering of high-energy ions due to AIC-modes15
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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
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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
RF 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
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18. Pitch angle scattering due to low frequency fluctuations
Limiter Floating Potential
Electrostatic Probe
Drift-type fluctuations
eeHED: high energy ion to the end
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
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19. Measurement of radial transport of high-energy ions due to low-frequency fluctuations19
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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
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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
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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
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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
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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 and 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
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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
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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
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