(EPM) and toroidal Alfvén eigenmode (TAE) are thought to be important in a future burning plasma because they may lead to degradation of energetic particle ...
9th IAEA Technical Meeting on Energetic Particles
in Magnetic Confinement Systems
November 9 - 11, 2005, Takayama, Japan
Studies on fast ion transport induced by energetic particle modes using fast particle diagnostics with high time resolution in CHS M. Isobe1, K. Toi1, H. Matsushita2, K. Goto3, N. Nakajima1, S. Yamamoto4, S. Murakami5, A. Shimizu1, C. Suzuki1, K. Nagaoka1, Y. Yoshimura1, T. Akiyama1, T. Minami1, M. Nishiura1, S. Nishimura1, K. Matsuoka1, S. Okamura1, D.S. Darrow6, D.A. Spong7, K. Shinohara8, M. Sasao9 and CHS team 1National
Institute for Fusion Science, Toki 509-5292 Japan 2The
Graduate University for Advanced Studies, Japan 3Nagoya University, Nagoya, Japan 4Osaka University, Japan 5Kyoto University, Japan 6Princeton Plasma Physics Laboratory, USA 7Oak Ridge National Laboratory, USA 8Japan Atomic Energy Agency, Japan 9Tohoku University, Japan.
Outline of talk 1. Motive 2. Compact Helical System (CHS) and fast ion diagnostics in CHS 3. Experimental results - Energetic particle modes (EPMs) driven by tangentially co-injected NB ions - Effect of EPMs on fast ion transport - On toroidal Alfvén eigenmode 4. Summary
Motive of this work Kinetically driven MHD modes such as energetic particle mode (EPM) and toroidal Alfvén eigenmode (TAE) are thought to be important in a future burning plasma because they may lead to degradation of energetic particle confinement.
My goal : Reveal effect of fast ion-driven MHD mode on fast ion transport in helical plasma of l=2.
Compact Helical System (CHS) CHS is a medium size helical device having number of toroidal field periods m=8 and multipolarity l=2. Machine parameters - R/a : 1.0 m/~0.2m - Bt : up to 2 T
Bt
IHF H+ plasma
NBI#2 (H): 32kV/0.8MW 1.5 Co-injected 1 1/q
Standard configuration Inward shifted(Rax=0.921m)
0.5
NBI#1 (H): 40kV/0.8MW, Co-injected
0
0
0.2
0.4
0.6 r/a
0.8
1
Arrangement of fast ion diagnostics in CHS NPA at equatorial plane - viewing angle variable from co- to counter-. - pulse counting mode, 5kHz
Scintillator-based lost fast ion probe(LIP) at outboard side - barely co-going transit ions
Co-going beam ions
LIP at inboard side - barely counter-going transit ions - helically trapped ions
The other important diagnostics for this study are : - Multi-channel Hα light detectors - Magnetic probes
CHS lost fast ion probe PMTs < 200 kHz
Study on MHD-induced fast ion losses
Beam splitter
Perspective view of probe tip C-MOS camera < 2 kHz Fast ion
Identify v///v and E of lost fast ions 50
Z (cm)
Bellows driving Probe shaft
ZnS(Ag) scintillator impact
orbit
tch i P
B
le g an
Gyroradius centroid - Only fast ions can enter the detector box and hit the scintillator surface.
0
R (cm) -50
Apertures
50
100
150
- Location of scintillation light due to impact of fast ions involves both information of pitch angle and energy of lost fast ions.
Typical bursting MHD modes observed in neutral beam-heated plasma of CHS Bursting MHD activities are often observed in CHS when high power neutral beam is tangentially coinjected into a relatively low density plasma. Magnetic probe signal
2
NBI (x2)
0 10 8 6 4 2 0 2
Frequency (kHz)
ECH
1
dBθ/dt
3
ne (x1019 m-3)
Ip (kA)
ECH/ NBIs (a.u.)
Bt/Rax=0.92T/0.974m (Outward shifted configuration) NB#1 : 39 keV / NB#2 : 32 keV / Total Pnb : 1.64 MW
shot#122765
1
m/n=3/2
0 0
50
100
Time (ms)
150
200
80
85
90
95
100
Time (ms)
Characteristics : - Periodic recurrence - Rapid frequency sweeping from 100 kHz down to ~ 50 kHz - Rotate in ion-diamag. direction - Mode number : m=3/n=2
Shear Alfvén continua (2D) in CHS Outward shifted, Rax=0.974 m β=0% equilibrium n=2, Zeff~3
1/q
1.5
1 0.5
- Experimentally observed mode frequency ranges from 100 kHz down to 50 kHz and is located below the TAE gap.
0 500
Frequency (kHz)
400
- It can be reasonably said that observed bursting MHD mode is so called energetic particle mode (EPM).
300 200 100 0
EPM 0
0.2
0.4
0.6 r/a
0.8
1
Effect of energetic particle mode on fast ion transport
0.4
shot#124868 Rax=0.962m/Bt=0.92T NB#1:Eb=39.5keV NB#2:Eb=31.9keV
Hα signal from small R side
3
0 80
85
90
95
100
105
dB/dt
0.2
110
Tangential view
Torus center
Hα (a.u.) NPA counts LIP (a.u.)
Time (ms)
Hα (a.u.)
Hα (a.u.)
Periodic increase on lost fast ions, fast CX neutral flux and Hα signals while EPM appears in CHS
m/n= 3/2
0 -3 2 1 0 50
E=39.1keV
0 0.4 0.2 2M-center 0 0.2 0.1 2M-outer 0 80
85
90
95
100
105
110
Time(ms)
- Periodic increase of LIP, NPA and Hα signals at the large R side coincide with each EPM burst. - On the other hand, no change in Hα emission from the edge at the inboard side. - It looks as if co-going beam ions diffuse toward the outside and are lost at large R side due to EPMs.
Enlarged pulses : time width = 3 ms dBθ/dt
shot#124868 Rax=0.962m/Bt=0.92T NB#1:Eb=39.5keV NB#2:Eb=31.9keV
Frequency (kHz)
- Fast ion loss signal begins after the mode amplitude reaches the maximum. - Time evolution of Hα signal is similar to that of fast ion loss rate.
Hα (a.u.) LIP (a.u.)
- Mode frequency becomes low as the fast ion loss rate increases. - Weak TAE mode appears between the two EMP bursts but no change on fast ion loss signal.
Lost fast ion probe
- It looks that EPM is stabilized after expulsion of beam ions. Hα emission from the edge 80
81
82 Time (ms)
83
Fast ion loss rate to the probe are more enhanced as magnetic fluctuation amplitude increases
3
Rax/Bt=0.962m/0.91T Pnb=~1.6 MW shot#124866, 124868
∆Γfastion (a.u.)
t=70-90ms
~ - There exists a threshold in Bθ (~4x10-5 T) for EPM-induced fast ion losses.
2
- Fast ion losses ~ due to EPM activities appear when Bθ is above 4x10-5 T and ~ increase as Bθ increases.
1 ~ ∆Γfastion ∝ Bθ3.0
0 0 No enhanced loss
5 10 ~ Bθ(x10-5 T)
15
Only fast CX neutral particles having energy close to Eb are affected by EPMs
0 3
NPA viewing angle : v///v = -0.85 at Rax
x1019 m-3
-> Limit angle in parallel direction
0 -3 20
E=41.1keV
NPA spectra shown by contour
10 0 50
Energy (keV)
NPA NPA NPA NPA NPA dB/dt
ne
1 0.5
NPA
shot#124868 Rax=0.962m/Bt=0.92T NB#1:Eb=39.5keV, NB#2:Eb=31.9keV Te(0) ~ 0.34 keV
E=39.1keV
25 0 80
E=35.3keV
40 0 80
E=31.4keV
42 40
36 E=25.5keV
34
20 0 75
44
38
40 0 40
46
80
85
90
Time (ms)
95
100
105
75
80
85
90 95 Time (ms)
100
105
- Increase of particle flux are observed only in energy range close to Eb. - Clear sign on slowing down of particles enhanced in flux has not been seen. - It looks as if only particles having E~Eb are affected by EPMs and are lost.
Dependence of fast CX neutral particle flux on NPA's viewing angle, i.e. pitch angle shot#124868 NB1:39.5kV / NB2:31.9kV / total Pnb=1.62MW
Rax=0.962cm/Bt=0.91T
NPA line of sight
angle 1 : more parallel
angle 2 : slightly perp.
Cogoing
angle 2 angle 1 0
Perp.
v///v
Pitch angle distribution along the line of sight
-0.5
Co0
0.2 0.4 0.6 0.8 r/a
1
0.5 0 15 10 5 0 3
1
NPA NPA NPA NPA NPA dB/dt Ip (kA) ne
IHF
NPA NPA NPA NPA NPA dB/dt Ip (kA) ne
1
-1
shot#124876 NB1:39.5kV / NB2:31.8kV / total Pnb=1.63MW
x1019 m-3
0 -3 20
E=41.1keV
10 0 50
E=39.1keV
25 0 80
E=35.3keV
40 0 80
E=31.4keV
40 0 40
E=25.5keV
20 0 75
80
85
90
Time (ms)
95
100
105
0.5 0 15 10 5 0 3
x1019m-3
0 -3 20
E=41.2keV
0 50
E=39.4keV
0 80
E=36.0keV
40 0 80
E=30.9keV
40 0 40
E=25.8keV
20 0 75
80
85
90
95
100
Time (ms)
- Flux of beam particles having parallel v///v is largely enhanced during EPM activities. - On the other hand, flux near Eb is quite small and no significant increase is observed when NPA is set to be slightly perpendicular. - It seems that beam ions (E~Eb) relatively parallel in v///v drives EMPs and are expelled.
105
Pitch angle distribution of beam ions at birth place 0
NPA
v///v
shot#124868 t=85ms Bt=0.91T/Rax=0.962m
600 Number of particles
Birth place of beam ions
Initial pitch angle as a function of r/a
-0.5
angle-1
Pitch angle spectrum
500 400 300 200 100
-0.5 -1 0
angle-2
-10
0.5 r/a
1
0 -1
-0.5 v///v
0
- Pitch angle range along NPA angle-1 is more parallel than that along angle-2 and is located at higher density domain of beam ions in pitch angle.
NB Eb=39.5keV
- Therefore, NPA set to be more parallel see significant effect of EPMs on beam ion transport because parallel beam component is thought to be driving source on excitation of EPMs.
Lost fast ion probe signals measured at large R side - Bright spot appears on the scintillator surface due to impact of fast ions Pitch angle (degrees) 100
EPM-quiescent phase t=74-75 ms
110 120
Co-, parallel 130 135 145 140 1.5 2.0 2.5 3.0 3.5 4.0 5.0
m/n=3/2
LIP
Primary spot
6.0
Hα
100
70
75
80 85 Time (ms)
90
EPM phase t=85-86 ms
- Primary loss spot appears in pitch angle of 130~133 degrees (v///v= -0.64 ~ -0.68). - Measured gyro-radius is consistent with that of ions having Eb. - During EPM, scintillation light intensity in more parallel pitch angle domain significantly increases.
110 120
Perp. 130 135 145 140 1.5 2.0 2.5 3.0 3.5 4.0 5.0 6.0
Gyroradius centroid (cm)
Frame rate : 1 kHz
Gyroradius centroid (cm)
Frequency (kHz) dBθ/dt
shot#122764 Rax/Bt=0.974 m/0.92 T
Orbits of fast ion reaching the probe Gyromotion-following orbits reaching the probe are calculated backward in time. Probe tip
Pitch angle : 142 degrees
Poincaré plot
- Barely passing - Guiding center still inside of LCFS
m/n=3/2
Pitch angle : 131 degrees - Barely passing but escaping orbit
Probe tip strike point
0.4
Probe tip 0.2
Z (m)
strike point
Projection on R-Z plane
0 -0.2 -0.4 0.6
0.8
1.0
R (m)
1.2
1.4
Effect of TAE modes on beam ions in CHS
TAE mode and its effect on beam ions - Toroidal Alfvén eigen (TAE) mode is excited by tangentially coinjected NB.
x1019 1
dBθ/dt
0
0 TAE
1
-2
EPM
-4 -6 0 87
m/n=2,3/1
87.5 88 88.5 Time (ms)
-8 89
TAE
EPM m/n=3/2
core-localized
Fluctuation amplitude
~ Bθ (T)
Hα (a.u.) LIP (a.u.)
Frequency (kHz)
2
2
m-3
Fast ion loss rate to probe (a.u.)
ne
Enlarged pulse t = 87-89 ms
shot#124870 Rax/Bt=96.2cm/0.91T
dBθ/dt (a.u.)
2
Amplitude ~ 1x10-5 T
Time (ms)
Time (ms)
- Significant increase of fast ion loss is not seen probably due to small amplitude of fluctuation.
TAE mode with higher fluctuation level shot#124870 Rax/Bt=96.2cm/0.91T
t = 130 - 142 ms
2
~ Bθ (T)
Fluctuation amplitude
amplitude :(3-4)x10-5 T
110
120
130 Time (ms)
140
150
Fast ion loss rate to probe (a.u.)
TAE
0 -2 1 -4 -6 -8 0 130 132 134 136 138 140 142 Time (ms)
- TAE can enhance fast on loss rate to probe when fluctuation level is above 3x10-5 T. - Threshold in fluctuation level to increase fast ion loss is ~2.5x10-5 T.
dBθ/dt (a.u.)
2
Summary Energetic particle mode (EPM) has been observed in CHS when high power neutral beam is tangentially coinjected into a relatively low density plasma. While EPM bursts appear, - Only fast CX neutral particles having energy close to Eb and parallel pitch angle are periodically increased. - Clear sign on slowing down of particles increased in neutral flux has not been seen. - Fast ions whose energy close to Eb are detected by scintillatorbased fast ion probe located at outside of plasma. These suggests that beam ions drive EPM and are consequently lost by EPM excited by themselves. TAE mode also enhances fast ion transport even in smaller fluctuation amplitude than EPM.
