plasma start-up, sustainment, and heating by rf waves in tst-2
DESCRIPTION
Plasma Start-up, Sustainment, and Heating by RF Waves in TST-2. Y. Takase, A. Ejiri, Y. Nagashima, O. Watanabe, Y. Adachi, B. An, H. Hayashi, S. Kainaga, H. Kobayashi, H. Kurashina, H. Matsuzawa, T. Oosako, J. Sugiyama, - PowerPoint PPT PresentationTRANSCRIPT
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Plasma Start-up, Sustainment, and Heating by RF Waves in TST-2
Y. Takase, A. Ejiri, Y. Nagashima, O. Watanabe, Y. Adachi, B. An, H. Hayashi,
S. Kainaga, H. Kobayashi, H. Kurashina, H. Matsuzawa, T. Oosako, J. Sugiyama,
H. Tojo, K. Yamada, T. Yamada, T. Yamaguchi, T. Masuda, Y. Ono, M. Sasaki
The University of Tokyo
Joint Meeting of the4th IAEA Technical Meeting on Spherical Tori
and the14th International Workshop on Spherical Torus
7-10 October 2008Frascati, Italy
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TST-2 Spherical Tokamak
Nominal parameters: R = 0.38 m a = 0.25 m Bt = 0.2 T Ip = 0.1 MA
HHFW 21 MHz400 kW
ECH 2.45 GHz5 kW
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Part I. Noninductive Ip Start-up and Sustainment
• Ip start-up by ECW (2.45 GHz)
– Three phases of Ip start-up
– Dynamics of closed flux surface formation
– MHD activity and Ip collapse
• Ip sustainment by RF (21 MHz) power alone
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Three Phases of Ip Start-up by ECH
3Phase 1 2
Open Field Lines Current Jump
Closed Flux Surfaces
z[m]00.4
-0.4
-0.40
0.40
0.4-0.4 x[m]y[m]
0 0.5
0
0.6
-0.6
will use dIp/dt(17ms ~ 22ms)
0 0.5
0
0.6
-0.6
00.1-0.1
00.4
-0.4 0 0.4-0.4
z[m]
x[m]y[m]
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RF (21MHz) power can induce a current jump.
Plasma current can be sustained by RF power alone.
Antenna excites waves with a broad spectrum of toroidal mode numbers, up to |n| ~ 20. But only |n| = 0, 1 can propagate to the core.
Ion heating is not expected due to high harmonic number (> 10). Ion (H/D, C, O) heating was not observed.
Electron heating is expected to be weak due to low T. Soft X-rays (~ 2 keV) were observed at high RF power (~ 30 kW).
RF onlyRF
RF sust.EC sust.
80 ms RF only
Sustainment by RF Power
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Initial Current Ramp-up Rate and Ip-Wk Trajectory
Dependence on various parameters are summarized by a scaling lawJ. Sugiyama et al., Plasma Fusion Res., 3, 026 (2008).
0.0
0.2
0.4
0.6
0.8
0 0.02 0.04 0.06 0.08 0.1
Pla
sma
curr
ent
[kA
]
Stored kinetic energy [J]
Z
KP BR
WI
2
open: before jump
closed: after jump
KP WI is confirmed by equilibrium analysis
RFEC
Single-particle orbit theory predicts
A. Ejiri and Y. Takase, Nucl. Fusion, 47, 403 (2007).
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mag. axis
jt
F term
p termTruncated boundary
LCFS
pol. flux tor. current force bal.
flux loops saddle loops pickup coils
pressure pol. field pol. flux
pol. angle pol. anglez
“Truncated equilibrium” was introduced to include finite pressure and current in the open field line region. A. Ejiri et. al., Nucl. Fusion 46, 709 (2006).
Truncated equilibrium can reproduce magnetic measurements (~80 channels), and can be used to analyze all three phases.
)()1( 00
000
0
FR
r
r
Rj
d
dff
Rd
dpRj
pp
Equilibrium Reconstruction
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Dynamics of EC Induced Current Jump
EC induced current jump occurs when Ip exceeds a critical value Ip,crit where
Ip after current jump is given by
Equilibrium reconstruction reveals slow and soft formation of initial closed flux surfaces. While Ip increases rapidly, Wk and Rjmax increase slowly.
0.0
0.4
0.8
Pla
sma
curr
ent
[kA
]
(a)
IP I
PLCFS
0.2
0.3
0.4
0.5
0.6
0
0.1
20 25 30 35 40 45 50
Rjm
ax
Sto
red
En
ergy
[J]
Time [ms]
(b)
Just before and after closed flux surface formation
kA/mT 5.0, z
critp
B
I
kA/mT 2.1z
p
B
I
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Conditions for RF Induced Current Jump and Sustainment
RF induced current jump occurs when the injected RF power exceeds a threshold, which is different for H and D.O. Watanabe, et al., Plasma Fusion Res. 3, 049 (2008) .
Injection timing should be just before a current jump.
Current jump does not occur by RF power alone, and Ip stays at a low level < 0.3 kA.
Only low n|| waves can propagate to the plasma core, and formation of high energy electrons is expected. Soft X-ray energy spectrum indicates the presence of high energy (2-3 keV) electrons. However, Ip can be sustained even when soft X-rays are not observed. 0
2
4
6
8
10
0.5 1 1.5 2 2.5 3Energy [keV]
Log (N)
Exp(-E/42eV)
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Comparison of Equilibria during Sustainment
Truncated boundary
LCFS
j
Inboard limiter
LCFS Outboardlimiter
#53773 50ms
Truncated boundary
LCFS
j
Inboard limiter LCFS Outboard
limiter
#53783 50ms
Truncated boundary
LCFS
j
Inboard limiter LCFS Outboard
limiter
#53197 90ms
RF sustained, Ip = 0.6 kA EC sustained, Ip = 0.6 kA EC sustained, Ip = 1.3 kA
%80/ ,70~ ,4.1[ms] 01.0~[eV], 20~
0
0
pLCFS
pLCFS
E
IIq
T
%93/ ,50~ ,1.1
[ms] 02.0~[eV], 45~
0
0
pLCFS
pLCFS
E
IIq
T
%70/ ,50~ ,0.1
[ms] 05.0~[eV], 180~
0
0
pLCFS
pLCFS
E
IIq
T
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Ip Collapses are Often Observed during RF Sustainment
Power spectra of inboard BZ
RF sustainmentw/o collapse
ECH alone
Inboard Bz
Outboard Bz
Ip
4 discharges with almost the same operational conditions
Low and high frequency components are observed for collapsed discharges
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Expansion of Open Field Line Region is Observed before MHD Activity
Phenomelogy of Ip collapse
Slow fluctuations
Rapid growth of high frequency
fluctuations
Ip collapse
EC
RF
Collapsed discharges are different from the beginning of RF pulse.
Inward shift and expansion of open field line
region
0.3
0.4
0.5
Rax
[m
]
0.0
0.4
0.8
42 44 46 48 50 52
Vol
ume
[m3 ]
Time [ms]
Inside LCFS
Whole
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Summary
• Sustainment of ST plasma by low frequency RF power was demonstrated.
• Equilibrium analysis revealed detailed information during each phase of discharge.
• Initial current formation phase is characterized by a slow increase in Ip, proportional to the stored energy.
• During the current jump phase, initial closed flux surfaces are formed gradually, and changes in Wk and Rjmax are small. soft dynamics
• Sustained ST plasma has high p>1 and high q0>30
• MHD instability often terminates the RF sustained plasma, but no such phenomenon is observed for the EC sustained plasma.
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Part II. HHFW Heating and Parametric Decay
• Electron heating
• Parametric Decay Instability (PDI)– Parameter dependences– Newly discovered sub-harmonic decay branch
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Introduction
• A degradation of heating efficiency is observed during high-harmonic fast wave (HHFW) heating of spherical tokamak plasmas when parametric decay instability (PDI) is observed. Understanding and suppression of PDI is necessary to make HHFW a reliable heating and current drive tool in high plasmas.
• In TST-2, wave measurements were made using a radially movable electrostatic probe (ion saturation current and floating potential), RF magnetic probes distributed both toroidally and poloidally, microwave reflectometry, and fast optical diagnostic.
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Typical Discharge Heated by HHFW(Inboard Shifted Plasma)
• Te = 140 170 eV over 0.4 ms after RF turn-on (PRF = 200 kW)
• PDI becomes stronger and Te decreases slowly after 0.4 ms (causality?)
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ES probe = 165
inner wallprobes
Reflectometer = -75
RF Diagnostics
HHFWAntenna
front surface of S.S. enclosureat R = 635mm
RF magnetic probes
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I
cos(p+t+RF)
sin(p+t+RF)
VCO6-10GHz
X4 5-20mW
DC-500MHzQLO RF
coaxial scalar horn
RF21MHzeit
Ep x Bt
Aeit Aeit+i
Digitizer (25MHz) or Oscilloscope (~250MHz)
F.G.
X5
X10
Gunn25.85 or 27.44 GHz
DC-100MHz
waveguide
D.C.-3dB
cutoff surface
Mirror
Second Mirror
~500mm
Launching horn
Receiving horn
24-40GHz100mW
Microwave Reflectometer
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0
100
200
300
HH
FW
[kW
]
0
20
40
60
4521845219452244522745189
I p
[kA
]
0
0.1
Sof
t X
-ray
[a.u
.]
0
5
10
Rad
iati
on[a
.u.]
0
20
40
PF
3[k
A]
45218
45219 45224
45227
45189
0
5
16 17 18 19 20 21 22
NL
[1018
m2 ]
Time [ms]
PDI Spectra Measured by Reflectometer
Reflectometer
H plasma
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f=1MHz
Pow
er [
10dB
]
1.7 MHz
19.3 MHz
21 MHz
22.7 MHz
#45244
20.0 20.5 21.0
Pow
er [
10dB
]
Time [ms]
1.7 MHz
19.3 MHz
21 MHz
22.7 MHz
#45227
HHFW
HHFW
10-7
10-6
10-5
10-7 10-6 10-5 10-4
P(-f ci
)
P(f0)
P(f0)2
Time Evolution and Power Correlation
Reflectometer
Sideband power varies quadratically with the pump wave power
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Electrostatic Probe
Digitizer channelsCh. 1: mag. probe at = 155Ch. 2-4: ES probe at = 165 2: f1
3: Iis
4:f2
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)()(2tY )()(
2tZ
)()()(
)()(22
2*
tZY
YZ
)(
)()(Re
)()(Imtan
*
*
1 tYZ
YZ
titi eZtzeYty )( ,)(
sampling rate: dt (2 ns)data window for FFT: N (10000)overlapping of data window: N/2 (5000)points for smoothing along time : m (49)
time resolution for : dtNm/2 = 0.49 ms tfZ2
)(
Spectral Analysis of RF Data
tfZ pump
2)(
tz ty
0t
pumpf 0
2)( tfZ
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Time Evolution and Power CorrelationB and Iis
B~
isI~
correlation becomes higher and phase shift becomes definite during second half of RF pulse
H plasma
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1
~f2
~f
Time Evolution and Power Correlationf1 and f2
correlation is nearly one and phase shift is almost zero for the pump wave
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isI~
1
~f
Time Evolution and Power Correlationis and f1
correlation is intermediate and phase shift is non-zero
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Newly Discovered Sub-Harmonic Decay Modes
f3
f2
f1
f0
f
deuterium
hydrogen
f increases with B
Two additional peaks were discovered between f0 and f0 – fcH in H plasmas(note that there is a dip at f0 – fcD)
– These modes may involve molecular ions or partially ionized impurity ions.
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R (mm)R (mm)
isI~
2
~f
Radial Fall-off is Steeper for is than f2
outboard limiter
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Phase Difference Between Neighboring RF Probes
2/
2/
t (ms)
t (ms)
2/
t (ms)
t (ms)
RF
B~
= - 65 = - 55
= - 0.5 corresponds to |n| = 18 Phase shift is not constant throughout the RF pulse.
B~
@
sampling rate: dt (2 ns)data window for FFT: N (500)
no window overlapping no smoothing time resolution = 1 s
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B Dependence of Frequency Spectrumat Different Locations
D plasma
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• RF probes on the outboard side have similar signal levels.
• RF probe on the inboard side has much smaller signal levels compared to the outboard side in low B discharges, but comparable in high B discharges.
• The vertical (poloidal) polarization is much weaker than the horizontal (toroidal) polarization.
• The frequency difference between the pump wave and the lower sideband wave increases with the magnetic field.
• The lower sideband becomes weaker, and the lower sideband peak becomes unresolved at low magnetic field.
Summary of RF Magnetic Probe B Scan
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Summary of PDI Observations
• The frequency spectrum exhibits peaks at ion-cyclotron harmonic sidebands f0 ± nfci and low-frequency ion-cyclotron harmonics nfci, consistent with the HHFW pump
wave decaying into the HHFW or ion Bernstein wave (IBW) sideband and the ion-cyclotron quasi-mode (ICQM).
– PDI becomes stronger at lower densities, and much weaker when the plasma is far away from the antenna.
– The lower sideband power was found to increase quadratically with the local pump wave power.
– The lower sideband power relative to the local pump wave power was larger for reflectometer compared to either electrostatic or magnetic probes.
– The radial decay of the pump wave amplitude in the SOL was much faster for Iis than for f.
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Conclusions