Transcript HT-7上逃逸电子行为的研究进展

HT-7
ASIPP
Progress of runaway electron behavior
study in the HT-7 tokamak
WPI Team, H.Lin, X.G.Zu and HT-7 Team
[email protected]
HT-7
Content
From 2005.3 to 2006.1
 Tools for runaways detection
 Suppression of runaways by high initial density
 Interaction of runaways with magnetic field ripple
 Interaction of runaways with LH waves
 Slide away discharges in HT-7
 Enhancement of runaways in LHCD discharges
 Suppression of runaways by LHCD
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Experimental setup
Schematic view of infrared
measurement system in HT-7
 NaI detectors(40*40mm)
HXR : 0.5～7MeV
 CdTe detector: tangential view
HXR : 0.3～1.2MeV
(a)View into HT-7 vessel with an infrared camera without plasma
(b)View into HT-7 vessel with a normal photocamera
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Ohmic discharge ＃77952&77962
Runaway discharge
Suppression of runaway by higher initial density
IR picture for #77952 at 0.7s
IR picture for #77962 at 0.7s
Effective avalanche time is derived by
fitting the IR intensity, it is about 170 ms.
(a) plasma current, (b) loop voltage and consumed magnetic flux, (c) plasma lineaveraged density, (d) HXR intensity measured by NaI detector (HXRI), (e) HXR
intensity measured by CdTe detector.
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＃77952 IR Pattern
Interaction of runway electrons with
magnetic field ripple
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The toroidal magnetic field is slightly modulated
due to finite number of coils. Runaway electrons
accelerated by the electric field can experience a
resonant interaction between their relativistic downshifted cyclotron frequency and the magnetic field
ripple. A resonance occurs when
When the runaways accelerated in the toroidal electric
field cannot cross a particular ripple resonance, they
pile up at this resonance energy as the observed energy
limit in the HXR spectra.
For HT-7: N=24, R=1.22m, Bt~1.8~2.2
ripple
Wmax
( MeV )  0.511
n the harmonic number
eBt R
30

( MeV )
nNme c
n
ASDXE: n=7
Ref: PRL 75 (1995) 4626
n＝8， Wripple＝ 3.75 MeV ；
n＝7， Wripple＝ 4.3 MeV ；
n＝6， Wripple＝ 5 MeV;
n＝5， Wripple＝ 6 MeV ；
n＝4， Wripple ＝ 7.5 MeV
According to theory, different harmonic number of ripple be
efficient in energy blocking for different electric field value.
.
Different harmonic resonance at different
electric field
Plasma current scan
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Different harmonic resonance during current HT-7
scan and density scan experiments
Current scan:Ne=0.5*1019m-3,Ip=60-80kA
Vl=1.3V
n= 8
Vl=1.4V
n= 5
Density scan:
Ip=80kA,
Ne=0.4-0.6*1019m-3
 With increasing current or decreasing density, the HXR spectra become more energetic.
 The HXR spectra show a energy cutoff.
 There is an energy gap in the HXR spectra near the energy limit.
Vl=1.53V
n= 4
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Wmax is the experimental energy limit derived from the HXR spectra.
E is the wide of energy gap. n is the harmonic number.
Wripple is the theory energy limit for runaways & ripple resonance with n .
The energy limit derived from the HXR spectra is consistent with runaways & ripple resonance energy.
 The energy gap become larger for resonance at small harmonic number is larger.
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Interaction of runaways with LH waves（Anomalous
Doppler resonance ）
Due to the relativistic decrease of the electron
cyclotron frequency, a cyclotron resonance may
appear between runaway electrons and lower
hybrid waves.
Ref：Phys. Plasma 9 (2002)1667
 k  n ce  k // v //
 
28 Bt n
 (1  N // )
LH power
ECE
In runaway discharges, after
the injection of LH waves,
the ECE increase step by
step, results from FPAS.
Interaction of runaways with
LH waves?
 n=0: Cerenkov resonance
 n<0: anomalous Doppler resonance.
( Parail-Pogutse instability ，FPAS)
 n>0: normal Doppler resonance.
The resonance energy for the interaction of
runaways with LH waves:
0.55 MeV for LHCD with: W＝2.45*2pi
GHz，Bt＝2T，N//＝4，n=-1;
1.08 MeV for OH target with: W~6GHz，Bt
＝2T，N//＝4，n=-1.
 when the LH power is zero, there is no inverse
sawtooth like structure in ECE signal.
 When the power has 20kW, there has inverse
sawtooth like structure in ECE signal.
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FPAS in runaway discharge
FPAS in current decay
phase from runaway
discharge
ECE increased by a large step, it is
different with slide away discharge.
Results from FPAS.
Interaction of runaways with LH
waves?
HT-7
IR pattern in #84276
0.3s
0.4s
0.5s
0.52s
0.54s
A island splits into bi-island！
0.56s
0.58s
0.66s
0.68s
0.6s
0.7s
0.62s
0.8s
0.64s
0.9s
Left is LFS
Runaways are built up since the current ramp-up phase due to low density. At 0.62s, the runaways
pattern is split into bi-island like structure！The physics mechanism is under investigating.
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Observed island during Pellet injection inTextor-94
Ref: PRL 1994
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Slide-away discharges in HT-7
逃逸放电下密度进一步降低, 放电进入slide-away region. 特征：ECE台阶式增长, 环电压下降, 约束变好(Ha).
理论上阈值条件为：
 pe / ce  1
即当电子等离子体频率低于回旋频率时，一些电子等离子体频率附近的波被激发，逃逸电子和波作用被散射，垂直能量增加。
HT-7实际上的阈值条件是 2.08 (#76750)；2.46 (#76757)
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HT-7上通过电流压缩也实现了slide-away放电.
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Enhancement of runaways in LHCD discharges
 LHCD with high loop
voltage induces runaways
Cut off in the FEB
spectra may be due to
the runaway of electron
with energy higher than
the critical energy.
Critical energy for electron run away
LHCD phase: 229keV
Subsequent OH phase: 87keV
Low energy runaways are built up during LHCD phase
due to the fast electron tail extends over the critical energy.
After the termination of LH power, energetic fast
electrons within slow down time are converted into
runaways due to the increased loop voltage.
IR intensity in runaway enhanced
discharges. The increasing of IR
intensity indicates the building up of
energetic runaways.
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Suppression of runaways by LHCD
 The fall of HXR and the IR intensity
observed at the time of LH injection indicates
that the reduction of the electric field by LHCD
produces a suppression of the existing runaway
population.
 After the termination of LH power, the
runaways recovered.
The energetic runaways can be efficiently
suppressed according to the evolution of HXR
flux and the IR intensity.
 The suppression of runaways by LHCD can
also be identified from the neutron flux. See
signal (h) in the waveforms.
 After the termination of LH power, the fast
electron population keeps at a high level, so the
loop voltage in subsequent OH phase is much
lower than the initial OH phase. See signal (c)
in the waveforms.
(a) is the plasma current, (b) the center line-averaged
density, (c) the loop voltage, (d) the LH power, (e) the
center line-integrated FEB emission intensity, (f) the
forward HXR emission intensity from HXRI, (g) the
backward HXR emission intensity from HXRIII, (h) the
neutron flux from photonnuclear reactions.
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Runaway Probability Function
“I noticed your RSI article. I have just a quick thought that if you continue to
make these interesting measurements, then since you have LHCD on HT-7, you
might also consider measuring the runaway probability function by varying the rf
phase velocity. To my knowledge, such a direct measurement has not yet been done,
though much of the physics has been inferred from current (magnetic)
measurements”. Nat Fisch
“The most information is generally extracted when you can compare a function,
rather than a point, and all of these Green functions can be put as a function of phase
velocity, or even observation time. This method of analysis might allow you to deduce
some of these other parameters that should not be coupled to the rf. You might want
to keep the loop voltage constant then to simplify your analysis. If you go this route,
there will be some theoretical work in generalizing what Karney and I did. You
probably are not up against this limit, but since you are doing the most
comprehensive runaway measurements in the world right now, you might want to see
if you can check on this theory”. Nat
Fisch
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Runaway Probability Function in LHCD plasma
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已经尝试的实验
 改变等离子体小半径，观察在不同磁场波纹强度下的逃逸－波纹共振。
（没有明显变化）
 高密度抑制低杂波关断后的逃逸。（failed，反而引发了雪崩逃逸）
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Next targets
 ECCD下的逃逸增强行为（和585合作）
 逃逸放电下ECCD抑制逃逸的效率（和585合作）
 破裂下的逃逸行为，尤其是破裂下逃逸的能量极限。
 探索抑制和迁移逃逸的有效手段（充气、外加磁扰动、wiggler）
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逃逸诊断使用经费
IR探测器（HgCdTe），液氮制冷，
波长：8～12μm 0.65万
CdTe探测系统：0.5～1.2MeV的 X-ray 1.9万
BGO探测器：范围1～20MeV， 1.5万
即将为EAST配置一个75mm的NaI探测器，大约0.85万
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Proposal (1)
 Runaway Probability Function by experiments （Fisch）
等离子体电流Ip＝100kA，密度ne＝1.2
低杂波相位扫描，n//=1.8-3.45 ；
功率两个平台P＝100kW；300kW
为了简化分析，需要不同的相位下的LHCD放电的环电压基本接近，但
由于不同相位下的驱动效率不同，在相同功率水平下环电压不同，因此
需要低杂波功率做适当的扫描-50kW～+50kW step of 5kW
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Conclusions
 Runaway electrons have been measured in combination of hard x-ray
detectors and thermographic camera in the HT-7 tokamak.
 The dynamics of runaways in the Ohmic and LHCD plasmas is monitored.
The interaction of runaways with magnetic field ripple was observed from
the HXR spectra.
 The enhancement of runaways in LHCD plasmas is identified from the
HXR and IR signals.
 The suppression of runaways by LHCD and high initial density is achieved
in HT-7.
 The interaction of runaways with LH waves is suggested.
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Thank you！
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放电结束后的IR信号
HT-7