HT-7 Progress of runaway electron behavior study in the HT-7 tokamak ASIPP WPI Team, H.Lin, X.G.Zu and HT-7 Team

HT-7 Content  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 From to

HT-7 Experimental setup (a)View into HT-7 vessel with an infrared camera without plasma (b)View into HT-7 vessel with a normal photocamera 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

HT-7 Ohmic discharge ＃ 77952&77962 Runaway discharge (a) plasma current, (b) loop voltage and consumed magnetic flux, (c) plasma line- averaged density, (d) HXR intensity measured by NaI detector (HXRI), (e) HXR intensity measured by CdTe detector. Suppression of runaway by higher initial density IR picture for #77952 at 0.7sIR picture for #77962 at 0.7s Effective avalanche time is derived by fitting the IR intensity, it is about 170 ms.

HT-7 ＃ IR Pattern

HT-7 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 down- shifted cyclotron frequency and the magnetic field ripple. A resonance occurs when For HT-7: N=24, R=1.22m, B t ~1.8~2.2 Ref: PRL 75 (1995) 4626 Interaction of runway electrons with magnetic field ripple n the harmonic number 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.. According to theory, different harmonic number of ripple be efficient in energy blocking for different electric field value. n ＝ 8 ， W ripple ＝ 3.75 MeV ； n ＝ 7 ， W ripple ＝ 4.3 MeV ； n ＝ 6 ， W ripple ＝ 5 MeV; n ＝ 5 ， W ripple ＝ 6 MeV ； n ＝ 4 ， W ripple ＝ 7.5 MeV ASDXE: n=7

HT-7 Different harmonic resonance at different electric field Plasma current scan

HT-7 Different harmonic resonance during current scan and density scan experiments Current scan:Ne=0.5*10 19 m -3,Ip=60-80kA Density scan: Ip=80kA, Ne= *10 19 m -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. V l =1.3V n= 8 V l =1.4V n= 5 V l =1.53V n= 4

HT-7 W max is the experimental energy limit derived from the HXR spectra. is the wide of energy gap. n is the harmonic number. W ripple 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.

HT-7 Interaction of runaways with LH waves （ Anomalous Doppler resonance ）  n=0: Cerenkov resonance  n<0: anomalous Doppler resonance. ( Parail-Pogutse instability ， FPAS)  n>0: normal Doppler resonance. Ref ： Phys. Plasma 9 (2002)1667 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. Due to the relativistic decrease of the electron cyclotron frequency, a cyclotron resonance may appear between runaway electrons and lower hybrid waves. ECE LH power In runaway discharges, after the injection of LH waves, the ECE increase step by step, results from FPAS. Interaction of runaways with LH waves?  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.

HT-7

ECE increased by a large step, it is different with slide away discharge. Results from FPAS. Interaction of runaways with LH waves? FPAS in runaway discharge FPAS in current decay phase from runaway discharge

HT-7 IR pattern in # s 0.52s0.5s0.4s0.54s 0.56s0.58s0.6s0.62s0.64s A island splits into bi-island ！ 0.68s 0.7s 0.66s 0.8s0.9s 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. Left is LFS

HT-7 Observed island during Pellet injection inTextor-94 Ref: PRL 1994

HT-7 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： 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)

HT-7 HT-7 上通过电流压缩也实现了 slide-away 放电.

HT-7 Enhancement of runaways in LHCD discharges Critical energy for electron run away LHCD phase: 229keV Subsequent OH phase: 87keV Cut off in the FEB spectra may be due to the runaway of electron with energy higher than the critical energy. IR intensity in runaway enhanced discharges. The increasing of IR intensity indicates the building up of energetic runaways.  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.  LHCD with high loop voltage induces runaways

HT-7 Suppression of runaways by LHCD (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.  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 suppression of runaways by LHCD can also be identified from the neutron flux. See signal (h) in the waveforms.  The energetic runaways can be efficiently suppressed according to the evolution of HXR flux and the IR intensity.  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.

HT-7 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

HT-7 Runaway Probability Function in LHCD plasma

HT-7 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）

HT-7 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 ）

HT-7 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

HT-7 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 // = ； 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

HT-7 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.

HT-7 Thank you ！

HT-7 放电结束后的 IR 信号

HT-7