Observation of trapped and passing runaway electrons by infrared camera in the EAST tokamak

In EAST, synchrotron radiation is emitted by runaway electrons in the infrared band, which can be observed by infrared cameras. This synchrotron radiation is mainly emitted by passing runaway electrons with tens of Me V energy. A common feature of radiation dominated by passing runaway electrons is that it is strongest on the high field side. However,the deeply trapped runaway electrons cannot reach the high field side in principle. Therefore, in this case, the high field side radiation is expected to be weak. This paper reports for the first time that the synchrotron radiation from trapped runaway electrons dominates that from passing runaway electrons and is identifiable in an image. Although the synchrotron radiation dominated by trapped runaway electrons can be observed in experiment, the proportion of trapped runaway electrons is very low.     

Energy limit of runaway electrons in the HT-7 tokamak

The energy limit of runaway electrons in the HT-7 tokamak is investigated by measuring the synchrotron radiation originated from the runaway electrons and the hard X-ray radiation (HXR) when they hit the first wall. An upper boundary on the runaway energy can appear due to the resonance between the electron gyromotion and the magnetic field ripple in the low field side. The experimental derived maximum energy in the core is about 26 MeV, and maximum energy in the edge region is blocked to no more than 5 MeV. This resonance interaction of runaways with the nth harmonic of the magnetic field ripple can account for the observed energy gap of the runaways.     

EAST װ�õ����������ݵ���ͬ��������Ϊ��ʵ���о�

通过红外可见内窥镜诊断系统对EAST等离子体芯部逃逸电子的同步辐射功率谱进行了分析,得出低能段逃逸电子同步辐射主要在红外波段,随着逃逸电子能量的增加,同步辐射向短波方向移动进入可见光波段。在欧姆放电条件下,对逃逸电子同步辐射所产生的的红外可见光进行了成像分析,同时研究了EAST等离子体在低杂波和中性束注入加热条件下的逃逸电子行为。实验结果显示,低杂波和 NBI 的投入总体抑制电子的逃逸,但低杂波投入初期产生的快电子对逃逸电子的产生具有促进作用。     

EAST 装置等离子体逃逸电子同步辐射行为的实验研究

通过红外可见内窥镜诊断系统对EAST 等离子体芯部逃逸电子的同步辐射功率谱进行了分析,得出低能段逃逸电子同步辐射主要在红外波段,随着逃逸电子能量的增加,同步辐射向短波方向移动进入可见光波段。在欧姆放电条件下,对逃逸电子同步辐射所产生的的红外可见光进行了成像分析,同时研究了EAST 等离子体在低杂波和中性束注入加热条件下的逃逸电子行为。实验结果显示,低杂波和NBI 的投入总体抑制电子的逃逸,但低杂波投入初期产生的快电子对逃逸电子的产生具有促进作用。     

Role of bremsstrahlung radiation in limiting the energy of runaway electrons in tokamaks

Bremsstrahlung radiation of runaway electrons is found to be an energy limit for runaway electrons in tokamaks. The minimum and maximum energy of runaway electron beams is shown to be limited by collisions and bremsstrahlung radiation, respectively. It is also found that a massive injection of a high-Z gas such as xenon can terminate a disruption-generated runaway current before the runaway electrons hit the walls.     

Investigation of fast pitch angle scattering of runaway electrons in the EAST tokamak

This paper reports that an experimental investigation of fast pitch angle scattering(FPAS) of runaway electrons in the EAST tokamak has been performed.From the newly developed infrared detector(HgCdTe) diagnostic system,the infrared synchrotron radiation emitted by relativistic electrons can be obtained as a function of time.The FPAS is analysed by means of the infrared detector diagnostic system and the other correlative diagnostic systems(including electron-cyclotron emission,hard x-ray,neutrons).It is found that the intensity of infrared synchrotron radiation and the electron-cyclotron emission signal increase rapidly at the time of FPAS because of the fast increase of pitch angle and the perpendicular velocity of the energetic runaway electrons.The Parail and Pogutse instability is a possible mechanism for the FPAS.     

Polymethyl methacrylate glow under the influence of runaway electron beams generated in a gas diode

Spectral and amplitude–time characteristics of PMMA radiation under the impact of runaway electrons with subnanosecond duration are studied. The PMMA radiation spectra for a subnanosecond electron beam pulse duration are determined for the first time. The studies show that radiation of the band with a maximum at about 490 nm the intensity of which decreases toward the short-wave spectral region is recorded in the glow spectra. The glow intensity of this band varies proportionally to the number of electrons in the beam, which allows the possibility of using this radiation for determination of the number of high energy electrons in electron beams.     

Quasi-linear stage of synchrotron instability. II. Monoenergetic initial electron distribution

Using the results obtained in the first part of the work, we analyze quasi-linear relaxation of an initial monoenergetic quasi-isotropic electron distribution embedded in a cold plasma. It is shown that in this case the radiation energy density increases due to synchrotron instability only at the initial stage of relaxation and after that reabsorption emerges. At the stage of reabsorption, electrons absorb the emitted radiation for the same characteristic time as the time of radiation energy density increase at the maser stage. We calculate the maximum fraction of initial electron energy which is converted into synchrotron radiation energy. Analytical expressions are obtained for the amplitude and width of the frequency spectrum at the instant of the maximum level of synchrotron radiation and for the maximum value and width of the electron distribution over momentum at the end of relaxation. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radio.zika, Vol. 49, No. 12, pp. 1063–1074, December 2006.     

Synchrotron radiation of electrons trapped in earth's magnetosphere

Relatively large fluxes of trapped electrons with energy above several tens of MeV in the inner radiation belt, established according to low altitude satellite measurements, invoked the question of their production of synchrotron radiation in geomagnetic field. Based on experimental data on electrons, rough estimate of the spectral density of synchrotron radiation at the Equator is obtained in the region of maximum emissivity. Relatively low value of the radiation flux implies that only precise measurements at 1 GHz allowing to measure the directivity of emissions may be relevant.     

逃逸电子和不稳定性波反常多普勒共振实验研究

利用电子回旋辐射诊断系统并结合其他相关诊断研究了HL-2A托卡马克中逃逸电子与波间的反常多普勒共振作用.结果显示:欧姆放电下提高等离子密度能抑制逃逸电子束的不稳定性,但等离子密度的再次降低导致逃逸电子又会激发不稳定性波,并耦合不稳定性波发生二次反常多普勒共振作用.利用统计方法分析了HL-2A上不同放电阶段逃逸电子反常多普勒共振阈值(ωpe/ωce)区间大致都在0.17-0.54范围内.此共振机制导致逃逸电子在速度空间被波散射,平行能量转化到垂直能量,pitch角增加,同步辐射功率增强,逃逸电子能量限制在反常多普勒效应的阈值能量附近.基于反常多普勒共振的逃逸抑制能有效减轻逃逸电子对装置第一壁的损坏.     

Experimental observation of increased threshold electric field for runaway generation due to synchrotron radiation losses in the FTU Tokamak

The threshold electric field for runaway generation has been investigated during runaway suppression experiments by means of electron-cyclotron-resonance heating in the flattop phase of FTU discharges. Runaway suppression has been experimentally found to occur at electric fields substantially larger than those predicted by the relativistic collisional theory of runaway generation, ER=nee3lnΛ/4πε0(2)mec2. These experimental results are consistent with an increase of the critical electric field due to the electron synchrotron radiation losses. No runaway electrons are found in FTU experiments below the radiation threshold. These results support evidence for a new threshold electric field for runaway generation that accounts for the effect of the synchrotron losses, and which should be considered when making predictions on runaway generation and mitigation in devices such as ITER.     

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利用硬X射线探测系统监测HT-7托卡马克装置中逃逸电子轰击到装置第一壁材料时所产生的高能硬X射线,研究了在放电平顶阶段提高等离子体密度对逃逸电子行为的影响。实验结果表明,通过提高放电平顶阶段等离子体密度,HXR强度迅速降到很低的水平,这意味着能有效减少这个阶段形成的逃逸电子的数目及能量。     

Scale size of magnetic turbulence in tokamaks probed with 30-MeV electrons

Measurements of synchrotron radiation emitted by 30-MeV runaway electrons in the TEXTOR-94 tokamak show that the runaway population decays after switching on neutral beam injection (NBI). The decay starts only with a significant delay, which decreases with increasing NBI heating power. This delay provides direct evidence of the energy dependence of runaway confinement, which is expected if magnetic modes govern the loss of runaways. Application of the theory by Mynick and Strachan [Phys. Fluids 24, 695 (1981)] yields estimates for the "mode width" (delta) of magnetic perturbations: delta<0.5 cm in Ohmic discharges, increasing to delta = 4.4 cm for 0. 6 MW NBI.     

Study of runaway electron behaviour during electron cyclotron resonance heating in the HL-2A Tokamak

During the current flat-top phase of electron cyclotron resonance heating discharges in the HL-2A Tokamak, the behaviour of runaway electrons has been studied by means of hard x-ray detectors and neutron diagnostics. During electron cyclotron resonance heating, it can be found that both hard x-ray radiation intensity and neutron emission flux fall rapidly to a very low level, which suggests that runaway electrons have been suppressed by electron cyclotron resonance heating. From the set of discharges studied in the present experiments, it has also been observed that the efficiency of runaway suppression by electron cyclotron resonance heating was apparently affected by two factors: electron cyclotron resonance heating power and duration. These results have been analysed by using a test particle model. The decrease of the toroidal electric field due to electron cyclotron resonance heating results in a rapid fall in the runaway electron energy that may lead to a suppression of runaway electrons. During electron cyclotron resonance heating with different powers and durations, the runaway electrons will experience different slowing down processes. These different decay processes are the major cause for influencing the efficiency of runaway suppression. This result is related to the safe operation of the Tokamak and may bring an effective control of runaway electrons.     

Observation of synchrotron radiation from electrons accelerated in a petawatt-laser-generated plasma cavity

The dynamics of plasma electrons in the focus of a petawatt laser beam are studied via measurements of their x-ray synchrotron radiation. With increasing laser intensity, a forward directed beam of x rays extending to 50 keV is observed. The measured x rays are well described in the synchrotron asymptotic limit of electrons oscillating in a plasma channel. The critical energy of the measured synchrotron spectrum is found to scale as the Maxwellian temperature of the simultaneously measured electron spectra. At low laser intensity transverse oscillations are negligible as the electrons are predominantly accelerated axially by the laser generated wakefield. At high laser intensity, electrons are directly accelerated by the laser and enter a highly radiative regime with up to 5% of their energy converted into x rays.     

Elektronenbeschleunigung in einem Plasmabetatron mit und ohne hohem Leitungsstrom

A previous paper by Drees and Paul reported measurements on a plasma betatron. The study was continued using a betatron field with a vector potential taking into account the self magnetic field of a plasma current. The plasma was produced by a high frequency electric quadrupole field without an azimuthal magnetic field. The bremsstrahlung intensity of the accelerated electrons was observed as a function of gas pressure and accelerating field. The maximum energy of the electrons was 1.3 MeV compared to 1.5 MeV given by the field parameters. The maximum number in this energy range was about 10¹⁰ per pulse corresponding to a circulating runaway current of ～ 1 A. The conduction current was drasticly reduced by coating the inner wall of the quartz glass torus with a thin layer of graphite. This change in the plasma current did not influence the γ radiation intensity.     

等离子体中相对论电子的同步曲率辐射特性研究

根据同步曲率辐射理论推导了在等离子体环境中,不同磁场条件下的相对论性电子的吸收系数和发射系数表达式，计算了电子的发射强度，并且在此基础上研究了同步曲率辐射机制的脉泽效应.研究了两种磁场位型，第一种是强度均匀但弯曲的磁场，第二种是偶极磁场，结果发现了一些偶极磁场下特有的辐射特性.考虑到在天体的环境下电子具有的不同的能谱分布，分别选用了三种典型的能谱分布(幂率分布，高斯分布，热分布)进行了研究，通过计算负吸收和脉泽放大效应在发射强度上的表现后， 发现在某些天体物理环境中，同步曲率辐射在等离子体中的确存在脉泽放大效应.这些研究结果对太阳系中行星外层辐射的研究和宇宙中的射电高亮温度等问题的研究可能提供有益的帮助. 关键词： 同步曲率辐射 负吸收 脉泽效应     

托卡马克中等离子体频率附近的增强辐射现象

实验提出托卡马克中的等离子体频率附近的增强辐射现象与高能逃逸电子有关,但是辐射的机制尚未完全弄清。本文讨论了两种可能的机制。一种是诱发辐射,这是来自逃逸电子与波的反常迴旋共振。另一种是自发辐射,主要来自契仑柯夫共振。前者要求逃逸电子的能量比后者为大,因此,自发辐射理论更引人注意。在很宽的参数变化范围内对辐射率作了数值计算。对于等离子体频率大于迴旋频率的情况也作了讨论。 关键词：     

非圆截面环流器硬X射线辐射强度的绝对测量

本文给出了逃逸电子在孔拦上轰击出的硬X射线能谱及其辐射强度,描述了硬X射线闪烁谱还原成能谱的分析处理法,并对NaI(T1)探测器的绝对光峰效率进行了测量。     

Quasi-linear stage of synchrotron instability I. The limit of low initial radiation energy density

We consider the initial problem of quasi-linear relaxation of synchrotron instability in a spatially homogeneous system of relativistic electrons embedded in a cold plasma. We analyze evolution of the energy spectrum of electrons and the frequency spectrum of radiation at the limit of low initial radiation energy density. At this limit, the problem is reduced to the solution of a diffusion equation with a stationary diffusion coefficient for the electron distribution over momentum which does not depend on the initial radiation energy density. The radiation frequency spectrum is approximated by the Gaussian profile, and its parameters are expressed via the electron energy density at the given and initial instants of time. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 11, pp. 964–976, November 2006.