Generation Of Runaway Electrons Research Articles

Space- and time-resolved characterization of nanosecond time scale discharge at pressurized gas

The phenomenon of ultra-fast electrical gas breakdown was investigated. Nanosecond high-voltage pulses with durations of 1 and 5 ns and amplitudes of 100 and 200 kV, respectively, were used to study the parameters of the discharge in a pressured (1-7) × 105 Pa air-filled gap. The development of the discharge and the plasma propagation velocity was examined using optical fast frame imaging. The generation of runaway electrons in the breakdown process was confirmed by electron imaging and time-resolved x-ray diagnostics. Runaway electron beam energy distribution was obtained for a 1 ns duration high-voltage pulse. The origin and the role of runaway electrons in the discharge initiation are also discussed.

Energy of electrons emitted as a result of separation of surfaces

Experiments indicating acceleration of charged particles as a result of separation of solid surfaces are analyzed. As a possible mechanism of such acceleration, generation of surface charge on the separated surfaces of a cleaved ionic crystal is considered. The maximum electric field generated due to the charging of the separated surfaces and the energy of electrons accelerated in such a field are estimated. It is shown that, for the maximum attainable electric field, conditions are created for the generation of runaway electrons that, even at atmospheric pressure, electrons are accelerated to high energies, not experiencing collisions with gas particles.

Experimental observation of runaway electrons near the axis of a Z-pinch in a high-Z medium

Generation of runaway electrons in the axial region of a Z-pinch (high-current vacuum spark) operating in a high-Z medium was observed experimentally using pulsed optical shadowgraphy and X-ray pinhole imaging.

Simulation of runaway electron generation during plasma shutdown by impurity injection in ITER

Disruptions in a large tokamak can cause serious damage to the device and should be avoided or mitigated. Massive gas or killer pellet injection are possible ways to obtain a controlled fast plasma shutdown before a natural disruption occurs. In this work, plasma shutdown scenarios with different types of impurities are studied for an ITER-like plasma. Plasma cooling, runaway generation and the associated electric field diffusion are calculated with a 1D-code taking the Dreicer, hot-tail and avalanche runaway generation processes into account. Thin, radially localized sheets with high temperature can be created after the thermal quench, and the Dreicer and avalanche processes produce a high runaway current inside these sheets. At high impurity concentration the Dreicer process is suppressed but hot-tail runaways are created. Favorable thermal and current quench times can be achieved with a mixture of deuterium and neon or argon. However, to prevent the avalanche process from creating a significant runaway current fraction, it is found to be necessary to include runaway losses in the model.

JET disruption studies in support of ITER

Plasma disruptions affect plasma-facing and structural components of tokamaks due to electromechanical forces, thermal loads and generation of high energy runaway electrons (REs). Asymmetries in poloidal halo and toroidal plasma current can now be routinely measured in four positions 90° apart. Their assessment is used to validate the design of the ITER vessel support system and its in-vessel components. The challenge of disruption thermal loads comes from both the short duration over which a large energy has to be lost and the potential for asymmetries. The focus of this paper will be on localized heat loads. Resonant magnetic perturbations failed to reduce the generation of REs in JET. An explanation of the limitations applying to these attempts is offered together with a minimum guideline. The REs generated by a moderate, but fast, Ar injection in limiter plasmas show evidence of milder and more efficient losses due to the high Ar background density.

Miniature UV lamp excited by subnanosecond voltage pulses

Energy, time, and spectral characteristics of emission of the second positive system of N2 molecules in gaseous nitrogen, Ar — N2 mixture, and air are investigated. An FPG-10 generator with voltage pulse FWHM of 200 and 400 ps and matched-load amplitudes of 14 and 6 kV, respectively, is used to excite gases. It is shown that excitation can be performed in two regimes using this generator. In the first regime a diffuse discharge is formed at atmospheric pressure, which opens ways to design miniature nanosecond UV lamps. A diffuse discharge is formed due to the generation of runaway electrons, with the aid of electrodes having a small radius of curvature and voltage pulses with a sharp leading edge. In the second regime an elevated average radiation power is obtained under excitation by a barrier discharge. However, the operating pressure is lower in this case, and the sizes of the emitting region and the UV pulse width significantly increase.

Measurement of high-energy electrons by means of a Cherenkov detector in ISTTOK tokamak

The paper concerns detectors of the Cherenkov radiation which can be used to measure high-energy electrons escaping from short-living plasma. Such detectors have high temporal (about 1 ns) and spatial (about 1 mm) resolution. The paper describes a Cherenkov-type detector which was designed, manufactured and installed in the ISTTOK tokamak in order to measure fast runaway electrons. The radiator of that detector was made of an aluminium nitride (AlN) tablet with a light-tight filter on its front surface. Cherenkov signals from the radiator were transmitted through an optical cable to a fast photomultiplier. It made possible to perform direct measurements of the runaway electrons of energy above 80 keV. The measured energy values and spatial characteristics of the recorded electrons appeared to be consistent with results of numerical modelling of the runaway electron generation process in the ISTTOK tokamak.

Runaway Electron Measurements in the EAST Tokamak

AbstractAn experimental study of runaway electrons in the EAST tokamak has been performed by a recently developed multi‐channel hard x‐ray diagnostics based on NaI(TL) scintillator detectors. It is found that in the current quench phase, the inductive loop voltage plays an important role in the generation of runaway electrons. And the avalanche mechanism was the main mechanism for runaway electrons after the disruptions. The distribution and transportation of runaway electrons were also investigated by multi‐channel hard x‐ray diagnostics. It is also found that the intensity of runaway electrons emission in the core plasma was much higher than those in the downside of the cross‐section, while the emission intensity of runaway electrons in the core plasma was almost the same. Calculated shrinking coefficient of runaway electrons emission after the plasma disruption was about 26 m/s according to the experimental data (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Runaway electrons behaviors during ion cycolotron range of frequency and lower hybrid wave plasmas in the HT-7 Tokamak

HT-7 Tokamak is equipped with a lower hybrid wave (LHW) system and an ion cyclotron range of frequency (ICRF) system. ICRF can accelerate ions effectively, while LHW can accelerate electrons effectively. The generation of runaway electrons during the LHW and ICRF plasmas, as well as the time evolution of electron temperature during the ICRF and LHW plasmas was investigated in this paper. The runaway critical energy for runaway electrons was also calculated according to the experimental data. It was observed that the combination of ICRF and LHW can produce a higher heating efficiency and a better coupling between ICRF and plasmas if the power of LHW exceeds a critical value. Therefore, the generation of runaway electrons and fusion neutrons are affected by ICRF.

Electric field and ionization-gradient effects on inertial-confinement-fusion implosions

The generation of strong, self-generated electric fields (108–109 V m−1) in direct-drive, inertial-confinement-fusion capsules has been reported (Li et al 2008 Phys. Rev. Lett. 100 225001). Various models are considered herein to explain the observed electric field evolution, including the potential roles of electron pressure gradients near the fuel–pusher interface and plasma polarization effects that are predicted to occur across shock fronts (Zel'dovich and Raizer 2002 Physics of Shock Waves and High-Temperature Hydrodynamic Phenomena (Mineola, NY: Dover) p 522). In the latter case, strong fields in excess of 1010 V m−1 and localized to 10–100 nm may be consistent with the data obtained from proton radiography. Such field strengths are similar in magnitude to the criterion for runaway electron generation that could lead to plasma kinetic effects and potential shock-front broadening. The observed electric field generation may also be partly due to plasma ionization gradients localized near the fuel–pusher interface. A model is proposed that allows for differing electron- and ion-density gradient scale lengths in the presence of ionization gradients while preserving overall charge neutrality. Such a redistribution of electrons compared with standard, charge-neutral, single-fluid radiation-hydrodynamics modelling may affect the interpretation of imploded-core x-ray diagnostics as well as alter alpha particle deposition in the thermonuclear fuel.

Runaway electron generation in tokamak disruptions

Runaway electrons can be generated in disruptions by the Dreicer, hot tail and avalanche mechanisms. Analytical and numerical results for hot tail runaway generation are included in a one-dimensional model of electric field, temperature and runaway current, which is applied to simulate disruptions and fast shutdown. The peaked shape of the runaway current density profile may cause tearing modes to become unstable. Fast shutdown is studied by prescribing varying amounts of injected impurities. Large argon content suppresses runaways in JET simulations but causes hot tail generation in ITER. A pellet code is coupled to the runaway model, and it is extended to enable simulations of carbon doped deuterium pellet injection. Such pellets are seen not to give enough cooling for a fast current quench.

Generation of runaway electrons during deterioration of lower hybrid power coupling in lower hybrid current drive plasmas in the HT-7 tokamak

Efficient coupling of lower hybrid (LH) power from the wave launcher to the plasma is a very important issue in lower hybrid current drive (LHCD) experiments. The large unbalanced reflections in the grill trigger the LH protection system, which will trip the power, resulting in the reduction of the coupled LH power. The generation of runaway electrons has been investigated in LHCD plasmas with deterioration of LH coupling in the HT-7 tokamak. The deterioration of LH coupling results in an increase of the loop voltage and a more energetic fast electron population. These two effects favor the generation of a runaway population. It is found that most of the fast electrons generated by LH waves through parallel electron Landau damping were converted into a runaway population through the acceleration from the toroidal electric field when significant deterioration of LH coupling occurs.

Magnetic fluctuation level in disruption plasmas in the TEXTOR tokamak

AbstractRunaway transport is used to probe the magnetic fluctuation level for TEXTOR disruption plasmas. A zero-dimensional model of the current quench including the generation of runaway electrons has been applied to simulate the experimental current evolution during major disruptions [Plasma Phys. Control. Fusion50 (2008), 105007]. According to the loss rate of runaway electrons used in the fitting parameter of the zero-dimensional model, the magnetic fluctuation level in TEXTOR is derived. It is found that the magnetic fluctuation level is in the order of 10−5, and it increases with the atom number mixed into the plasma.

Magnetic field threshold for runaway generation in tokamak disruptions

Experimental observations show that there is a magnetic field threshold for runaway electron generation in tokamak disruptions. In this work, two possible reasons for this threshold are studied. The first possible explanation for these observations is that the runaway beam excites whistler waves that scatter the electrons in velocity space prevents the beam from growing. The growth rates of the most unstable whistler waves are inversely proportional to the magnetic field strength. Taking into account the collisional and convective damping of the waves it is possible to derive a magnetic field threshold below which no runaways are expected. The second possible explanation is the magnetic field dependence of the criterion for substantial runaway production obtained by calculating how many runaway electrons can be produced before the induced toroidal electric field diffuses out of the plasma. It is shown, that even in rapidly cooling plasmas, where hot-tail generation is expected to give rise to substantial runaway population, the whistler waves can stop the runaway formation below a certain magnetic field unless the postdisruption temperature is very low.

Effect of gas puffing on current ramp-down ohmic discharges in HT-7 tokamak

A new behaviour of current ramp-down ohmic deuterium discharges has been observed by additional deuterium fueling/gas puffing in HT-7 tokamak. It has been investigated, that the additional gas puffing converts the fast pitch angle scattering (FPAS) event to normal pitch angle scattering (NPAS) event characterized by higher electron energies and shifting the trigger time of the event, which is dependent on the pulse injection time. The attempt has also been made to investigate the scaling of critical values for runaway electrons generation and determination of their related energies along with trigger parameters of the instability and conversion relation between NPAS and FPAS events by ramping-down plasma current and changing the toroidal magnetic field in low-density ohmic discharges. The direct and inverse relationship has been observed for critical trigger value ω p e / ω c e of runaways generation with the plasma current and the toroidal magnetic field strength respectively. The runaway instability regime is characterized by relaxations in the electron cyclotron emission due to relativistic anomalous Doppler resonance (ADR) effect, which transfers energy from parallel to perpendicular motion.

Revisiting the momentum-space study of runaway electrons during lightning initiation

The runaway electron flow in momentum space in weakly ionized air is being revisited. On studying the runaway electron flow in momentum space it was shown that including the scattering of electrons shifts the critical energy for runaway electron generation to higher values.

Use of Cherenkov-type detectors for measurements of runaway electrons in the ISTTOK tokamak

Gas, fluid, or solid Cherenkov-type detectors have been widely used in high-energy physics for determination of parameters of charged particles, which are moving with relativistic velocities. This paper presents experimental results on the detection of runaway electrons using Cherenkov-type detectors in the ISTTOK tokamak discharges. Such detectors have been specially designed for measurements of energetic electrons in tokamak plasma. The technique based on the use of the Cherenkov-type detectors has enabled the detection of energetic electrons (energies higher than 80 keV) and determination of their spatial and temporal parameters in the ISTTOK discharges. Obtained experimental data were found in adequate agreement to the results of numerical modeling of the runaway electron generation in ISTTOK.

Generation and suppression of runaway electrons in disruption mitigation experiments in TEXTOR

Runaway electrons represent a serious problem for the reliable operation of the future experimental tokamak ITER. Due to the multiplication factor of exp(50) in the avalanche even a few seed runaway electrons will result in a beam of high energetic electrons that is able to damage the machine. Thus suppression of runaway electrons is a task of great importance, for which we present here a systematic study of runaway electrons following massive gas injection in TEXTOR.Argon injection can cause the generation of runaways carrying up to 30% of the initial plasma current, while disruptions triggered by injection of helium or of mixtures of argon (5%, 10%, 20%) with deuterium are runaway free. Disruptions caused by argon injection finally become runaway free for very large numbers of injected atoms.The appearance/absence of runaway electrons is related to the fraction of atoms delivered to the plasma centre. This so-called mixing efficiency is deduced from a 0D model of the current quench. The estimated mixing efficiency is 3% for argon, 15% for an argon/deuterium mixture and about 40% for helium.A low mixing efficiency of high-Z impurities can have a strong implication for the design of the disruption mitigation system for ITER. However, a quantitative prediction requires a better understanding of the mixing mechanism.

Hot tail runaway electron generation in tokamak disruptions

Hot tail runaway electron generation is caused by incomplete thermalization of the electron velocity distribution during rapid plasma cooling. It is an important runaway electron mechanism in tokamak disruptions if the thermal quench phase is sufficiently fast. Analytical estimates of the density of produced runaway electrons are derived for cases of exponential-like temperature decay with a cooling rate lower than the collision frequency. Numerical simulations, aided by the analytical results, are used to compare the strength of the hot tail runaway generation with the Dreicer mechanism for different disruption parameters (cooling rate, post-thermal quench temperature, and electron density) assuming that no losses of runaway electrons occur. It is seen that the hot tail runaway production is going to be the dominant of these two primary runaway mechanisms in ITER [R. Aymar et al., Plasma Phys. Controlled Fusion 44, 519 (2002)].

Suppression of Runaway Electrons by Resonant Magnetic Perturbations in TEXTOR Disruptions

The generation of runaway electrons in the international fusion experiment ITER disruptions can lead to severe damage at plasma facing components. Massive gas injection might inhibit the generation process, but the amount of gas needed can affect, e.g., vacuum systems. Alternatively, magnetic perturbations can suppress runaway generation by increasing the loss rate. In TEXTOR disruptions runaway losses were enhanced by the application of resonant magnetic perturbations with toroidal mode number n=1 and n=2. The disruptions are initiated by fast injection of about 3x10{21} argon atoms, which leads to a reliable generation of runaway electrons. At sufficiently high perturbation levels a reduction of the runaway current, a shortening of the current plateau, and the suppression of high energetic runaways are observed. These findings indicate the suppression of the runaway avalanche during disruptions.