A large number of runaway electrons(REs)generated during disruption can cause significant damage to next-generation large-scale tokamaks.The influence of three-dimensional(3D)helical magnetic perturbations on the supp...A large number of runaway electrons(REs)generated during disruption can cause significant damage to next-generation large-scale tokamaks.The influence of three-dimensional(3D)helical magnetic perturbations on the suppression of RE generation was explored using a set of 3D helical coils in J-TEXT tokamak,which can excite m/n=-2/2 helical magnetic perturbations.Experimental evidence shows that the-2/2 magnetic perturbations caused by the opposite coil current direct plasma toward the high-field side,simultaneously enhancing the magnetic fluctuations,which would enhance the radial loss of REs and even prevent RE generation.On the other hand,-2/2 magnetic perturbations can also reduce the cooling time during the disruption phase and generate a population of high-energy REs,which can interact with high-frequency magnetic fluctuations and in turn suppress RE generation.The critical helical coil current was found to correlate with electron density,requiring higher coil currents at higher densities.According to the statistical analysis of RE generation at different electron densities,the applied-2/2 magnetic perturbations can increase the magnetic fluctuations to the same level at lower electron densities,which can decrease the threshold electron density for RE suppression.This will be beneficial for RE mitigation in future large tokamak devices.展开更多
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） diagnost...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.展开更多
The formation of runaway electron pre-ionized diffuse discharges at the pressures 0.05-0.7 MPa of in air, argon, nitrogen, and SF6 in an inhomogeneous electric field was investigated. Dynamics of intensity of the disc...The formation of runaway electron pre-ionized diffuse discharges at the pressures 0.05-0.7 MPa of in air, argon, nitrogen, and SF6 in an inhomogeneous electric field was investigated. Dynamics of intensity of the discharge plasma radiation from the different discharge gap regions in the gas pressure range （0.05-0.7 MPa） was established. It was shown that, the breakdown is occurred owing to the ionization wave, which starts from the electrode with small radius of curvature at both polarity of high voltage pulses. It is seen that formation of bright spots on the fiat electrode at the negative polarity of the electrode with small radius of curvature are observed during the changing of the discharge current polarity. It was shown that, at positive polarity of electrode with a small radius of curvature, the bright spots on the flat electrode arise due to the participation of the dynamic displacement current in the gap conductance.展开更多
We have observed reduction of the runaway electron synchrotron radiation, hard X-ray （HXR） intensity, and HXR energy after applying 110 GHz 2nd harmonic electron cyclotron resonant heating （ECRH） during runaway el...We have observed reduction of the runaway electron synchrotron radiation, hard X-ray （HXR） intensity, and HXR energy after applying 110 GHz 2nd harmonic electron cyclotron resonant heating （ECRH） during runaway electron （RE） discharges at low density with startup runaway electrons. However, we did not see a significant reduction of X-rays from 170 GHz 2nd harmonic ECRH at a higher field. A recently installed IR TV camera was used to observe the forward cone of synchrotron radiation from high energy REs in KSTAR. We have observed changes to the synchrotron images and reduction of the HXR by application of resonant magnetic perturbations （RMP） from in-vessel control coils （IVCC） installed inside KSTAR in the n----1 configuration.展开更多
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 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： electroh 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.展开更多
A detailed analysis of the synchrotron radiation intensity and energy of runaway electrons is presented for the Experimental Advanced Superconducting Tokamak（EAST）. In order to make the energy of the calculated runa...A detailed analysis of the synchrotron radiation intensity and energy of runaway electrons is presented for the Experimental Advanced Superconducting Tokamak（EAST）. In order to make the energy of the calculated runaway electrons more accurate, we take the Shafranov shift into account. The results of the analysis show that the synchrotron radiation intensity and energy of runaway electrons did not reach the maximum at the same time. The energy of the runaway electrons reached the maximum first, and then the synchrotron radiation intensity of the runaway electrons reached the maximum.We also analyze the runaway electrons density, and find that the density of runaway electrons continuously increased. For this reason, although the energy of the runaway electrons dropped but the synchrotron radiation intensity of the runaway electrons would continue rising for a while.展开更多
Runaway electron current generated during the current quench phase of tokamak disruptions could result in severe damage to future high performance devices.To control and mitigate such runaway electron current,it is im...Runaway electron current generated during the current quench phase of tokamak disruptions could result in severe damage to future high performance devices.To control and mitigate such runaway electron current,it is important to accurately describe the runaway electron current dominated equilibrium,based on which further stability analysis could be carried out.In this paper,we derive a Grad-Shafranov-like equation solving for the axisymmetric drift surfaces of the runaway electrons instead of the magnetic flux surfaces for the simple case that all runaway electrons share the same parallel momentum.This new equilibrium equation is then numerically solved with simple rectangular wall with ITER-like and MAST-like geometry parameters.The deviation between the drift surfaces and the flux surfaces is readily obtained,and runaway electrons are found to be well confined even in regions with open field lines.The change of the runaway electron parallel momentum is found to result in a horizontal current center displacement without any changes in the total current or the external field.The runaway current density profile is found to affect the susceptibility of such displacement,with flatter profiles result in more displacement by the same momentum change.With up-down asymmetry in the external poloidal field,such displacement is accompanied by a vertical displacement of runaway electron current.It is found that this effect is more pronounced in smaller,compact device and weaker poloidal field cases.The above results demonstrate the dynamics of current center displacement caused by the momentum space change in the runaway electrons,and pave a way for more sophisticated runaway current equilibrium theory in the future with more realistic consideration on the runaway electron momentum distribution.This new equilibrium theory also provides foundation for future stability analysis of the runaway electron current.展开更多
This paper presents an experimental investigation into the runaway electron spectrum with a gas diode composed of a rough spherical cathode and plane anode under the excitation of a nanosecond-pulse generator in atmos...This paper presents an experimental investigation into the runaway electron spectrum with a gas diode composed of a rough spherical cathode and plane anode under the excitation of a nanosecond-pulse generator in atmospheric air.The runaway electron beams are measured by a collector covered with aluminum foil with a thickness from 0μm(mesh grid)to 50μm.The energy spectrum is calculated by an improved Tikhonov regularization called the maximum entropy method.The experimental results show that the transition state of the discharge consisted of multiple streamer channels stretched from the cathode with glow-like plasma uniformly distributed over the anode.The number of runaway electrons measured by the collector is in the order of 1010 in atmospheric pressure air with a gap spacing of 5 mm and applied voltages of70–130 kV.The cathode with a rough surface creates a more inhomogeneous electric field and larger emission site for the runaway electrons around the cathode,providing conditions for the coexistence of filamentary streamer and diffuse discharge.The reconstructed spectra show that the energy distribution of the runaway electrons presents a single-peak profile with energies from eU_(m/2)–2 eU_(m/3)(U_(m)is maximal voltage across the gap).展开更多
Two different types of MHD instabilities with rapidly chirping frequency were found to arise in the Globus-M2 spherical tokamak in substantially different frequency ranges.The first type arises at frequencies of an or...Two different types of MHD instabilities with rapidly chirping frequency were found to arise in the Globus-M2 spherical tokamak in substantially different frequency ranges.The first type arises at frequencies of an order of 1 MHz in ohmic plasmas at relatively low density(n_(e))<2×10^(19) m^(-3) in a wide range of toroidal magnetic fields and plasma currents.This type of instability was identified as compressional Alfven waves,driven by electrons,accelerated during a sawtooth crush.It was found that the mode frequency is sweeping in time,according to the Berk-Breizman hole-clump nonlinear chirping model.The second type of wave arises in a specific single-swing regime of the central solenoid current with a very narrow plasma column,when the plasma tends to decay at extremely low density(n_(e))<2×10^(18) m^(-3) and,in fact,is an instability of the runaway electron beam.The exited modes cover the whole observed frequency range and are divided into several(two or three)frequency regions:approximately 0-30 MHz,60-120 MHz and sometimes 30-60 MHz.Reconnection of the branches was also observed.Single chirps are more rapid than for 1 MHz Alfven instability and follow an exponential law.This paper,to our knowledge,is the first report of frequency chirping instabilities excited by accelerated electrons at a spherical tokamak.展开更多
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 MeV e...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 MeV 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.展开更多
Operation of HT-7 tokamak in a multicycle alternating square wave plasma current regime is reported. A set of AC operation experiments, including LHW heating to enhance plasma ionization during the current transition ...Operation of HT-7 tokamak in a multicycle alternating square wave plasma current regime is reported. A set of AC operation experiments, including LHW heating to enhance plasma ionization during the current transition and current sustainment, is described. The behaviour of runaway electrons is analysed by four HXR detectors tangentially viewing the plasma in the equatorial plane, within energy ranges 0.3-1.2 MeV and 0.3-7 MeV, separately. High energy runaway electrons （~MeV） axe found to circulate predominantly in the opposite direction to the plasma current, while the number of low energy runaway electrons （~tens to hundreds of keV） circulating along the plasma current is comparable to that in the direction opposite to the plasma current. AC operation with lower hybrid current drive （LHCD） is observed to have an additional benefit of suppressing the runaway electrons if the drop of the loop voltage is large enough.展开更多
The effect of tearing modes on the confinement of runaway electrons is studied in Experimental Advanced Superconducting Tokamak(EAST).The general tendency of the radial diffusion coefficient of runaway electrons(REs)D...The effect of tearing modes on the confinement of runaway electrons is studied in Experimental Advanced Superconducting Tokamak(EAST).The general tendency of the radial diffusion coefficient of runaway electrons(REs)Dr is derived based on the time response relation between the tearing modes and runaway electrons.The results indicate that,the magnetic fluctuations of tearing modes will enhance the radial diffusion of runaway electrons when the magnetic island is small.Following the increasing of the magnetic fluctuations of the tearing modes,the formed large magnetic island may weaken the radial diffusion of runaway electrons.The results can be important to understand the confinement of runaway electrons when large magnetic islands exist in the plasma.展开更多
In the experiments of actively triggering plasma disruption by massive gas injection, the externally applied resonant magnetic perturbation has been used to mitigate the hazard of runaway electron(RE). Motivated by th...In the experiments of actively triggering plasma disruption by massive gas injection, the externally applied resonant magnetic perturbation has been used to mitigate the hazard of runaway electron(RE). Motivated by the experiment of multimode coupling to suppress REs on J-TEXT, some typical simulation cases with non-ideal MHD with rotation-open discussion(NIMROD) code are carried out to explore the influential mechanism of different relative phases between m/n =2/1 and m/n = 3/1 magnetic islands on the confinement of REs. Results show that the RE confinement is drastically affected by the relative phase between 2/1 and 3/1 magnetic islands. When the O point phase of 2/1 and 3/1 magnetic islands is toroidal 330°, REs can be effectively lost. The fitting curve of the remaining ratio of REs vs. the relative toroidal phase is predicted to approximate a sine-like function dependence. Further studies indicate that the phase difference between coexisting 2/1 and 3/1 islands can affect the radial transport of impurities. The loss of runaway electrons is closely related to the deposition effect of impurity. The impurity is easier to spread into the core region with smaller poloidal phase difference between the radial velocity of impurity and the impurity quantity of Ar.展开更多
A simple one-dimensional numerical model including generation, acceleration and loss effects for runaway electrons are used to deduce the runaway energy εr. The simulation results are presented in a form of a scaling...A simple one-dimensional numerical model including generation, acceleration and loss effects for runaway electrons are used to deduce the runaway energy εr. The simulation results are presented in a form of a scaling law of εr on plasma parameters. The scaling of εr and therefore the runaway confinement time εr and runaway electron diffusivity Dr have been studied in HL-1M tokamak, by measuring the hard-X ray spectra under different experimental conditions. A tentative explanation for the scaling of obtained data based on the effects from magnetic turbulence is presented.展开更多
The generation of runaway electrons(REs)is observed during the low-density helium ohmic plasma discharge in the Experimental Advanced Superconducting Tokamak(EAST).The growth rate of hard x-ray(HXR)is inversely propor...The generation of runaway electrons(REs)is observed during the low-density helium ohmic plasma discharge in the Experimental Advanced Superconducting Tokamak(EAST).The growth rate of hard x-ray(HXR)is inversely proportional to the line-average density.Besides,the RE generation in helium plasma is higher than that in deuterium plasma at the same density,which is obtained by comparing the growth rate of HXR with the same discharge conditions.The potential reason is the higher electron temperature of helium plasma in the same current and electron density plateau.Furthermore,two Alfvén eigenmodes driven by REs have been observed.The frequency evolution of the mode is not fully satisfied with the Alfvén scaling and when extension of the Alfvén frequency is towards 0,the high frequency branch is~50 kHz.The different spatial position of the two modes and the evolution of the helium concentration could be used to understand deviation between theoretical and experimental observation.展开更多
The results of theoretical simulation of runaway electron generation in high-pressure pulsed gas discharge with inhomogeneous electric field are presented.Hydrodynamic and kinetic approaches are used simultaneously to...The results of theoretical simulation of runaway electron generation in high-pressure pulsed gas discharge with inhomogeneous electric field are presented.Hydrodynamic and kinetic approaches are used simultaneously to describe the dynamics of different components of low-temperature discharge plasma.Breakdown of coaxial diode occurs in the form of a dense plasma region expanding from the cathode.On this background there is a formation of runaway electrons that are initiated by the ensemble of plasma electrons generated in the place locally enhanced electric field in front of dense plasma.It is shown that the power spectrum of fast electrons in the discharge contains electron group with the so-called“anomalous”energy.展开更多
Runaway electrons in tokamaks have been widely studied theoretically and experimentally. The runaway confinement time τr in ohmic and additionally heated tokamak plasmas presents an anomalous behavior compared with t...Runaway electrons in tokamaks have been widely studied theoretically and experimentally. The runaway confinement time τr in ohmic and additionally heated tokamak plasmas presents an anomalous behavior compared with theoretical predictions based on neoclassical models. A one-dimensional numerical model including generation, acceleration and loss effect of runaway electrons is used to deduce the runaway energy εr dependence on the runaway confinement time.展开更多
The major disruption can not only lead to the great heat loads and produce the large electromagnetic force on the first wall and divertor plates, but also can generate the runaway electrons. The runaway electrongenera...The major disruption can not only lead to the great heat loads and produce the large electromagnetic force on the first wall and divertor plates, but also can generate the runaway electrons. The runaway electrongeneration is a dangerous event for tokamak operation.展开更多
Runaway electrons in tokamaks have been widely studied theoretically and experimentally. The runaway confinement time τ1 in ohmic and additionally heated tokamak plasmas presents an anomalous behavior when compared w...Runaway electrons in tokamaks have been widely studied theoretically and experimentally. The runaway confinement time τ1 in ohmic and additionally heated tokamak plasmas presents an anomalous behavior when compared with theoretical predictions based on neoclassical models. Runaway electrons have received lately a great attention due to several reasons： （a） the possibility to study electromagnetic turbulence by measuring the runaway flux fluctuations and its energy spectra, and （ b ） the runaway electrons are powerful diagnostics capable of yielding valuable information on the actual distribution function of fusion experiments.展开更多
1 Generation of vertical instability Tokamak experiments show that the energy confinement time and performance are better, and the larger plasma current can be achieved for non-circular cross-sectional shape of plasma...1 Generation of vertical instability Tokamak experiments show that the energy confinement time and performance are better, and the larger plasma current can be achieved for non-circular cross-sectional shape of plasma than circular cross-section. However, the external magnetic fields which are used to produce the non-circular cross sectional shape also cause the confined plasma to become unstable to small vertical displacement. In general, the ratio of elongation k is larger, the possibility of instability is more. In practice, this vertical displacement mode stabilized by feedback control system or other provided external radial magnetic field to balance out the plasma motion. Under some of situations the control system may be fail due to rapid growth rate of instability exceeding the ability of controlling. The plasma will then move vertically upwards or downwards depending upon the characteristics of instability and control failure.展开更多
基金supported by the National Magnetic Confinement Fusion Energy R&D Program of China (Nos.2018YFE0309103 and 2019YFE03010004)National Natural Science Foundation of China (Nos.12475222,12205122,and 51821005)Hubei International Science and Technology Cooperation Projects (No.2022EHB003)。
文摘A large number of runaway electrons(REs)generated during disruption can cause significant damage to next-generation large-scale tokamaks.The influence of three-dimensional(3D)helical magnetic perturbations on the suppression of RE generation was explored using a set of 3D helical coils in J-TEXT tokamak,which can excite m/n=-2/2 helical magnetic perturbations.Experimental evidence shows that the-2/2 magnetic perturbations caused by the opposite coil current direct plasma toward the high-field side,simultaneously enhancing the magnetic fluctuations,which would enhance the radial loss of REs and even prevent RE generation.On the other hand,-2/2 magnetic perturbations can also reduce the cooling time during the disruption phase and generate a population of high-energy REs,which can interact with high-frequency magnetic fluctuations and in turn suppress RE generation.The critical helical coil current was found to correlate with electron density,requiring higher coil currents at higher densities.According to the statistical analysis of RE generation at different electron densities,the applied-2/2 magnetic perturbations can increase the magnetic fluctuations to the same level at lower electron densities,which can decrease the threshold electron density for RE suppression.This will be beneficial for RE mitigation in future large tokamak devices.
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 10935004 and 10775041)partly by JSPS-CAS Core University Program in the field of "Plasma and Nuclear Fusion"
文摘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.
基金Acknowledgments The work was supported by the grant from the Russian Science Foundation, project No. 14-29-00052.
文摘The formation of runaway electron pre-ionized diffuse discharges at the pressures 0.05-0.7 MPa of in air, argon, nitrogen, and SF6 in an inhomogeneous electric field was investigated. Dynamics of intensity of the discharge plasma radiation from the different discharge gap regions in the gas pressure range （0.05-0.7 MPa） was established. It was shown that, the breakdown is occurred owing to the ionization wave, which starts from the electrode with small radius of curvature at both polarity of high voltage pulses. It is seen that formation of bright spots on the fiat electrode at the negative polarity of the electrode with small radius of curvature are observed during the changing of the discharge current polarity. It was shown that, at positive polarity of electrode with a small radius of curvature, the bright spots on the flat electrode arise due to the participation of the dynamic displacement current in the gap conductance.
文摘We have observed reduction of the runaway electron synchrotron radiation, hard X-ray （HXR） intensity, and HXR energy after applying 110 GHz 2nd harmonic electron cyclotron resonant heating （ECRH） during runaway electron （RE） discharges at low density with startup runaway electrons. However, we did not see a significant reduction of X-rays from 170 GHz 2nd harmonic ECRH at a higher field. A recently installed IR TV camera was used to observe the forward cone of synchrotron radiation from high energy REs in KSTAR. We have observed changes to the synchrotron images and reduction of the HXR by application of resonant magnetic perturbations （RMP） from in-vessel control coils （IVCC） installed inside KSTAR in the n----1 configuration.
基金Project supported by the National Natural Science Foundation of China (Grant Nos 10675124,10775041 and 10775045)
文摘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： electroh 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.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11775263 and 11405219)the JSPS-NRF-NSFC A3 Foresight Program in the Field of Plasma Physics,China(Grant No.11261140328)the National Magnetic Confnement Fusion Science Program of China(Grant No.2015GB102004)
文摘A detailed analysis of the synchrotron radiation intensity and energy of runaway electrons is presented for the Experimental Advanced Superconducting Tokamak（EAST）. In order to make the energy of the calculated runaway electrons more accurate, we take the Shafranov shift into account. The results of the analysis show that the synchrotron radiation intensity and energy of runaway electrons did not reach the maximum at the same time. The energy of the runaway electrons reached the maximum first, and then the synchrotron radiation intensity of the runaway electrons reached the maximum.We also analyze the runaway electrons density, and find that the density of runaway electrons continuously increased. For this reason, although the energy of the runaway electrons dropped but the synchrotron radiation intensity of the runaway electrons would continue rising for a while.
基金Project supported by the National MCF Energy Research and Development Program of China (Grant No. 2019YFE03010001)the National Natural Science Foundation of China (Grant No. 11905004)
文摘Runaway electron current generated during the current quench phase of tokamak disruptions could result in severe damage to future high performance devices.To control and mitigate such runaway electron current,it is important to accurately describe the runaway electron current dominated equilibrium,based on which further stability analysis could be carried out.In this paper,we derive a Grad-Shafranov-like equation solving for the axisymmetric drift surfaces of the runaway electrons instead of the magnetic flux surfaces for the simple case that all runaway electrons share the same parallel momentum.This new equilibrium equation is then numerically solved with simple rectangular wall with ITER-like and MAST-like geometry parameters.The deviation between the drift surfaces and the flux surfaces is readily obtained,and runaway electrons are found to be well confined even in regions with open field lines.The change of the runaway electron parallel momentum is found to result in a horizontal current center displacement without any changes in the total current or the external field.The runaway current density profile is found to affect the susceptibility of such displacement,with flatter profiles result in more displacement by the same momentum change.With up-down asymmetry in the external poloidal field,such displacement is accompanied by a vertical displacement of runaway electron current.It is found that this effect is more pronounced in smaller,compact device and weaker poloidal field cases.The above results demonstrate the dynamics of current center displacement caused by the momentum space change in the runaway electrons,and pave a way for more sophisticated runaway current equilibrium theory in the future with more realistic consideration on the runaway electron momentum distribution.This new equilibrium theory also provides foundation for future stability analysis of the runaway electron current.
基金supported by the National Science Fund for Distinguished Young Scholars(Grant No.51925703)National Natural Science Foundation of China(Grant Nos.52022096 and 51907190)the Royal Society–Newton Advanced Fellowship,UK(Grant No.NAF\R2\192117)。
文摘This paper presents an experimental investigation into the runaway electron spectrum with a gas diode composed of a rough spherical cathode and plane anode under the excitation of a nanosecond-pulse generator in atmospheric air.The runaway electron beams are measured by a collector covered with aluminum foil with a thickness from 0μm(mesh grid)to 50μm.The energy spectrum is calculated by an improved Tikhonov regularization called the maximum entropy method.The experimental results show that the transition state of the discharge consisted of multiple streamer channels stretched from the cathode with glow-like plasma uniformly distributed over the anode.The number of runaway electrons measured by the collector is in the order of 1010 in atmospheric pressure air with a gap spacing of 5 mm and applied voltages of70–130 kV.The cathode with a rough surface creates a more inhomogeneous electric field and larger emission site for the runaway electrons around the cathode,providing conditions for the coexistence of filamentary streamer and diffuse discharge.The reconstructed spectra show that the energy distribution of the runaway electrons presents a single-peak profile with energies from eU_(m/2)–2 eU_(m/3)(U_(m)is maximal voltage across the gap).
文摘Two different types of MHD instabilities with rapidly chirping frequency were found to arise in the Globus-M2 spherical tokamak in substantially different frequency ranges.The first type arises at frequencies of an order of 1 MHz in ohmic plasmas at relatively low density(n_(e))<2×10^(19) m^(-3) in a wide range of toroidal magnetic fields and plasma currents.This type of instability was identified as compressional Alfven waves,driven by electrons,accelerated during a sawtooth crush.It was found that the mode frequency is sweeping in time,according to the Berk-Breizman hole-clump nonlinear chirping model.The second type of wave arises in a specific single-swing regime of the central solenoid current with a very narrow plasma column,when the plasma tends to decay at extremely low density(n_(e))<2×10^(18) m^(-3) and,in fact,is an instability of the runaway electron beam.The exited modes cover the whole observed frequency range and are divided into several(two or three)frequency regions:approximately 0-30 MHz,60-120 MHz and sometimes 30-60 MHz.Reconnection of the branches was also observed.Single chirps are more rapid than for 1 MHz Alfven instability and follow an exponential law.This paper,to our knowledge,is the first report of frequency chirping instabilities excited by accelerated electrons at a spherical tokamak.
基金the National Natural Science Foundation of China(Grant No.11775263)the National Magnetic Confinement Fusion Energy Research Project of China(Grant No.2015GB111003).
文摘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 MeV 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.
基金Project supported by the National Natural Science Foundation of China (Grant Nos.10935004 and 10775041)
文摘Operation of HT-7 tokamak in a multicycle alternating square wave plasma current regime is reported. A set of AC operation experiments, including LHW heating to enhance plasma ionization during the current transition and current sustainment, is described. The behaviour of runaway electrons is analysed by four HXR detectors tangentially viewing the plasma in the equatorial plane, within energy ranges 0.3-1.2 MeV and 0.3-7 MeV, separately. High energy runaway electrons （~MeV） axe found to circulate predominantly in the opposite direction to the plasma current, while the number of low energy runaway electrons （~tens to hundreds of keV） circulating along the plasma current is comparable to that in the direction opposite to the plasma current. AC operation with lower hybrid current drive （LHCD） is observed to have an additional benefit of suppressing the runaway electrons if the drop of the loop voltage is large enough.
基金Project partly supported by the Youth Innovation Promotion Association of Chinese Academy of Sciences(Grant No.2021445)the Science Foundation of Institute of Plasma Physics of Chinese Academy of Sciences(Grant No.DSJJ-2022-05)partly supported by the Comprehensive Research Facility for Fusion Technology Program of China(Grant No.2018-000052-73-01-001228).
文摘The effect of tearing modes on the confinement of runaway electrons is studied in Experimental Advanced Superconducting Tokamak(EAST).The general tendency of the radial diffusion coefficient of runaway electrons(REs)Dr is derived based on the time response relation between the tearing modes and runaway electrons.The results indicate that,the magnetic fluctuations of tearing modes will enhance the radial diffusion of runaway electrons when the magnetic island is small.Following the increasing of the magnetic fluctuations of the tearing modes,the formed large magnetic island may weaken the radial diffusion of runaway electrons.The results can be important to understand the confinement of runaway electrons when large magnetic islands exist in the plasma.
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 12175078 and 51821005)
文摘In the experiments of actively triggering plasma disruption by massive gas injection, the externally applied resonant magnetic perturbation has been used to mitigate the hazard of runaway electron(RE). Motivated by the experiment of multimode coupling to suppress REs on J-TEXT, some typical simulation cases with non-ideal MHD with rotation-open discussion(NIMROD) code are carried out to explore the influential mechanism of different relative phases between m/n =2/1 and m/n = 3/1 magnetic islands on the confinement of REs. Results show that the RE confinement is drastically affected by the relative phase between 2/1 and 3/1 magnetic islands. When the O point phase of 2/1 and 3/1 magnetic islands is toroidal 330°, REs can be effectively lost. The fitting curve of the remaining ratio of REs vs. the relative toroidal phase is predicted to approximate a sine-like function dependence. Further studies indicate that the phase difference between coexisting 2/1 and 3/1 islands can affect the radial transport of impurities. The loss of runaway electrons is closely related to the deposition effect of impurity. The impurity is easier to spread into the core region with smaller poloidal phase difference between the radial velocity of impurity and the impurity quantity of Ar.
文摘A simple one-dimensional numerical model including generation, acceleration and loss effects for runaway electrons are used to deduce the runaway energy εr. The simulation results are presented in a form of a scaling law of εr on plasma parameters. The scaling of εr and therefore the runaway confinement time εr and runaway electron diffusivity Dr have been studied in HL-1M tokamak, by measuring the hard-X ray spectra under different experimental conditions. A tentative explanation for the scaling of obtained data based on the effects from magnetic turbulence is presented.
基金Project supported by the National Key R&D Program of China(Grant Nos.2017YFE0301205 and 2022YFE03050003)the Youth Innovation Promotion Association of Chinese Academy of Sciences(Grant No.Y2021116)+1 种基金the National Natural Science Foundation of China(Grant Nos.12005262,12105186,12175277,and 11975271)the Users of Excellence Program of Hefei Science Center CAS(Grant No.2021HSC-UE016).
文摘The generation of runaway electrons(REs)is observed during the low-density helium ohmic plasma discharge in the Experimental Advanced Superconducting Tokamak(EAST).The growth rate of hard x-ray(HXR)is inversely proportional to the line-average density.Besides,the RE generation in helium plasma is higher than that in deuterium plasma at the same density,which is obtained by comparing the growth rate of HXR with the same discharge conditions.The potential reason is the higher electron temperature of helium plasma in the same current and electron density plateau.Furthermore,two Alfvén eigenmodes driven by REs have been observed.The frequency evolution of the mode is not fully satisfied with the Alfvén scaling and when extension of the Alfvén frequency is towards 0,the high frequency branch is~50 kHz.The different spatial position of the two modes and the evolution of the helium concentration could be used to understand deviation between theoretical and experimental observation.
基金This work is supported by Russian Fund of Basic Research(projects 15-08-03983 and 15-58-53031).
文摘The results of theoretical simulation of runaway electron generation in high-pressure pulsed gas discharge with inhomogeneous electric field are presented.Hydrodynamic and kinetic approaches are used simultaneously to describe the dynamics of different components of low-temperature discharge plasma.Breakdown of coaxial diode occurs in the form of a dense plasma region expanding from the cathode.On this background there is a formation of runaway electrons that are initiated by the ensemble of plasma electrons generated in the place locally enhanced electric field in front of dense plasma.It is shown that the power spectrum of fast electrons in the discharge contains electron group with the so-called“anomalous”energy.
文摘Runaway electrons in tokamaks have been widely studied theoretically and experimentally. The runaway confinement time τr in ohmic and additionally heated tokamak plasmas presents an anomalous behavior compared with theoretical predictions based on neoclassical models. A one-dimensional numerical model including generation, acceleration and loss effect of runaway electrons is used to deduce the runaway energy εr dependence on the runaway confinement time.
文摘The major disruption can not only lead to the great heat loads and produce the large electromagnetic force on the first wall and divertor plates, but also can generate the runaway electrons. The runaway electrongeneration is a dangerous event for tokamak operation.
文摘Runaway electrons in tokamaks have been widely studied theoretically and experimentally. The runaway confinement time τ1 in ohmic and additionally heated tokamak plasmas presents an anomalous behavior when compared with theoretical predictions based on neoclassical models. Runaway electrons have received lately a great attention due to several reasons： （a） the possibility to study electromagnetic turbulence by measuring the runaway flux fluctuations and its energy spectra, and （ b ） the runaway electrons are powerful diagnostics capable of yielding valuable information on the actual distribution function of fusion experiments.
文摘1 Generation of vertical instability Tokamak experiments show that the energy confinement time and performance are better, and the larger plasma current can be achieved for non-circular cross-sectional shape of plasma than circular cross-section. However, the external magnetic fields which are used to produce the non-circular cross sectional shape also cause the confined plasma to become unstable to small vertical displacement. In general, the ratio of elongation k is larger, the possibility of instability is more. In practice, this vertical displacement mode stabilized by feedback control system or other provided external radial magnetic field to balance out the plasma motion. Under some of situations the control system may be fail due to rapid growth rate of instability exceeding the ability of controlling. The plasma will then move vertically upwards or downwards depending upon the characteristics of instability and control failure.
