Tokamak Disruptions Research Articles

Analytical Analysis of the Electromagnetic Response for the Vacuum Vessel of EAST

During plasma disruptions and vertical displacement events (VDEs), time-varying eddy currents are induced on Vacuum Vessel (VV) and Plasma Facing Components (PFCs) of Tokamak. In this paper, we calculate eddy currents induced forces on VV and inner limiter (one of the PFC) during plasma disruption in Experimental Advanced Superconducting Tokamak (EAST). Various plasma transients—VDEs and major disruptions (MDs) are considered. And the study includes assessments of integral values (net vertical, hoop forces), time evolution, peaks of the force distribution. It is shown that the distribution pattern of the eddy currents for different scenarios differ greatly, therefore the resulting EMFs and torques cause different mechanical response.

Asymmetric wall force and toroidal rotation in tokamak disruptions

This paper investigates the scaling of asymmetric wall force and toroidal rotation during tokamak disruptions. It is shown by simulations and analysis how these effects depend on the displacement in an asymmetric vertical displacement event (AVDE) and on 3D magnetic field perturbation amplitude. The simulations include a sequence of both upward and downward displacements. It is verified that the asymmetric force is proportional to the vertical current moment, which in turn is proportional to the product of vertical displacement and the magnetic perturbation amplitude. It is verified how the scaling of toroidal rotation in AVDE disruptions depends on magnetic perturbation amplitude, vertical displacement, and βN.

On the avalanche generation of runaway electrons during tokamak disruptions

A simple zero dimensional model for a tokamak disruption is developed to evaluate the avalanche multiplication of a runaway primary seed during the current quench phase of a fast disruptive event. Analytical expressions for the plateau runaway current, the energy of the runaway beam, and the runaway energy distribution function are obtained allowing the identification of the parameters dominating the formation of the runaway current during disruptions. The effect of the electromagnetic coupling to the vessel and the penetration of the external magnetic energy during the disruption current quench as well as of the collisional dissipation of the runaway current at high densities are investigated. Current profile shape effects during the formation of the runaway beam are also addressed by means of an upgraded one-dimensional model.

Mechanisms of plasma disruption and runaway electron losses in the TEXTOR tokamak

Based on the analysis of data from the numerous dedicated experiments on plasma disruptions in the TEXTOR tokamak the mechanisms of the formation of runaway electron (RE) beams and their losses are proposed. The plasma disruption is caused by a strong stochastic magnetic field formed due to nonlinearly excited low-mode-number magneto-hydro-dynamics (MHD) modes. It is hypothesized that the RE beam is formed in the central plasma region confined by an intact magnetic surface due to the acceleration of electrons by the inductive toroidal electric field. In the case of plasmas with the safety factor $q(0)<1$ the most stable RE beams are formed by the outermost intact magnetic surface located between the magnetic surface $q=1$ and the closest low-order rational surface $q=m/n>1~(q=5/4,q=4/3,\dots )$. The thermal quench (TQ) time caused by the fast electron transport in a stochastic magnetic field is calculated using the collisional transport model. The current quench (CQ) stage is due to the particle transport in a stochastic magnetic field. The RE beam current is modelled as a sum of a toroidally symmetric part and a small-amplitude helical current with a predominant $m/n=1/1$ component. The REs are lost due to two effects: (i) by outward drift of electrons in a toroidal electric field until they touch the wall and (ii) by the formation of a stochastic layer of REs at the beam edge. Such a stochastic layer for high-energy REs is formed in the presence of the $m/n=1/1$ MHD mode. It has a mixed topological structure with a stochastic region open to the wall. The effect of external resonant magnetic perturbations on RE loss is discussed. A possible cause of the sudden MHD signals accompanied by RE bursts is explained by the redistribution of runaway current during the resonant interaction of high-energetic electron orbits with the $m/n=1/1$ MHD mode.

The ITPA disruption database

A multi-device database of disruption characteristics has been developed under the auspices of the International Tokamak Physics Activity magneto-hydrodynamics topical group. The purpose of this ITPA disruption database (IDDB) is to find the commonalities between the disruption and disruption mitigation characteristics in a wide variety of tokamaks in order to elucidate the physics underlying tokamak disruptions and to extrapolate toward much larger devices, such as ITER and future burning plasma devices. In contrast to previous smaller disruption data collation efforts, the IDDB aims to provide significant context for each shot provided, allowing exploration of a wide array of relationships between pre-disruption and disruption parameters. The IDDB presently includes contributions from nine tokamaks, including both conventional aspect ratio and spherical tokamaks. An initial parametric analysis of the available data is presented. This analysis includes current quench rates, halo current fraction and peaking, and the effectiveness of massive impurity injection. The IDDB is publicly available, with instruction for access provided herein.

Novel approaches for mitigating runaway electrons and plasma disruptions in ADITYA tokamak

This paper summarizes the results of recent dedicated experiments on disruption control and runaway mitigation carried out in ADITYA, which are of the utmost importance for the successful operation of large size tokamaks, such as ITER. It is quite a well-known fact that disruptions in tokamaks must be avoided. Disruptions, induced by hydrogen gas puffing, are successfully avoided by two innovative techniques in ADITYA using a bias electrode placed inside the last closed flux surface and applying an ion cyclotron resonance pulse with a power of ∼50 to 70 kW. These experiments led to better understanding of the disruption avoidance mechanisms and also can be thought of as one of the options for disruption avoidance in ITER. In both cases, the physical mechanism seems to be the control of magnetohydrodynamic modes due to increased poloidal rotation of edge plasma generated by induced radial electric fields. Real time avoidance of disruption with identifying proper precursors in both the mechanisms is successfully attempted. Further, analysing thoroughly the huge database of different types of spontaneous and deliberately-triggered disruptions from ADITYA, a significant contribution has been made to the international disruption database (ITPA). Furthermore, the mitigation of the runaway electron generated mainly during disruptions remains a challenging topic in present tokamak research as these high-energy electrons can cause severe damage to in-vessel components and the vacuum vessel. A simple technique has been implemented in ADITYA to mitigate the runaway electrons before they can gain high energies using a localized vertical magnetic field perturbation applied at one toroidal location to extract runaway electrons.

Numerical calculation of ion runaway distributions

Ions accelerated by electric fields (so-called runaway ions) in plasmas may explain observations in solar flares and fusion experiments; however, limitations of previous analytic work have prevented definite conclusions. In this work, we describe a numerical solver of the 2D non-relativistic linearized Fokker-Planck equation for ions. It solves the initial value problem in velocity space with a spectral-Eulerian discretization scheme, allowing arbitrary plasma composition and time-varying electric fields and background plasma parameters. The numerical ion distribution function is then used to consider the conditions for runaway ion acceleration in solar flares and tokamak plasmas. Typical time scales and electric fields required for ion acceleration are determined for various plasma compositions, ion species, and temperatures, and the potential for excitation of toroidal Alfvén eigenmodes during tokamak disruptions is considered.

Mechanism of runaway electron beam formation during plasma disruptions in tokamaks

A new physical mechanism of formation of runaway electron beams during plasma disruptions in tokamaks is proposed. The plasma disruption is caused by a strong stochastic magnetic field formed due to nonlinearly excited low-mode number magnetohydrodynamic (MHD) modes. It is conjectured that the runaway electron beam is formed in the central plasma region confined inside the intact magnetic surface located between q = 1 and the closest low–order rational magnetic surfaces [q = 5/4 or q = 4/3,…]. It results in that runaway electron beam current has a helical nature with a predominant m/n = 1/1 component. The thermal quench and current quench times are estimated using the collisional models for electron diffusion and ambipolar particle transport in a stochastic magnetic field, respectively. Possible mechanisms for the decay of the runaway electron current owing to an outward drift electron orbits and resonance interaction of high–energy electrons with the m/n = 1/1 MHD mode are discussed.

Development of an ITER prototype disruption mitigation valve

Disruptions in tokamaks seem to be unavoidable. Consequences of disruptions are (i) high heat loads on plasma-facing components, (ii) large forces on the vacuum vessel, and (iii) the generation of runaway electron beams. In ITER, the thermal energy of the plasma needs to be evenly distributed on the first wall in order to prevent melting, forces from vertical displacement events have to be minimized, and the generation of runaway electrons suppressed. Massive gas injection using fast valves is a concept for disruption mitigation which is presently being explored in many tokamaks. Fast disruption mitigation valves based on an electromagnetic eddy current drive have been developed in Jülich since the 1990s and models of various sizes have been built and are in operation in the TEXTOR, MAST, and JET tokamaks. A disruption mitigation valve for ITER is of necessity larger with an estimated injected gas volume of ∼20kPam3[7] for runaway electron suppression and all materials used have to be resistant to much higher levels of neutron and gamma radiation than in existing tokamaks. During the last 5 years, the concept for an ITER prototype disruption mitigation valve has been developed up to the stage that a fully functional valve could be built and tested. Special emphasis was given to the development and functional testing of some critical items: (i) the injection chamber seal, (ii) the piston seal, (iii) the eddy current drive, and (iv) a braking mechanism to avoid too fast closure of the valve, which could damage the injection chamber seal. The concept of the valve will be introduced and detailed solutions and testing results for the critical items presented.

Adaptive Grids in Simulations of Toroidal Plasma Starting from Magneto-Hydrodynamic Equilibrium

This paper introduces the notion of Tokamak Magneto-Hydrodynamics (TMHD), which explicitly reflects the anisotropy of a high temperature tokamak plasma. The set of TMHD equations is formulated for simulation of macroscopic plasma dynamics and disruptions in tokamaks. Free from the Courant restriction on the time step, this set of equations is adequate to plasma dynamics with realistic parameters of high performance plasmas and does not require any extension of the MHD plasma model. At the same time, TMHD requires the use of magnetic field aligned numerical grids. Examples of their use in 2-dimensional cases of tokamak equilibria and dynamics of the wall touching kink mode are presented. For the 3-dimensional case of an ergodic magnetic field, this paper introduces the reference magnetic coordinates as a practical algorithm for generating adaptive grids for TMHD.

Characterization of plasma current quench during disruption in EAST tokamak**Project supported by the National Magnetic Confinement Fusion Science Program of China (Grant Nos. 2014GB103000 and 2013GB102000) and the National Natural Science Foundation of China (Grant No. 11205199 and 11205192).

A preliminary analysis of plasma current quenching is presented in this paper based on the disruption database. It demonstrates that 26.8% of discharges have been disrupted in the last 2012 campaign, in addition, the plasma disruptive rate grows with the increase of plasma current. The best-fit linear and instantaneous plasma current quench rate is extracted from the recent EAST disruptions, showing that an 80%–30% interval of the maximum plasma current is well fit for the EAST device. The lowest area-normalized current quench time is 3.33 ms/m2 with the estimated plasma electron temperature being 7.3 eV,∼9.5 eV. In the disruption case the maximum eddy current goes up to 400 kA, and a fraction of currents are respectively driven on the upper and lower outer plate with nearly 100 MPa–200 MPa stress in the leg.

Overvoltage protection for magnetic system during disruption in tokamak

During a plasma disruption the magnetic flux in the tokamak changes rapidly, which in most cases will cause high-voltage surges among the magnetic systems and may bring severe damage to the components if there is no overvoltage protection. This paper investigates the way to limit the plasma disruption overvoltage and absorb the energy with the use of metal oxide varistors (MOVs). An overvoltage model of plasma disruption is introduced which can be used for the simulation of plasma disruption and the analysis of the overvoltage. The effectiveness of the overvoltage protection system is validated with disruption experiments. It shows that by optimizing the varistors voltage, the overvoltage during plasma disruption can be limited to an ideal low value. Now the overvoltage protection system has been deployed in J-TEXT tokamak and serves well for daily experiments.

Research on Plasma Interaction with Materials for Thermonuclear Reactors in the Russian Federation

The short review of art of state of research in Russian Federation in the field of plasma surface interactions (PSI) related to the problems of plasma-faced materials in controlled fusion devices is presented. The most activity in this field in RF is concentrated on the theoretical, computer and experimental simulations of processes taking place at plasma and its components interactions with condensed matter. The main recent results in PSI obtained in RF are shortly listed including studies of elementary processes accompanied steady state plasma interactions with materials and components as well as results obtained during high power pulsed plasma simulations of transients and disruptions in tokamaks. Detailed list of recent publications is also presented.

Metal Erosion under Plasma Flow Typical for ITER Transient Regimes

Theoretical models of movement of a fast melt metal layer (up to v ∼ 10 m/s) and droplet erosion are developed for power density pulses typical for tokamak plasma edge localized modes (ELM) and disruption. Fast melt metal movement and droplet erosion were shown to be possible only at plasma flow pressure P > 1 atm. The secondary shielding plasma near the target has a high density, a relatively low temperature and a much higher pressure than that of the original plasma flow. Even if initial plasma flow has a pressure below the threshold of fast melt layer movement and droplet erosion, the secondary shielding plasma can cause fast melt metal movement and droplet erosion.

Toroidal current asymmetry in tokamak disruptions

It was discovered on JET that disruptions were accompanied by toroidal asymmetry of the toroidal plasma current Iϕ. It was found that the toroidal current asymmetry was proportional to the vertical current moment asymmetry with positive sign for an upward vertical displacement event (VDE) and negative sign for a downward VDE. It was observed that greater displacement leads to greater measured Iϕ asymmetry. Here, it is shown that this is essentially a kinematic effect produced by a VDE interacting with three dimensional MHD perturbations. The relation of toroidal current asymmetry and vertical current moment is calculated analytically and is verified by numerical simulations. It is shown analytically that the toroidal variation of the toroidal plasma current is accompanied by an equal and opposite variation of the toroidal current flowing in a thin wall surrounding the plasma. These currents are connected by 3D halo current, which is π/2 radians out of phase with the n = 1 toroidal current variations.

Alfvénic instabilities driven by runaways in fusion plasmas

Runaway particles can be produced in plasmas with large electric fields. Here, we address the possibility that such runaway ions and electrons excite Alfvénic instabilities. The magnetic perturbation induced by these modes can enhance the loss of runaways. This may have important implications for the runaway electron beam formation in tokamak disruptions.

Inter-machine comparison of the termination phase and energy conversion in tokamak disruptions with runaway current plateau formation and implications for ITER

The termination of the current and the loss of runaway electrons following runaway current plateau formation during disruptions have been investigated in the JET, DIII-D and FTU tokamaks. Substantial conversion of magnetic energy into runaway kinetic energy, up to ∼10 times the initial plateau runaway kinetic energy, has been inferred for the slowest current terminations. Both modelling and experiment suggest that, in present devices, the efficiency of conversion into runaway kinetic energy is determined to a great extent by the characteristic runaway loss time, τdiff, and the resistive time of the residual ohmic plasma after the disruption, τres, increasing with the ratio τdiff/τres. It is predicted that, in large future devices such as ITER, the generation of runaways by the avalanche mechanism will play an important role, particularly for slow runaway discharge terminations, increasing substantially the amount of energy deposited by the runaways onto the plasma-facing components by the conversion of magnetic energy of the runaway plasma into runaway kinetic energy. Estimates of the power fluxes on the beryllium plasma-facing components during runaway termination in ITER indicate that for runaway currents of up to 2 MA no melting of the components is expected. For larger runaway currents, minimization of the effects of runaway impact on the first wall requires a reduction in the kinetic energy of the runaway beam before termination and, in addition, high plasma density ne and low ohmic plasma resistance (long τres) to prevent large conversion of magnetic into runaway kinetic energy during slow current terminations.

Effect of irradiation with pulsed fluxes of high-temperature deuterium plasma and fast ions on the structure of the surface layer of W-1% La2O3 alloy

Scanning electron microscopy, X-ray electron probe analysis, and X-ray phase analysis are used to investigate the surface structure of W-1% La2O3 alloy after irradiation with submicrosecond pulses of deuterium ions and high-temperature deuterium plasma in a Plasma Focus installation for simulation of a plasma disruption in the ITER tokamak. It is established that melting of the tungsten surface and formation of micro-cracks take place under irradiation with a power flux density up to 1012 W/cm2. Introduction of La2O3 particles in the alloy leads to the formation of complex La-W-O oxides with lower melting points, which results in the formation of a nonuniform structure affecting the high-temperature response of the material.

Transient snakes in an ohmic plasma associated with a minor disruption in the HT-7 tokamak

A transient burst (ms, an order of the fast-particle slowdown timescale) of a spontaneous snake is observed for the first time in a HT-7 heavy impurity ohmic plasma. The features of the low-Z impurity snake are presented. The flatten electron profile due to the heavy impurity reveals the formation of a large magnetic island. The foot of the impurity accumulation is consistent with the location of the transient snake. The strong frequency-chirping behaviors and the spatial structures of the snake are also presented.

Measurement of synchrotron radiation from runaway electrons during the TEXTOR tokamak disruptions

Investigations of runaway electrons (REs) during induced disruptions are performed at the TEXTOR tokamak. The synchrotron radiation generated by REs in the plasma core is detected using an infrared camera. The measurements enable the observation of the structure and dynamics of the runaway beam. In particular, the runaway beam is investigated as a function of the vertical and horizontal control fields. From the plasma current a runaway number of 4.33 × 1016 was calculated. The number of REs with energies exceeding 25 MeV is 6.4 × 1015 according to the synchrotron measurement at the end of the current quench phase. The mean pitch angle of the >25 MeV runaways is found to be 52 mrad which is smaller than in the low density discharge scenario. In addition, the synchrotron measurements show for the first time that a significant number of the high energy REs can survive after the end of the current plateau phase. In order to understand this, runaway orbit calculations have been performed which are in good agreement with the measurements.