Investigation on the Arc Ignition Characteristics and Energy Absorption of Liquid Metal Current Limiter Based on Self-Pinch Effect

The GaInSn liquid metal current limiter based on the fluid pinch effect has broad application prospects due to its particular properties.However,the limited rated current and ability of power dissipation are the critical problems for its wide application.Firstly,the temperature distribution of the liquid metal current limiter (LMCL) was obtained by experiments with a rated current of 1 kA and the arc ignition phenomenon was observed with 1.5 kA,which indicates that the rated current is mainly limited by the arc rather than the high temperature compared to the traditional switchgears.Furthermore,an improved method is proposed by adding the paralleled pure resistance,impedance or another LMCL element to protect the setup from the fault energy concentration in the setup.The problem of a slower arc voltage increasing rate can be solved by adding a paralleled impedance with suitable parameters.Finally,the current limiting properties based on the improved method were investigated and the alternating oscillating current was found between two paralleled LMCL elements owing to their deviation of arc ignition in reality.     

Simulation of the Effect of a Metal Vapor Arc on Electrode Erosion in Liquid Metal Current Limiting Device

The effect of arc plasma on electrode erosion in a liquid metal current limiter(LMCL)is studied.Based on a simplified two-dimensional magnetohydrodynamic model,the elongated GaInSn metal vapor arc and its contraction process in a liquid metal current limiter are simulated.The distributions of temperature,pressure and velocity of the arc plasma are calculated.The simulation results indicate that the electrode erosion is mainly caused by two high temperature gas jet flows arising from the pressure gradient,which is a result of the non-uniform arc temperature distribution.The gas flows,which act as jets onto the electrode surface,lead to the evaporation of the electrode material form the surface.A redesign structure of the electrode is proposed and implemented according to the analysis,which greatly increased the service life of the electrode.     

Suppression of runaway current by electrode biasing and limiter biasing on J-TEXT

The avoidance of runaway electrons (REs) generated during plasma disruption is of great concern for the safe operation of tokamak devices. Experimental study on the suppression of runaway current by electrode biasing (EB) and limiter biasing (LB) has been performed on the J-TEXT tokamak, which could be an alternative way to suppress the runaway current. The experimental results show that the higher the voltage value, the smaller the runaway current in both EB and LB experiments. The runaway current can be completely suppressed at an electrode biased voltage of +450 V and a limiter biased voltage of +300 V. The comparison of the energy spectra during the runaway plateau phase shows that the maximum energy max (ERE) and radiation temperature THXR of hard x-rays (HXRs) are significantly reduced after the application of +200 V limiter biased voltage. The electric field generated by the biased voltage may be the key factor to suppress the runaway current, and the measured radial electric field increases obviously after the voltage is applied. This may result in an increase in the loss of REs to realize the suppression of runaway current.     

Modelling of carbon erosion and re-deposition for the EAST movable limiter

The movable limiter at the mid-plane of the Experimental Advanced Superconducting Tokamak(EAST)with carbon coatings on the surface was exposed to edge plasma to study the material erosion and re-deposition.After the experiments,the carbon erosion and re-deposition is modelled using the 3D Monte Carlo code ERO.The geometry of the movable limiter,3D configuration of the plasma parameters and electromagnetic fields under both limiter and divertor configurations have been implemented into the code.In the simulations,the main uncertain parameters such as carbon concentrationρ_c in the background plasma and cross-field transport coefficient D_⊥in the vicinity of surface according to the'funneling model',have been studied in comparison with experiments.The parameterρ_c mainly influences the net erosion and deposition profiles of the two sides of the movable limiter,while D_⊥mostly changes the profiles on the top surface.     

Evidence of vapor shielding effect on heat flux loaded on flowing liquid lithium limiter in EAST

A lithium (Li) vapour layer was formed around a flowing liquid Li limiter to shield against the plasma incident power and reduce limiter heat flux in the EAST tokamak. The results revealed that after a plasma operation of a few seconds, the layer became clear, which indicated a strong Li emission with a decrease in the limiter surface temperature. This emission resulted in a dense vapour around the limiter, and Li ions moved along the magnetic field to form a green shielding layer on the limiter. The plasma heat flux loaded on the limiter, measured by the probe installed on the limiter, was approximately 52% lower than that detected by a fast-reciprocating probe at the same radial position without the limiter in EAST. Additionally, approximately 42% of the parallel heat flux was dissipated directly with the enhanced Li radiation in the discharge with the liquid metal infused trenches (LIMIT) limiter. This observation revealed that the Li vapour layer exhibited an excellent shielding effect to liquid Li on plasma heat flux, which is a possible benefit of liquid-plasma-facing components in future fusion devices.     

The application of limiter target electrostatic measurement system in J-TEXT tokamak

The limiter target electrostatic measurement system including limiter grounding current sensors and Langmuir probes have been newly developed for the measurement of the limiter target area on the Joint-Texas Experimental tokamak(J-TEXT).Current sensors fixed between graphite limiters and the vacuum vessel walls are used to measure the currents between limiters and vessel wall.Simultaneously,a rectangular poloidal array containing 54 Langmuir probes is embedded in the graphite tiles of limiters for a more localized measurement.Based on this system,the effect of both the plasma's inherent behavior,including plasma motion and the 2/1 tearing mode,and the electrode biasing on probe and sensor signals have been observed and analyzed in the experiments.     

Review of FTU results with the liquid lithium limiter

Since the end of 2005 a liquid lithium limiter has been installed on FTU. The liquid lithium is confined by capillarity in a mat of stainless steel or other refractory material and the capillary force counteracts the electromagnetic force. In this paper we review some of the most important results obtained in the experimental campaigns led on FTU. Peaked electron density profiles are spontaneously formed when the density exceeds 1.0 × 10²⁰ m⁻³. Despite to the fact that FTU is a fully metallic machine with a TZM toroidal limiter, the only impurity that is present in the plasma is lithium so that very clean plasma are obtained and the beneficial effects are discussed. Heat loads in excess of 5 MW/m² are withstood by the limiter without any damage also because the radiative losses from the evaporated lithium are able to dissipate most of the incoming heat flux.     

Preliminary design and performance study of EAST liquid lithium limiter based on CPS

Lithium has the ability of H recycling suppression and impurities absorption and it can be used as plasma facing material (PFM) in tokamaks. Lithium conditioning experiments were launched on EAST, HT-7 and some other tokamaks for many years by using the methods of GDC, IRCF and evaporation. Liquid lithium has better performances in effective lifetime and heat removal aspects compared to non-liquid lithium. While, applying liquid lithium in the tokamak would cause the safety problem as the lithium can react with many substances violently and the magnetohydrodynamic behavior is difficult to be handled. EAST liquid lithium limiter (LLL) system is under developing and will be applied in EAST to study the main technologies of the liquid lithium application. The normal operation temperature of the limiter is expected as 230–550 °C under the active cooling of water. Capillary porous system (CPS) is used to prevent the lithium from splashing under large electromagnetic force by increasing the surface tension of the lithium. In order to investigate the cooling performance of the cooling design, the thermal-hydraulic analysis was done which shows that with 3 m/s flowing velocity, the water can keep the limiter under 550 °C all the time if the heat flux is lower than 0.7 MW/m². Under heat flux of 1 MW/m², the limiter should be retreated within 7 s to avoid erosion. The pressure drop of the coolant under 3 m/s is less than 40 kPa with temperature difference nearly 34 °C which meet the design requirements very well. The key manufacture process and technologies like vacuum bonding between the CuCrZr heat sink and 316L guide plate were well studied in the R&D process. The heating test on the test bench showed that the CPS can be heated efficiently by the heaters attached into the heat sink.     

Heat loads on FTU liquid lithium limiter

The choice of the best material exposed to the plasma in a future reactor is still an open question. One of main requirements to be satisfied is the capability to withstand high heat loads, in the range 10–20 MW/m², during normal operations in a future reactor, as well as the peak power released by ELMs in H-mode operation. On FTU, since the end of 2005, we have started an innovative program having as main goal the possibility to expose a liquid surface to the plasma. The small wetted area, of the FTU three liquid lithium limiter units, does not allow to use it as main limiter for all the duration of the discharge so that it is always set in the shadow of the main toroidal limiter. In this condition, heat loads up to 2 MW/m² are normally withstood by the liquid lithium limiter without any surface damage and problems to the FTU operations. In order to increase the heat load on the liquid lithium limiter for a controlled limited period, the plasma column is shifted towards the liquid lithium limiter during the discharge. The surface temperature remains constant although the plasma column is pushed on the liquid lithium limiter. This saturation of the surface temperature can be understood considering the dependence of the evaporation rate versus the surface temperature between 250 °C and 550 °C that increases by five orders of magnitude. The evaporated lithium forms a strongly radiative cloud all around the three units limiting the power load on the surface. We do not observe any accumulation of lithium into the discharge as it can be also inferred from the time evolution of the Li III line growing up until the temperature is reaching the maximum value and then remaining almost constant.     

Modeling of heat deposition on the W/Cu movable limiter in HT-7

The thermal behavior of a directly water-cooled W/Cu movable poloidal limiter was investigated in HT-7, a medium-sized superconducting tokamak with limiter configuration, major radius R = 1.22 m, and minor radius a = 0.27 m. The W/Cu movable limiter (ML) was exposed to the plasma at various radial positions at r < a. The surface and bulk temperatures were monitored by an IR-camera and the thermocouples, respectively. The heat flux deposited on the limiter was evaluated by an ANSYS code using the measured surface temperatures as boundary conditions. It was found that the maximum heat flux incident on the ML was less than 1 MW/m² in the Ohmic discharges, but reached up to 5-7 MW/m² in the discharges with lower hybrid current drive (LHCD). A simple model was developed to understand heat transport to the W/Cu ML, taking into account the “funnel effect”.     

Low-temperature operating regime of the tokamak evacuating limiter

Conclusions The conditions for realizing the regime of strong recycling of a cold dense plasma of an evacuating limiter were determined based on a previously proposed model for describing the limiter layer of a tokamak. The scaling for the dependence of the gas pressure in the evacuation system on the average plasma density in the limiter layer was found, and agreed quantitatively with the results of measurements on the Alcator and ISX-B tokamaks.For the tokamak reactor of the INTOR scale the calculations show that the low-temperature operating regime of the evacuating limiter can be realized with a quite low pumping rate. It has the advantages of reduced erosion of the limiter and small fluxes of impurities into the working volume of the reactor. In addition, the relative concentration of the helium ash in the limiter layer does not exceed 2–3%, but the density of the main plasma is comparable to the proposed average density in the reactor. The concept of a stochastic limiter is of interest for lowering the plasma density in the limiter layer and lowering the thermal loads on the limiter.Translated from Atomnaya Énergiya, Vol. 61, No. 6, pp. 440–443, December, 1986.     

Evaporation Erosion During the Relay Contact Breaking Process Based on a Simplified Arc Model

Evaporation erosion of the contacts is one of the fundamental failure mechanisms for relays.In this paper,the evaporation erosion characteristics are investigated for the copper contact pair breaking a resistive direct current (dc) 30 V/10 A circuit in the air.Molten pool simulation of thc contacts is coupled with the gas dynamics to cMculate the evaporation rate.A simplified arc model is constructed to obtain the contact voltage and current variations with time for the prediction of the current density and the heat flux distributions flowing from the arc into the contacts.The evaporation rate and mass variations with time during the breaking process are presented.Experiments are carried out to verify the simulation results.     

Behavior of an indigenously fabricated transferred arc plasma furnace for smelting studies

The utilization of industrial solid waste for metal recovery requires high-temperature tools due to the presence of silica and alumina, which is reducible at high temperature. In a plasma arc furnace, transferred arc plasma furnace(TAP) can meet all requirements, but the disadvantage of this technology is the high cost. For performing experiments in the laboratory, the TAP was fabricated indigenously in a laboratory based on the different inputs provided in the literature for the furnace design and fabrication. The observed parameters such as arc length, energy consumption, graphite electrode consumption, noise level as well as lining erosion were characterized for this fabricated furnace. The nitrogen plasma increased by around 200 K(200 ℃) melt temperature and noise levels decreased by ~10 d B compared to a normal arc.Hydrogen plasma offered 100 K(100 ℃) higher melt temperature with ~5 d B higher sound level than nitrogen plasma. Nitrogen plasma arc melting showed lower electrode and energy consumption than normal arc melting, whereas hydrogen plasma showed lower energy consumption and higher electrode consumption in comparison to nitrogen plasma. The higher plasma arc temperature resulted in a shorter meltdown time than normal arc with smoother arcing. Hydrogen plasma permitted more heats, reduced meltdown time, and lower energy consumption, but with increased graphite consumption and crucible wear. The present study showed that the fabricated arc plasma is better than the normal arc furnace with respect to temperature generation, energy consumption, and environmental friendliness. Therefore, it could be used effectively for smelting-reduction studies.     

Thermal load on the TFR Tokamak limiter during additional heating pulses and major plasma disruptions

Inconel and graphite have been tested as limiter materials in the TFR Tokamak. Their behaviour during MHD activity and plasma current disruptions in high density low impurity content plasmas has been studied. The discharge energy balance with auxiliary heating has been established by using infrared measurements of the limiter temperature increase and bolometric techniques. Measurements of the temperature distribution on different limiters show that the maximum limiter temperature can be reduced by a large factor by an appropriate choice of the limiter shape and of its total area. The characteristics of the scrape-off layer in the limiter shadow has also been investigated in different limiter configurations. The experimental results and observations indicate a low physical and chemical sputtering of the graphite limiters during plasma discharges and their good properties for thermal shocks during major disruptions.     

Critical Length Criterion and the Arc Chain Model for Calculating the Arcing Time of the Secondary Arc Related to AC Transmission Lines

The prompt extinction of the secondary arc is critical to the single-phase reclosing of AC transmission lines,including half-wavelength power transmission lines.In this paper,a lowvoltage physical experimental platform was established and the motion process of the secondary arc was recorded by a high-speed camera.It was found that the arcing time of the secondary arc rendered a close relationship with its arc length.Through the input and output power energy analysis of the secondary arc,a new critical length criterion for the arcing time was proposed.The arc chain model was then adopted to calculate the arcing time with both the traditional and the proposed critical length criteria,and the simulation results were compared with the experimental data.The study showed that the arcing time calculated from the new critical length criterion gave more accurate results,which can provide a reliable criterion in term of arcing time for modeling and simulation of the secondary arc related with power transmission lines.     

Arc characteristics during the instability stage on transverse magnetic field contacts

The arcing process greatly affects the breaking ability after current zero. The instability stage is the transition stage from the ignition to the movement stage, which affects the arc movement characteristics. In this paper, the arc characteristics during the instability stage on spiral-type contacts were investigated using a high-speed video camera. A multi-column parallel instability mode and a single-column instability mode were found during the instability stage. The arc appearance and constriction degree changed rapidly. The arc voltage usually increased accompanied by fluctuations. In addition, it was found that the current significantly influenced the arc mode and duration in the instability stage. With increased peak current, the probability of a single-column instability mode increased, and the fluctuation range and average time decreased.     

Dynamic physical characteristics of DC arc on arcing horn for HVDC grounding electrode line

The dynamic physical characteristics of a DC arc on an arcing horn for a high voltage direct current(HVDC) grounding electrode line are significantly different from those of the switching device arc,secondary arc,AC fault arc and pantograph-catenary arc.In this work,an experimental platform for the DC arc on the arcing horn was built,and mechanisms of the arc column short circuit and arc root movement were studied.This work further analyzes the characteristics and mechanisms of the arc motion wh...     

Experience and technical issues of liquid lithium application as plasma facing material in tokamaks

The following critical issues of liquid lithium used in tokamak conditions are considered: major physical properties of lithium, physico-chemical aspects of lithium interaction and compatibility with structural materials of fusion reactors. Lithium capillary-porous system (CPS) is considered as advanced plasma facing material for power fusion reactor and its main properties are presented. Review of plasma facing element (PFE) structures based on lithium CPS and tests results in T-11M, T-10 and FTU tokamaks are included. Brief review of projects of lithium limiter of FTU with active system for thermal stabilization and module of lithium divertor for KTM tokamak with liquid metal (Na-K) cooling system based on the lithium CPS use are presented.     

Characterization of plasma current quench during disruptions at HL-2A

The most essential assumptions of physics for the evaluation of electromagnetic forces on the plasma-facing components due to a disruption-induced eddy current are characteristics of plasma current quenches including the current quench rate or its waveforms.The characteristics of plasma current quenches at HL-2A have been analyzed during spontaneous disruptions.Both linear decay and exponential decay are found in the disruptions with the fastest current quenches.However,there are two stages of current quench in the slow current quench case.The first stage with an exponential decay and the second stage followed by a rapid linear decay.The faster current quench rate corresponds to the faster movement of plasma displacement.The parameter regimes on the current quench time and the current quench rates have been obtained from disruption statistics at HL-2A.There exists no remarkable difference for distributions obtained between the limiter and the divertor configuration.This data from HL-2A provides basic data of the derivation of design criteria for a large-sized machine during the current decay phase of the disruptions.     

Beneficial influence of the weldment rupture of cask impact limiter case on the impact absorbing behavior

In the cask impact limiter design, the functions of steel case should be evaluated for the protection of the filler materials and the impact energy absorption by the buckling deformation in both the fire and impact accidents. The objective of this paper is to evaluate the beneficial influence of the intermittent weldment of the cask impact limiter case on the cask impact behavior. This paper describes the test results for the joint strength evaluation of intermittent weldment and the cask impact analysis considering the weldment rupture of the impact limiter case. The weldment rupture of the impact limiter case causes to lose their constraining effect for the wood blocks, which are filled into the metal incasement between the case and the gussets. The weldment rupture of the impact limiter case reduces the impact force which acts on the impact target significantly in vertical and horizontal drop impacts. Therefore the beneficial effect of weldment rupture should be considered in the impact limiter design and the cask impact analysis.