Prefill Pressure Research Articles

Development of full electromagnetic plasma burn-through model and validation in MAST

This paper describes the improvement of the electromagnetic plasma burn-through model. Full circuit equations describing the currents in solenoid, poloidal field coils, and toroidally conducting passive structures have been integrated into the differential equation system of the plasma energy and particle balances in DYON. This enables consistent calculation of the time-evolving loop voltage at a plasma position only using operation signals in a control room, which are current (or voltage) waveforms in solenoid and poloidal field coils and prefill gas pressure. The synthetic flux loop data calculated in the modelling agrees well with the measurement in MAST, confirming the validity of the loop voltage calculation. The electromagnetic modelling also enables calculation of 2D time-evolving poloidal magnetic flux map, thereby modelling the plasma volume evolution during the plasma break-down and burn-through phase. Only using the control room operation signals used in 34 ohmic start-up discharges with the direct induction start-up scenario in MAST, the electromagnetic plasma burn-through modelling has reproduced the time-evolution of plasma current, electron density and temperature, and plasma volume, showing a reasonable level of agreement with experimental measurement.

Impact of the electron density and temperature gradient on drift-wave turbulence in the Large Plasma Device

In this paper we present an experimental study of edge turbulence in the Large Plasma Device at UCLA. We utilize a scan of discharge power and prefill pressure (neutral density) to show experimentally that turbulent density fluctuations decrease with decreasing density gradient, as predicted for resistive drift-wave turbulence (RDWT). As expected for RDWT, we observe that the cross-phase between the density and potential fluctuations is close to 0. Moreover, the addition of an electron temperature gradient leads to a reduction in the amplitude of the density fluctuations, as expected for RDWT. However, counter to theoretical expectations, we find that the potential fluctuations do not follow the same trends as the density fluctuations for changes either in density gradients or the addition of a temperature gradient. The disconnect between the density and potential fluctuations is connected to changes in the parallel flows as a result of differences in the prefill pressure, i.e. neutral density. Further analysis of the density and potential fluctuation spectra show that the electron temperature gradient reduces the low frequency fluctuations up to $10 \,{\rm kHz}$ and the introduction of a temperature gradient leads to an unexpected ${\sim }{\rm \pi}$ shift of the density–potential cross-phase at ${\sim }10\,{\rm kHz}$ , while maintaining the typical resistive drift-wave cross-phase at lower frequencies. These experiments partly confirm existing knowledge on resistive drift-wave turbulence, but also introduce new observations that indicate a need for dedicated nonlinear three-dimensional turbulence simulations that include neutrals.

Numerical Study on the Effect of Plasma Density on Runaway Electron Suppression in the ADITYA-U Tokamak

Runaway Electrons (REs) generated during plasma disruptions in fusion grade tokamaks have the potential to severely damage the plasma-facing components. Designing optimal plasma discharge scenarios for RE suppression in future experiments requires interpretative modeling of current experiments. Multiple experiments have been carried out on ADITYA-U tokamak to design optimal plasma discharge scenarios for RE avoidance and suppression. In this article, we have numerically studied two representative pairs of ADITYA-U plasma discharges. In the first pair of discharges, plasma density was increased by gas puffs during the flap-top phase which is shown to suppress RE generation. In the second pair of the representative discharges, the effect of a lower ratio of peak electric field during breakdown to the pre-fill pressure is shown on RE generation. We simulate these plasma discharges using the PREDICT code to study the dynamics of Res. The results are consistent with the experimental Hard X-Ray diagnostic observations for both the pair of representative discharges. Additionally, the lowest RE-current is obtained in the discharges with a low ratio of the peak electric field during breakdown to the pre-fill pressure and high plasma density during the flap-top phase due to gas puffs. The suppression of REs is demonstrated by showing the effect of increasing plasma density on the separatrix in momentum space between thermal electrons and REs.

Studies of EC pre-ionization in DIII-D to support development of ITER plasma initiation

New experiments have been conducted at DIII-D to improve the physics understanding of plasma initiation assisted by electron cyclotron (EC) wave injection, allowing better extrapolation to ITER. This has been achieved by applying an EC pulse prior to start of the inductive plasma initiation (i.e. the generation of a loop voltage). A pre-plasma was formed during the EC pulse that was characterized in terms of the maximum density and temperature. Parametric scans were performed to study the influence of the EC injected power, EC injection angle, and pre-fill gas pressure on the pre-plasma creation process. These experiments showed that pre-ionized plasma of good quality can have a significant effect on the subsequent V loop induced plasma initiation process, i.e. a high density pre-plasma, increases the plasma current rise and speed at which ionization is achieved when the V loop is applied. A good quality pre-plasma is one that achieved a significant degree of ionization, mainly obtained by providing sufficient ECH power in DIII-D of the order of 1 MW. It was found that a minimum EC power of 0.5 MW was required in DIII-D to create ionization, and this would scale to a minimum power of roughly 6.5 MW for ITER.

Analysis of runaway electron discharge formation during Joint European Torus plasma start-up

Joint European Torus (JET) plasma initiations that form a significant quantity of runaway electrons have been studied. It is shown that there is no direct relationship between the prefill pressure and breakdown electric field and signs of runaway electrons during the plasma initiation. Runaway electron generation is determined by the electric field and density development at and after burn-through. A clear criterion of density and electric field at one given point in time, which would ensure the avoidance of runaway electron generation, cannot be determined, because the timescales for the formation of runaway electrons and for the dynamics of the density differ significantly. Moreover, the formation process can be reversed, reducing the influence of runaway electrons on the discharge. Ensuring a high enough density will reduce the likelihood that runaway electron discharges are formed. It is also found that at JET the electric field often exceeds the critical electric field during the early stages of the current ramp-up phase, even when no signs of runaway electrons are present. Expected runaway current dynamics have been analysed using the discharge circuit equation. The comparison of the expected runaway electron current dynamics shows it to be significantly slower compared to theoretical expectations in the presence of a hot and dense thermal background plasma. This could be explained by an enhanced critical electric field and/or a reduced confinement of runaway electrons. The latter is shown to be affected by bursts of magnetohydrodynamic activity that are characteristic during the current ramp-up. The development of discharges in which the current is fully carried by runaway electrons happens on a slow timescale of several seconds, limited by the available flux. Such timescales are sufficient for improved active control of these events, avoiding runaway currents at plasma initiation exceeding values at which they could damage in-vessel components. The results provide insight into the improvement and interpretation of self-consistent modelling of runaway electron generation during the start-up of International Thermonuclear Experimental Reactor discharges.

Parameter space validation through OOPS simulations of plasma burnthrough and discharge evolution in the SST-1 tokamak

Plasma burnthrough and current ramp-up phases in an SST-1 superconducting tokamak are simulated by the OOPS code. The main purpose of this study is to optimize the operation regime in SST-1 through the input parameter scan for OOPS for both successful and failed shots from the SST-1 database. SST-1 is now equipped with carbon plasma facing components (PFCs). Hence, PFCs are expected to offer a carbon dominated impurity environment. An electrically continuous vacuum vessel and cryostat hinder sufficient loop voltage during start-up, thereby rendering electron cyclotron resonance heating (ECRH) pre-ionization an absolute requirement. OOPS simulation is optimized for the essential plasma initiation parameters like the seed electron density ne = 3 × 1017/m3 and the initial neutral density n0 = 1–4 × 1018/m3 (corresponding to a tailored pre-fill pressure) and for an error magnetic field Berr ≤ 20 G. The simulation shows that the ECRH pre-ionization threshold power required for the successful plasma start-up under low available loop voltage conditions is ≥180 kW.

ITER breakdown and plasma initiation revisited

This paper revisits a number of key aspects of plasma initiation, aiming to clarify concepts, provide definitions and improve understanding, in view of ITER First Plasma operation. It shows that size matters and that breakdown and plasma initiation differ in larger devices. The large thick and conductive ITER vessel slows down the prefill process, the development of the toroidal electric field and affects the dynamics of the poloidal magnetic field at breakdown. The large vacuum vessel requires more than 1 s for the prefill gas to spread over the vessel. It slows down the development of the toroidal electric field, applied to ionize the prefill gas, by about 1 s and complicates the control of the magnetic configuration required for plasma initiation. It is shown that the avalanche process that provides the initial ionization slows down towards the end, if the cross-section of the toroidal discharge is larger. On the other hand, a larger plasma volume is beneficial for the subsequent burn-through of main-species ionization and impurity line-radiation. ITER First Plasma operation aims to reach a minimum plasma current of 100 kA, which will require a full avalanche and a partial burn-through to reach electron temperatures of at least 10–20 eV. The burn-through limits the ITER prefill pressure upper range of about 1 mPa. These predictions are based on 0D models of plasma initiation that furthermore assume the plasma volume remains constant. As burn-through is a critical process, models that consider radial profiles and/or volume dynamics, could provide further insight in the constraints of ITER plasma initiation. Electron cyclotron heating (ECH) high-power microwaves can be used to assist the ITER plasma initiation process. Although the ECH burn-through assist is reasonably well understood, no model exist that can predict the behaviour of ECH pre-ionization on ITER plasma initiation. ECH burn-through assist is shown to be ineffective for ITER First Plasma operations. Low prefill pressures are thought to increase the likelihood to develop a highly energetic supra-thermal electron discharge during the plasma initiation process. Considering supra-thermal electrons could affect the assumptions for the plasma resistance or even the ionization rate coefficient in plasma initiation models, hence altering the expected dynamics. Furthermore, only a qualitative picture of the processes that cause such supra-thermal discharges exists and a dedicated model is needed to improve ITER predictions.

Optical emission spectroscopy investigation of the current sheet in a small-bore parallel-plate electromagnetic plasma accelerator

The optical emission spectrum of the propagating current sheet in a small-bore parallel-plate electromagnetic plasma accelerator has been studied. The accelerator is powered by a fourteen stage pulse forming network, which yields a damped oscillation square wave of current with a pulse width of 20.5 μs. The movement of the first current sheet and the second current sheet is identified using three single magnetic probes placed at various axial positions. Current sheet canting is measured by two double magnetic probes. The spectrum of the current sheet is measured using a spectroscopic system. The electron density of the current sheet plasma is determined via Stark broadening of the hydrogen Hα line. The current sheet is almost perpendicular to the electrode. The current sheet electron density increases as the current sheet propagates. The current sheet electron density is mainly determined by the prefill pressure rather than the discharge current. The impurity charged particles caused by ablation of the back wall insulator exists in the current sheet. Residual particles are generally left behind the first current sheet and are swept and ionized by the second current sheet. The estimated current sheet sweeping efficiency is greater than 80%. The parallel-plate electromagnetic plasma accelerator developed in this paper shows great potential in generating high electron density plasma jets.

Current sheet characteristics of a parallel-plate electromagnetic plasma accelerator operated in gas-prefilled mode

The axial characteristics of a current sheet in a parallel-plate electromagnetic plasma accelerator operated in gas-prefilled mode are reported. The accelerator is powered by a fourteen stage pulse forming network. The capacitor and inductor in each stage are 1.5 μF and 300 nH, respectively, and yield a damped oscillation square wave of current with a pulse width of 20.6 μs. Magnetic probes and photodiodes are placed at various axial positions to measure the behavior of the current sheet. Both magnetic probe and photodiode signals reveal a secondary breakdown when the current reverses the direction. An increase in the discharge current amplitude and a decrease in pressure lead to a decrease in the current shedding factor. The current sheet velocity and thickness are nearly constant during the run-down phase under the first half-period of the current. The current sheet thicknesses are typically in the range of 25 mm to 40 mm. The current sheet velocities are in the range of 10 km/s to 45 km/s when the discharge current is between 10 kA and 55 kA and the gas prefill pressure is between 30 Pa and 800 Pa. The experimental velocities are about 75% to 90% of the theoretical velocities calculated with the current shedding factor. One reason for this could be that the idealized snowplow analysis model ignores the surface drag force.

Study on ECH-assisted start-up using trapped particle configuration in KSTAR and application to ITER

ECH-assisted start-up using trapped particle configuration (TPC) is firstly studied in a superconducting, conventional tokamak, KSTAR. First, improved and efficient start-up using TPC than conventional field null configuration (FNC) is achieved by enhanced pre-ionization plasma quality. TPC shows the broader operation window in terms of the poloidal field quality and the deuterium prefill pressure than that of FNC. Surprisingly the particle trapping enhances the plasma start-up performance even with much lower particle trapping ratio than that of spherical torus. Reliability of TPC with low trapping ratio is investigated by 0D plasma evolution code, TECHP0D. Second, the characteristics of TPC start-up are explored with experiments and modellings. Two kind of start-up failure conditions are identified by the magnetic pitch and prefill pressure scan experiments: (i) low ionization rate; and (ii) low density condition. These experimental observation has consistency with TECHP0D modeling result. Finally, reliable ITER-relevant low toroidal electric field start-up using TPC is achieved with drastically improved success rate of start-up in KSTAR. In this experiments, validity of the ITER-like toroidally inclined ECH/ECCD injection is also expected. Time dependent ITER start-up scenario using TPC is proposed and the superiority of it than FNC is discussed.

Second harmonic ECRH breakdown experiments in the TJ-II stellarator

A summary of the second harmonic ECRH breakdown experiments performed in the TJ-II stellarator is given here. The goal of these experiments is to determine the dependence of second harmonic X-mode plasma breakdown efficiency on several relevant parameters as the EC power, wave polarization, beam injection angle, tokamak-like induced electric field, neutral gas pre-fill pressure and discharge creation time with respect to the ramp-up of the currents in the coils. For each different scenario, the breakdown properties are determined, as well as the initial pressure conditions and the level of initial radiation before the injection of the ECRH power, which plays an essential role in the breakdown process.

Experimental characterization of plasma start-up using ECRH in preparation of W7-X operation

The upcoming operation of Wendelstein 7-X (W7-X) will be supported by an Electron Cyclotron Resonance Heating (ECRH) system working at 140 GHz in second harmonic at the nominal magnetic field of 2.5T. Because the optimization of the plasma breakdown is crucial to ensure a successful plasma build-up, dedicated plasma start-up experiments were performed on three stellarator/heliotron devices: Heliotron J, LHD and WEGA. Start-up behavior and dependencies on ECRH injected power, neutral gas pressure and rotational transform were obtained in X2 heating. Plasma start-up delay time decreases with the increase in ECRH input power. However, this behavior saturates when low pre-fill neutral gas pressure conditions are met. Both the delay time and the electron density are an increasing function of the gas pressure. On Heliotron J and WEGA devices, the higher the rotational transform is, the faster the start-up and the higher the plasma density are. Analysis of the temporal evolution of the plasma start-up shows that plasma start-up on stellarators is a two-step process. In addition, off-axis heating experiments are characterized by a longer plasma start-up duration compared to on-axis heating discharges. Third harmonic in X-mode has been attempted on LHD for different neutral gas puffing settings but no plasma breakdown was achieved. This multi-machine study was useful to define ECRH start-up scenarios for W7-X.

Energy Analysis of a Pulsed Inductive Plasma Through Circuit Simulation

Current profiles of a cylindrical ringing thetapinch are compared with SPICE simulations of an established circuit model and a least squares estimate is performed to determine plasma resistance and inductance for argon, hydrogen, and xenon plasmas with prefill pressures ranging from 10 to 100 mTorr. Plasma resistance is found to vary from 25.8 to 51.6 mQ with the lowest resistance occurring at 10 mTorr. Argon and xenon follow a similar trend with the xenon resistance averaging 4.2-mΩ (12.3%) larger than argon from 40 to 100 mTorr. Hydrogen resistance is found to increase rapidly as prefill pressure increases above 40 mTorr. Calculated plasma resistivity of 214-429 Ω-μm agrees with established literature. Plasma inductance varies from 41.3 to 47 nH and is minimized at 30 mTorr for argon and hydrogen, whereas xenon inductance is minimized at 20 mTorr. Hydrogen yields the highest inductance, averaging 1.9 nH (4.5%) more than argon over the pressure range tested. Temporal evolution of the energy partitioning into capacitive, inductive, and resistive loads is presented. Plasma inductive energy is found to be maximized when discharge current reaches its peak negative value of -23.5 kA. Xenon shows the greatest amount of inductive energy storage with a peak of 6.4 J (8.1%) of the initial 79.2 ± 0.1 J while argon dissipates the least energy through ohmic losses at most pressures. Hydrogen has the least inductive energy storage at all pressures and greatest ohmic losses above 60 mTorr. Xenon presents the largest ohmic losses over the 10-60-mTorr range.

Study of Plasma Startup on HL-2A Tokamak

For pure Ohmic drive both plasma breakdown and startup on HL-2A are on the low field side where the field null configuration is located, and the intensity of Dα emission confirms the plasma formation area. The current of vertical field coils is an important factor to facilitate plasma breakdown and a certain value is required for prompt plasma formation. The effect of the prefill pressure on plasma breakdown has been studied by scanning the prefill pressure. High and low prefill pressure will cause failure of the discharge and only in an appropriate range of the pressure can obtain a reproducible prompt discharge. A new plasma startup scenario has been introduced for bad wall conditions using early formation of a divertor configuration, the result demonstrates that it can be successful startup even if the plasma facing components have absorbed a lot of impurities.

Physics of plasma burn-through and DYON simulations for the JET ITER-like wall

This paper presents the DYON simulations of the plasma burn-through phase at Joint European Torus (JET) with the ITER-like wall. The main purpose of the study is to validate the simulations with the International Thermonuclear Experimental Reactor (ITER)-like wall, made of beryllium. Without impurities, the burn-through process of a pure deuterium plasma is described using DYON simulations, and the criterion for deuterium burn-through is derived analytically. The plasma burn-through with impurities are simulated using wall-sputtering models in the DYON code, which are modified for the ITER-like wall. The wall-sputtering models and the validation against JET data are presented. The impact of the assumed plasma parameters in DYON simulations are discussed by means of parameter scans. As a result, the operation space of prefill gas pressure and toroidal electric field for plasma burn-through in JET is compared with the Townsend avalanche criterion.

First experiments of plasma start-up assisted by ECRH on Tore Supra

ECRH pre-ionization and assisted start-up will be necessary in ITER due to the low electric field available (⩽0.3 V m−1). This paper reports on the results recently obtained on assisted start-up at the fundamental EC resonance on the tokamak Tore Supra (BT = 3.85 T, R = 2.4 m, a = 0.72). The Tore Supra ECRH/ECCD system is based on two 118 GHz gyrotrons (EC resonance at R ∼ 2.17 m). The power is injected into the plasma as Gaussian beams by an antenna located on the low field side which, using actively cooled mirrors inside the vacuum vessel, allows extensive control of both poloidal and toroidal injection angles. During the last experimental campaign, the ECRH system has been used for the first time to assist the plasma start-up. The usual start-up voltage on Tore Supra is 25 V (1.7 V m−1) with deuterium prefill pressure of around 20 mPa. The minimum stray field value (B⊥) in most of the vacuum vessel is lower than 5 mT, which gives toroidal connection lengths (a0BT/B⊥) longer than 770 m. In these conditions, attempts have been made to lower the start-up voltage. Sustained breakdowns were obtained down to ∼0.4 V m−1 without external assistance at 5 mPa. Besides, ∼300 kW of ECRH has been applied during 50 ms (one gyrotron in O-mode). Pre-ionization was successfully achieved at the resonance location and ∼30 kA of current was established in 10 ms. Successful plasma initiation has been obtained down to ∼0.28 V m−1 at 5 mPa.

Electron cyclotron heating assisted startup in JT-60U

Electron cyclotron heating (ECH)-assisted startup experiments have been performed in JT-60U. The breakdown loop voltage was successfully reduced from 25 to 4 V (=0.26 V m−1) by 200 kW ECH. This is lower than the 0.3 V m−1, which corresponds to the maximum electric field in ITER. Parameter scans of ECH power, prefill pressure, resonance position and polarization were carried out. The sensitivity of the breakdown to polarization and resonance position was observed. A prefilling gas pressure scan showed that the initial breakdown density increases with prefill pressure when it is is lower than 8 × 10−5 Torr. Higher harmonic ECH was also attempted. The second harmonic ECH-assisted startup was possible with higher ECH power injection. However, the third harmonic ECH-assisted startup was not successful.

Vacuum electric field created by microwaves inside magnetic confinement devices

The knowledge of the electric field created by microwave power injected inside the vacuum vessel is fundamental to understand second harmonic ECRH plasma breakdown in magnetic confinement devices. The simple photon model described in this letter allows to determine the electric field assuming that a homogeneous and stationary energy density state is reached during the first nanoseconds after the wave injection. According to second harmonic breakdown theory, the measurement of the maximum prefill pressure for which breakdown is achieved allows an indirect determination of the required electric field strength. Experiments performed in HSX show that the field value obtained in this way is in very good agreement with the value calculated with the model.

Second harmonic electron cyclotron plasma breakdown in TJ-II

A kinetic model for second harmonic electron cyclotron breakdown in stellarators, based on the results obtained from non-linear wave-particleinteraction theory, is presented. A set of particle balance equations that describes the H2 ionization chain by means of an electron momentumspace discretization is introduced and solved by numerical integration. The time evolutionof the various species densities is determined and breakdown relatedpredictions are obtained for different prefill pressure conditions and different values of injected power. Analysis of the TJ-IIstellarator experimental data shows good agreement with the theoretical results. This is the first time, to the authors' knowledge, that precise predictions are directly compared with experimental behaviour.

ECRH-assisted start-up in ITER

In ITER, the electric field applied for ionization and to ramp up the plasma current may be limited to . In this case, based on established theories of the avalanche process, it is shown that ohmic breakdown in ITER is only possible over a narrow range of pressure and magnetic error field. Therefore, ECRH may be necessary to provide robust and reliable start-up. ECRH can ensure prompt breakdown over a wide range of prefill pressure and error field and can also give control over the initial time and location of breakdown. For ECRH-assisted start-up in ITER, the power and pulse length requirements are essentially determined by the need to ensure burnthrough, i.e. complete ionization of hydrogen and the transition to high ionization states of impurities. A zero-dimensional (0D) code (with inclusion of some 1D effects) has been developed to analyse burnthrough in ITER. The 0D simulations indicate that control of the deuterium density is the key factor for ensuring successful start-up in ITER, where the effects of neutral screening and dynamic fuelling by the ex-plasma volume are also crucial. It is concluded that without ECRH, successful start-up will only be possible over a very restricted range of parameters but 3 MW of absorbed ECRH power will ensure reasonably robust start-up for a broad range of conditions with beryllium impurity. In the case of carbon impurity, even with an absorbed ECRH power of 5 MW one may be restricted to low prefill pressure and/or low carbon concentration for successful start-up.