Tokamak Start-up Research Articles

Binary Nature of Collisions Facilitates Runaway Electron Generation in Weakly Ionized Plasmas.

Dreicer generation is one of the main mechanisms of runaway electron generation in weakly ionized plasmas. It is often described as a diffusive flow from the Maxwellian core into high energies under the effect of the electric field. In this Letter we demonstrate a critical role of the binary nature of inelastic collisions in weakly ionized plasma during tokamak startup, where some electrons experience virtually no collisions during acceleration to the critical energy. We show that using the Fokker-Planck collisional operator can underestimate the Dreicer generation rate by several orders of magnitude.

Automated shot-to-shot optimization of the plasma start-up scenario in the TCV tokamak

Plasma start-up is typically achieved manipulating poloidal magnetic fields, gas injection and possibly auxiliary heating. Model-based design techniques have been gaining increasing attention in view of future large tokamaks which have more stringent constraints and less room for trial-and-error. In this paper, we formulate the tokamak start-up scenario design problem as a constrained optimization problem and introduce a novel shot-to-shot correction algorithm, based on the Iterative Learning Control concept, to compensate for unavoidable modeling errors based on experimental data. The effectiveness of the approach is demonstrated in experiments on the TCV tokamak showing that the target ramp-up scenario could be obtained in a small number of shots with a rough electromagnetic model.

Numerical study of plasmas start-up by electron cyclotron waves in NCST spherical tokamak and CN-H1 stellarator

According to the physics of tokamak start-up, this study constructs a zero-dimensional (0D) model applicable to electron cyclotron (EC) wave assisted start-up in NCST spherical torus (spherical tokamak) and CN-H1 stellarators. Using the constructed 0D model, the results obtained in this study under the same conditions are compared and validated against reference results for pure hydrogen plasma start-up in tokamak. The results are in good agreement, especially regarding electron temperature, ion temperature and plasma current. In the presence of finite Ohmic electric field in the spherical tokamak, a study on the EC wave assisted start-up of the NCST plasma at frequency of 28 GHz is conducted. The impact of the vertical magnetic field B v on EC wave assisted start-up, the relationship between EC wave injection power P inj, Ohmic electric field E, and initial hydrogen atom density n H0 are explored separately. It is found that under conditions of Ohmic electric field lower than ITER (~ 0.3 V m−1), EC wave can expand the operational space to achieve better plasma parameters. Simulating the process of 28 GHz EC wave start-up in the CN-H1 stellarator plasma, the plasma current in the zero-dimensional model is replaced with the current in the poloidal coil of the stellarator. Plasma start-up can be successfully achieved at injection powers in the hundreds of kilowatts range, resulting in electron densities on the order of 1017–1018 m–3.

Simulation study of the influences of beryllium on the tokamak start-up process

Tokamak start-up is strongly dependent on the state of the initial plasma formed during plasma breakdown. To acquire a better understanding of the process and to estimate the influence of the impurity of beryllium on the ohmic heating tokamak start-up process, one-dimensional particle-in-cell coupled with a Monte Carlo collision method has been developed. The main aim is to investigate the plasma performance under various amounts of beryllium with different discharge parameters. Tokamak breakdown with the impurity of beryllium in the ohmic heating strategy has been simulated. The simulation results show that with the impurity of beryllium, the increase of plasma density is suppressed compared with the case without beryllium. The breakdown time is delayed by the impurity. Moreover, the successful breakdown has a much higher requirement on discharge parameters with a low electric field operational scenario, since in the low electric field discharge the influence of beryllium impurity is greater. As the plasma density increases, the effect of beryllium impurity on plasma becomes more critical. It indicates that impurities cannot be neglected in the high plasma density.

Cross-machine comparison of runaway electron generation during tokamak start-up for extrapolation to ITER

A cross-machine comparison of global parameters that determine the runaway electron (RE) generation and loss process during tokamak start-up was carried out with the aim to extrapolate these to ITER. The study found that all considered discharges, also those that do not show signs of RE, are non-thermal at the start, i.e. have a streaming parameter larger than 0.1. During the current ramp-up the electric field, E, remains above the critical value, E c, that allows RE in the plasma. The distinction to be made is not if RE can form but, if sufficient RE can form fast enough such that they are detected or start to dominate the dynamics of the tokamak discharge. The dynamics of the value of E, density and temperature during tokamak are key to the formation of RE. It was found that larger devices operate with E closer to E c, due to their higher temperatures, hence the RE generation is relatively slower. The slower time scales for the formation of RE, estimated to be of the order of 100s of ms in ITER simplifies the development of avoidance schemes. The RE confinement time is also an important determinant of the entire process and is found to increase with the device size. The study also revealed that drift orbit losses, a mechanism often attributed as the main RE loss mechanism during the early tokamak discharge, are actually more difficult to achieve. RE losses might be more likely attributed to RE diffusion due to magnetic turbulence.

Experimental study of electron cyclotron heating assisted start-up on J-TEXT

Second harmonic X-mode (X2 mode) electron cyclotron heating (ECH) assisted start-up has been studied experimentally on the J-TEXT device to determine the minimum ECH power requirements to assist breakdown and develop a better physical description of the process. Results indicate that the minimum toroidal electric field for a successful start-up on J-TEXT changed from to , and that injecting of X2-mode ECH power can ensure robust breakdown. The critical ECH power for successful start-up was determined to be approximately . At lower powers ECH was observed to cause ionization, but this did not necessarily result in successful start-up. The effects of varying ECH power, pulse-width and toroidal magnetic field on start-up were also investigated. Higher ECH power leads to quicker, stronger ionization and CIII emission, and is beneficial for burn-through. Higher pre-plasma density caused by high ECH power can decrease the required toroidal electric field for ohmic breakdown, while the enhanced CIII emission may not be good for start-up. The characteristics of the pre-plasma formed by ECH prior to application of the loop voltage were also studied. The toroidal magnetic field affects the initial location where pre-plasma forms and this also affected the subsequent tokamak start-up. The analysis of spatial density showed that the pre-plasma can radially develop at a velocity of . Furthermore, it was found that injecting ECH power at the appropriate time with a low power of can achieve similar pre-ionization results to the high-power case. The transition from ECH plasma to ohmic plasma suggests that the ECH assisted start-up modified the process of purely ohmic breakdown.

The New PEGASUS-III Experiment

Developing attractive means of initiating current without using magnetic induction from a central solenoid is a critical scientific and technical challenge facing the spherical tokamak (ST). The PEGASUS program has focused on developing the physics basis and predictive models for nonsolenoidal tokamak startup using local helicity injection (LHI) and has demonstrated startup to ~0.2 MA in a low-field, near-unity aspect ratio ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$A$ </tex-math></inline-formula> ) ST. The PEGASUS facility is being upgraded into a solenoid-free ST called PEGASUS-III. Major features include: increased toroidal field (TF) to 0.6 T for up to 100 ms, improved shape control, and retaining the low-aspect ratio geometry of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$A \sim 1.2$ </tex-math></inline-formula> . This TF directly supports the new mission to expand the breadth of solenoid-free research on the facility with multiple reactor-relevant techniques. The implemented TF upgrade is comprised of a new center rod, outer TF coil system, torque plate assemblies, and TF interconnects between the center rod and outer conductors. The center rod is comprised of 24 water-cooled, insulated wedge conductors inside a new inner vacuum wall. There is no ohmic solenoid, which allows additional TF conductor for the 48-kA/turn TF current. The outer TF conductors are 12 pairs of air-cooled, reinforced Al plate conductors. Pairwise crossed conductor links connect the outer C-plates to the central rod conductor to eliminate the need for a toroidal compensation wind-back coil. Torque plate assemblies on top and bottom mechanically secure both the outer C-conductor and the interconnecting links, counteract torsional magnetic loads during a pulse, and provide compliance for vertical displacement. The assembly accommodates magnetic and thermal forces, limiting the axial excursion of the central assembly to <1 mm.

Runaway electron generation during tokamak start-up

Tokamak start-up is characterized by low electron densities and strong electric fields, in order to quickly raise the plasma current and temperature, allowing the plasma to fully ionize and magnetic flux surfaces to form. Such conditions are ideal for the formation of superthermal electrons, which may reduce the efficiency of ohmic heating and prevent the formation of a healthy thermal fusion plasma. This is of particular concern in ITER where engineering limitations put restrictions on the allowable electric fields and limit the prefill densities during start-up. In this study, we present a new 0D burn-through simulation tool called STREAM (STart-up Runaway Electron Analysis Model), which self-consistently evolves the plasma density, temperature and electric field, while accounting for the generation and loss of relativistic runaway electrons. After verifying the burn-through model, we investigate conditions under which runaway electrons can form during tokamak start-up as well as their effects on the plasma initiation. We find that Dreicer generation plays a crucial role in determining whether a discharge becomes runaway-dominated or not, and that a large number of runaway electrons could limit the ohmic heating of the plasma, thus preventing successful burn-through or further ramp-up of the plasma current. The runaway generation can be suppressed by raising the density via gas fuelling, but only if done sufficiently early. Otherwise a large runaway seed may have already been built up, which can avalanche even at relatively low electric fields and high densities.

Magnetic Turbulence and Current Drive during Local Helicity Injection.

Magnetic measurements during dc helicity injection tokamak startup indicate Alfvénic turbulence in the injected current streams mediates magnetic relaxation and results in macroscopic plasma current drive. Localization of such activity to the injected current streams, a bias voltage dependence to its onset, and higher-order spectral analysis indicate super-Alfvénic electrons excite instabilities that drive the observed turbulence. Measured fluctuation helicity is consistent with an α-dynamo electromotive force driving net current comparable to the macroscopic equilibrium current density. These results imply new constraints for scaling local helicity injection to larger devices.

Control of electron acceleration process during merging start-up of spherical tokamak

The axial plasma merging method is one of the start-up schemes to form spherical tokamak (ST) plasma without use of a center-solenoid coil. It involves a magnetic reconnection process in the presence of high toroidal magnetic field parallel to the reconnection electric field and has the potential to accelerate electrons directly along the magnetic field lines. In the paper, the role of boundary conditions of the reconnection downstream region was investigated in the UTST experiment. Self-generated axial electric field in the reconnection downstream region was observed to grow up to 20–30 times as large as the toroidal electric field where the MHD condition holds, but was largely suppressed inside the area where the magnetic field lines are contacted to the inboard limiter conductors equipped on the center stack. This suppression effect was attributed to the short circuit connection of the magnetic field lines to hinder the axial charge separation and then a significant part of the parallel electric field remained. These experimental results suggest that the merging start-up method has the capability to enhance electron acceleration efficiency by magnetic reconnection and may have the potential to produce high electron temperature ST plasma applicable for additional heating and current drive techniques.

Mitigation of the error field and eddy currents for the start-up of spherical tokamak using compensation and reverse coils

The tokamak is the most popularly accepted device among the fusion community for future fusion reactors. It has an assembly of toroidal field (TF), central solenoid (CS), poloidal field (PF) coils, and compensation coils (CC) for sustaining plasma in the vacuum vessel (VV). Eddy currents are crucial for the steady-state operation of a tokamak. Major sources of eddy current in the tokamak are edge localized modes and transient currents in the CS and TF coils. Eddy currents strongly affect the tokamak startup scenarios. Eddy currents are produced due to flux changes, the vertical component of the magnetic field (VCMF) of the CS coil, and also by the fast rise of current in the TF coil in the initial phase. This study is aimed to estimate the eddy current density and magnitude of the VCMF by experimental and simulation tools. Eddy current density and VCMF are mitigated on the central cylinder and vacuum vessel, using a pair of six turn CC connected in series with CS and a reverse coil connected in series with TF coil. The current direction in the reverse coil is opposite to the direction of current in the Z-shaped connectors of the TF coil. Tokamak startup is optimized by decreasing the eddy current density, VCMF, and the resulting electromagnetic (EM) forces with the installation of CC and reverse coils.

Validity of models for Dreicer generation of runaway electrons in dynamic scenarios

Runaway electron modelling efforts are motivated by the risk these energetic particles pose to large fusion devices. The sophisticated kinetic models can capture most features of the runaway electron generation but have high computational costs, which can be avoided by using computationally cheaper reduced kinetic codes. This paper compares the reduced kinetic and kinetic models to determine when the former solvers, based on analytical calculations assuming quasi-stationarity, can be used. The Dreicer generation rate is calculated by two different solvers in parallel in a workflow developed in the European integrated modelling framework, and this is complemented by calculations of a third code that is not yet integrated into the framework. Runaway Fluid, a reduced kinetic code, NORSE, a kinetic code using non-linear collision operator, and DREAM, a linearized Fokker–Planck solver, are used to investigate the effect of a dynamic change in the electric field for different plasma scenarios spanning across the whole tokamak-relevant range. We find that on time scales shorter than or comparable to the electron–electron collision time at the critical velocity for runaway electron generation, kinetic effects not captured by reduced kinetic models play an important role. This characteristic time scale is easy to calculate and can reliably be used to determine whether there is a need for kinetic modelling or cheaper reduced kinetic codes are expected to deliver sufficiently accurate results. This criterion can be automated, and thus it can be of great benefit for the comprehensive self-consistent modelling frameworks that are attempting to simulate complex events such as tokamak start-up or disruptions.

Observation and Investigation of Runaway Electrons in the Start-Up Phase of Taban Tokamak

The plasma current start-up in the Taban tokamak has been achieved via an electron cyclotron resonance (ECR) pre-ionization system. In the present work, the produced runaway electrons (REs) during ECR pre-ionization have been measured and investigated by hard X-ray (HXR) detection under different discharge conditions. The relationship between HXR emission and the loop voltage as well as the filling pressure in the Taban tokamak start-up phase has been reported.

Effects of reconnection downstream conditions on electron parallel acceleration during the merging start-up of a spherical tokamak

The axial merging method is one of the candidates to provide a center-solenoid-free start-up of high-beta spherical tokamak (ST) plasma. Two initially formed STs merge through magnetic reconnection in the presence of the guide (toroidal) magnetic field, which is perpendicular to the reconnection (poloidal) magnetic field. During ST merging start-up, electrons are effectively accelerated near the reconnection point where the reconnection electric field is almost parallel to the magnetic field. In order to evaluate the effectivity of this acceleration process on electron heating, the temporal and spatial distributions of generated energetic electrons are observed by a soft x-ray fast imaging system equipped on the UTST device. The energetic electrons were generated not only in the vicinity of the reconnection point but transiently in the inboard-side downstream region until the static electric field by charge separation grew to cancel the reconnection electric field component parallel to the magnetic field line. Adequate control of the downstream condition could enhance the generation of energetic electrons and provide a more effective conversion from the released magnetic energy to electron energy.

Investigation of fine structure formation of guide field reconnection during merging plasma startup of spherical tokamak in TS-3U

Ion heating/transport and its fine structure formation process through magnetic reconnection have been investigated by high guide field tokamak merging experiments in TS-3 and TS-3U. In addition to the previously reported demonstration of high-temperature plasma startup without center solenoid, the detailed fine structure formation process of reconnection heating has been revealed using new 96CH/320CH ultra-high-resolution 2D ion Doppler tomography diagnostics. By identifying the double-axis field configuration with the X-point on the midplane using in situ magnetic probe diagnostics, the detailed measurement successfully revealed that the ion temperature profile forms two types of characteristic heating structure, both around the X-point and downstream. The former is affected by the Hall effect to form a tilted heating profile, while the latter is affected by the transport process which a forms a poloidal double-ring-like structure. The achieved ion heating mostly depends on the reconnecting component of the magnetic field, and the contribution of the guide field to decrease the heating efficiency tends to be saturated in the high guide field regime. Under the influence of better toroidal confinement with higher guide field, the downstream ion heating is transported vertically, mostly by parallel heat conduction, and finally forms a poloidal ring-like hollow distribution aligned with the closed flux surface at the end of merging.

Electron Cyclotron collisionless interaction during EC-assisted tokamak start-up

In the initial phase of a plasma discharge with EC-assisted breakdown, the wave-particle interaction is nonlinear and wave trapping provides the mechanism for transition from a very low energy state to a much larger energy state. A Hamiltonian adiabatic approach provides the condition for the energy variation to occur in a rigorous way, together with quantitative estimate as a function of the wave frequency, harmonic number, polarisation and EC power and beam width, for the first, and second cyclotron harmonic. The interaction is highly localized in space close to the EC resonance. The spatial profile of the energy gain are reported together with the estimates of the width of the radial region where the interaction takes place in the case of a tokamak configuration. The analysis provides a physics based description of the process as well as indications for an optimized experimental setup.

On the breakdown modes and parameter space of ohmic tokamak start-up

Tokamak start-up is strongly dependent on the state of the initial plasma formed during plasma breakdown. We have investigated through numerical simulations the effects that the pre-filling pressure and induced electric field have on pure ohmic heating during the breakdown process. Three breakdown modes during the discharge are found, as a function of different initial parameters: no breakdown mode, successful breakdown mode and runaway mode. No breakdown mode often occurs with low electric field or high pre-filling pressure, while runaway electrons are usually easy to generate at high electric field or low pre-filling pressure (${&lt;}1.33\times 10^{-4}$ Pa). The plasma behaviours and the physical mechanisms under the three breakdown modes are discussed. We have identified the electric field and pressure values at which the different modes occur. In particular, when the electric field is $0.3~\text{V}~\text{m}^{-1}$ (the value at which ITER operates), the pressure range for possible breakdown becomes narrow, which is consistent with Lloyd’s theoretical prediction. In addition, for $0.3~\text{V}~\text{m}^{-1}$, the optimal pre-filling pressure range obtained from our simulations is $1.33\times 10^{-3}\sim 2.66\times 10^{-3}$ Pa, in good agreement with ITER’s design. Besides, we also find that the Townsend discharge model does not appropriately describe the plasma behaviour during tokamak breakdown due to the presence of a toroidal field. Furthermore, we suggest three possible operation mechanisms for general start-up scenarios which could better control the breakdown phase.

Initiation and sustainment of tokamak plasmas with local helicity injection as the majority current drive

Local helicity injection (LHI) is a non-solenoidal current drive capable of achieving high- tokamak startup with non-invasive current injectors in the plasma scrape-off layer. The choice of injector location within the edge region is flexible but has a profound influence on the nature of the current drive in LHI discharges. New experiments on the Pegasus ST with injection in the high-field-side, lower divertor region produce plasmas dominated by helicity injection current drive, static plasma geometry, and negligible inductive drive. Peak plasma current up to 200 kA, and a sustained plasma current of 100 kA for up to 18 ms, is demonstrated. Maximum achievable plasma current is found to scale approximately linearly with the effective loop voltage from LHI. A newly-observed MHD regime for LHI-driven plasmas in which large-amplitude fluctuations at 20–50 kHz are abruptly reduced on the outboard side results in improved current drive. A simultaneous increase in high frequency fluctuations (>400 kHz) inside the plasma edge suggests short wavelength turbulence as an important current drive mechanism during LHI.

Non-inductive build-up of a hot plasma by X-wave ECH/ECCD to remove initial high loop-voltage from start-up of fusion tokamaks

The possibility of non-inductive build-up of a hot plasma by X-wave ECH/ECCD to remove initial high loop-voltage stage from start-up of fusion tokamaks is theoretically investigated, using ITER parameters. There are two injection methods for X-wave ECH/ECCD in ITER start-up plasmas. The first is oblique X-wave injection from the low-field side, referred to as the DX method. For the low density start-up plasma at , the wave with GHz and is damped away via cyclotron resonance absorption before encountering the cyclotron cutoff layer when the electron temperature is as high as eV. The wave power is deposited into the resonance electrons of which parallel velocity is as fast as , which enables an efficient ECCD. The second method is oblique O-wave injection from the low-field side, referred to as the OX method. The waves pass through the ECR layer with negligible power deposition when eV and are mode-converted into oblique X-waves upon reflection on the inboard wall. The X-waves propagate toward the ECR layer without cutoff, and are cyclotron-damped away even when the temperature is as low as eV. The OX method is appropriate for the initial open field stage and transition stage of the discharge, while the DX method is appropriate for the final tokamak stage, where Te is high. Discussions are given on the choice of appropriate BV and location of ECR layer for initiation of closed flux surfaces. Nine model plasmas from an initial open field plasma to a final hot tokamak plasma, consisting of a series of snapshots of a build-up discharge, are generated using an equilibrium theory, enabling estimation of required RF power to drive the current. These investigations suggest that an injection power MW could build up a hot plasma with kA, , keV, m and m.

Application of Townsend avalanche theory to tokamak startup by coaxial helicity injection

The Townsend avalanche theory is employed to model and interpret plasma initiation in NSTX by Ohmic heating and coaxial helicity injection (CHI). The model is informed by spatially resolved vacuum calculations of electric field and magnetic field line connection length in the poloidal cross-section. The model is shown to explain observations of Ohmic startup including the duration and location of breakdown. Adapting the model to discharges initiated by CHI offers insight into the causes of upper divertor (absorber) arcs in cases where the discharge fails to start in the lower divertor gap. Finally, upper and lower limits are established for vessel gas fill based on requirements for breakdown and radiation. It is predicted that CHI experiments on NSTX-U should be able to use as much as four times the amount of prefill gas employed in CHI experiments in NSTX. This should provide greater flexibility for plasma start-up, as the injector flux is projected to be increased in NSTX-U.