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

Kim, Hyun-Tae; Sips, A. C. C.; Contributors, Efda‐Jet

doi:10.1088/0029-5515/53/8/083024

Cited by 22 publications

(34 citation statements)

References 28 publications

(64 reference statements)

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“…2012)), in which case is approximately constant with a value based on plasma surface experiment data, or physical sputtering, when depends on the incident ion energy (i.e. ITER wall) (Kim, Sips & Contributors 2013).…”

Section: Burn-through Modelmentioning

confidence: 99%

Runaway electron generation during tokamak start-up

Hoppe

et al. 2022

J. Plasma Phys.

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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.

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Section: Burn-through Modelmentioning

confidence: 99%

Runaway electron generation during tokamak start-up

Hoppe

et al. 2022

J. Plasma Phys.

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“…Later, based on the same code, Kim has developed a simulator, DYON (DYnamic 0D model of Non-fully ionized plasma) (Kim et al 2012) which contains plasma-surface interaction effects and shows good agreement with carbon-wall JET data. Afterwards, DYON was used to simulate plasma burn through at JET with an ITER-like wall (Kim, Sips & Fundamenski 2013b) and to make predictions on ITER operation modes (Kim, Sips & Contributors 2013a). Recently, a two-dimensional particle simulation code has been developed to specifically target ohmic breakdown in RZ plane which is in a cylindrical coordinate system (Yoo et al 2014(Yoo et al , 2017, in which only the space charge effect in the poloidal direction is considered.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2018

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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 (${<}1.33\times 10^{-4}$ 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.

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“…In fact, ITER-like superconductive facilities like EAST [10] and KSTAR [11] have some problems leading to unsuccessful start-up at initial operations [1]. For several years now, nearly all the experimental work conducted on tokamaks all over the worldincluding DIII-D [12] in USA, JET [13,14], ASDEX [15] and FTU [16] in EU, JT60 [17,18] in Japan, KSTAR [11] and VEST [19] in Korea, SST-1 [20] in India, HL-2A [21], EAST [1] and J-Text [22] in China-has been to investigate the optimum operational parameters in the start-up phase. Understanding and optim ization of parameters in the start up phase is critical not only for the future successful ITER startup scenario [7], but also for saving the valuable volt-second [9], which prompts more rapid burn-through and ultimately extends the pulse length of the discharge.…”

Section: Introductionmentioning

confidence: 99%

Numerical modeling of tokamak breakdown phase driven by pure Ohmic heating under ideal conditions

et al. 2016

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We have simulated tokamak breakdown phase driven by pure Ohmic heating with implicit particle in cell/Monte Carlo collision (PIC/MCC) method. We have found two modes can be differentiated. When performing breakdown at low initial gas pressure, we find that it works at lower density and current, but higher temperature, and requires lower heating power, compared to when having a high initial pressure. Further, two stages can be distinguished during the avalanche process. One is the fast avalanche stage, in which the plasma is heated by induced toroidal electric field. The other is the slow avalanche stage, which begins when the plasma density reaches 1015 m−3. It has been shown that ions are mainly heated by ambipolar field and become stochastic in the velocity distribution. However, when the induced electric field is low, there exists a transition phase between the two stages. Our model simulates the breakdown and early hydrogen burn-through under ideal conditions during tokamak start-up. It adopted fewer assumptions, and can give an idealized range of operative parameters for Ohmic start-up. Qualitatively, the results agree well with certain experimental observations.

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

Cited by 22 publications

References 28 publications

Runaway electron generation during tokamak start-up

Runaway electron generation during tokamak start-up

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

Numerical modeling of tokamak breakdown phase driven by pure Ohmic heating under ideal conditions

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