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

Kim, Hyun-Tae; Casson, F. J.; Meyer, H.; Cunningham, G.; Scannell, R.; Kogan, L. A.; Harrison, James R.; Kim, Seong-Cheol; Gwak, Jin-Woo; Na, Yong-Su; Lee, Jeong‐Won; Litaudon, X.; Falchetto, G.

doi:10.1088/1741-4326/ac9194

Cited by 6 publications

(6 citation statements)

References 23 publications

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“…Comparing this shot with the other two shots shown in figure 5, the pure ohmic heating start-up still does not have initial electrons, while shot #1068924 has a central line-integrated density of about 0.035 × 10 19 m −2 . As the loop voltage (or E loop ) consists of the sum of the plasma resistive component (I p R p ) and the inductive component (L c dI p /dt), where I p is the plasma current, R p the plasma resistance and L c the plasma inductance [14,25,26], these electrons can enhance the conductivity of the pre-fill gas (hydrogen) but contribute less to the reduction of the inductive component, relaxing the requirement of loop voltage for the avalanche process during the ohmic heating breakdown. As the E min of ECH assisted start-up is 29% of the ohmic heating start-up, the consumption in volt seconds by ECH assisted start-up is 16% of that of ohmic heating start-up until t = 10 ms.…”

Section: Typical Ech Assisted Start-upmentioning

confidence: 99%

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

et al. 2023

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The 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 physics description of the process. Results indicate that the minimum toroidal electric field for a successful start-up on J-TEXT was expanded from 2.5 V m−1 to 0.56 V m−1, and that injecting 300 kW of X2-mode ECH power can ensure robust breakdown. The critical ECH power for successful start-up was determined to be approximately 200 kW. 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 the Ohmic breakdown, while the enhanced CIII emission may be not good for the startup. The characteristics of the pre-plasma formed by ECH prior to the 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 startup. The analysis of spatial density showed that the pre-plasma can radially develop at a velocity of 600 m s−1. Furthermore, it was found that injecting ECH power at the appropriate time with a low power of 150 kW can achieve similar pre-ionization results as 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.

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Section: Typical Ech Assisted Start-upmentioning

confidence: 99%

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

et al. 2023

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“…The present study analyses the plasma power balance during the build-up of the plasma current, which has not been addressed in the earlier work and further strengthens the physics basis for plasma initiation in JT-60SA. Similar analyses of the plasma burn-through phase of tokamak startups have been performed using zero-dimensional (0D) [6][7][8][9][10][11][12][13][14][15] and onedimensional (1D) models [16,17]. In particular, the 0D models of the SCENPLINT code [7] and the DYON code [8] have comprehensively considered multiple impurity species and their sources associated with plasma-wall interaction (PWI).…”

Section: Introductionmentioning

confidence: 99%

Modelling of ohmic startup and runaway electron formation in support of JT-60SA initial operation

Matsuyama¹,

Wakatsuki²,

Inoue³

et al. 2022

Nucl. Fusion

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The plasma startup with ITER-relevant sub-mPa neutral pressures in the JT-60SA superconducting tokamak is studied using a zero-dimensional (0D) power balance model for the plasma burn-through phase. In the ohmic startup scenario assumed here, the lower limit of the operational neutral pressure is determined by the condition for the Townsend avalanche at a high $E/p$ limit, whereas the upper limit depends on the available ohmic heating power for the burn-through with the base Power Supply (PS) of the superconducting coils, corresponding to 0.2-0.3 V/m. The operational conditions that lead to RE generation are evaluated to mitigate the risk of RE discharges. The simulation shows that startup REs are generated during the temperature rise immediately after the burn-through under relatively low-density conditions, and then the RE current value is gradually increased in balance with the RE loss processes on the timescale of $I_p$ ramp-up. Therefore, it is expected that REs can be avoided or mitigated if the electron density during the early build-up phase is tuned by a combination of prefill gas and additional fuelling. However, a startup with an excessive prefill gas will lose the margin of the density control, and a full RE discharge can be triggered when the plasma burn-through fails owing to a high concentration of impurities such as oxygen, if the operational point is marginal to the burn-through limit. The present modelling work implies that preparing the operation with sufficient margins against the failure of impurity burn-through is important for mitigating the risk of unintended plasma termination. Although the carbon wall environment may have a lower risk of RE generation, to alleviate the operational uncertainties of the new device, strategies for expanding the operational window are also discussed in support of JT-60SA initial operation.

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“…where j RE is the RE current density, e is the charge of an electron and c is the speed of light. An expensive computational cost of the full electromagnetic plasma burn-through model [26] as well as a demand for flexible scenario development [14] has urged usage of the fluid description for start-up RE modelling. Models of the Dreicer generation rate were validated in dynamic scenarios with the plasma parameters fixed [27].…”

Section: Introductionmentioning

confidence: 99%

Kinetic modelling of start-up runaway electrons in KSTAR and ITER

et al. 2023

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Understanding formation of start-up Runaway Electrons (REs) is essential to ensure successful plasma initiation in ITER. The design of ITER start-up scenarios requires not only predictive simulations but also validation of assumptions. There are two generation mechanisms of REs: the Dreicer generation and runaway avalanche. Steady-state models for the Dreicer generation are likely to be valid due to the slower evolution of the plasma parameters during the plasma start-up. This is confirmed through the kinetic simulation, which demonstrate that the Dreicer generation can be extracted from the distribution function in dynamic scenarios. However, the runaway avalanche growth rate can differ from its steady state. There are two stages of evolution in the runaway avalanche growth rate before it reachs the steady state. In stage I, REs incapable of participating in the RE amplification reduce the runaway avalanche growth rate. In stage II, REs with finite energy can increase the runaway avalanche growth rate, but the effect is not significant. An error in the RE density due to the runaway avalanche is usually insignificant in most start-up RE modelling. The kinetic modelling of the KSTAR standard ohmic scenario confirms that the RE density can be modelled using the analytic formulas if it is the runaway-free discharge. However, the runaway-dominant discharge may demand the kinetic modelling. The scan of the Coulomb logarithm for the nonstandard ITER plasma start-up suggests that the timing of runaway current takeover predicted by the fluid simulation may change depending on the choice of the Coulomb logarithm.

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

Cited by 6 publications

References 23 publications

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

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

Modelling of ohmic startup and runaway electron formation in support of JT-60SA initial operation

Kinetic modelling of start-up runaway electrons in KSTAR and ITER

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