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

Sinha, Joyeeta; Vries, P. C. de; Walker, Mike; Battaglia, D. J.; Turco, F.; Hyatt, A.W.; Kim, Hyun-Tae; Stöber, J.; Yoneda, Ryota; Gribov, Y.; Haskey, S. R.; Bykov, I.; Hollmann, E. M.; Herfindal, J. L.; Glass, F.; Chen, J.; McLean, A.G.

doi:10.1088/1741-4326/ac59ea

Cited by 10 publications

(16 citation statements)

References 20 publications

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“…At t = −1 ms, the density peak is at the center of the vessel, where the ECH resonance layer is located. This phenomenon means that pre-plasma generates (or forms) near the resonance layer, as observed in many tokamaks [10,23,24]. At t = 1.6 ms, the plasma density becomes radially asymmetric.…”

Section: Typical Ech Assisted Start-upmentioning

confidence: 87%

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: 87%

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

et al. 2023

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“…However, the plasma startup with lower voltages is more demanding in terms of a technical challenge for the stray field optimisation [36] in the presence of large eddy currents. Another option is EC wave injection before the voltage-induced plasma initiation to pre-ionise the prefill gas [82]. In JT-60SA, preionization using trapped particle configuration with EC waves [83,84] has also been prepared, which may provide an alternative path to achieve low-voltage plasma breakdown.…”

Section: Avoidance Of Startup Res and Expansion Of The Operational Wi...mentioning

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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“…A more accurate expression, proposed by Farina, shows the maximum energy gain of an electron at second harmonic as W max,n=2 ∝ √ P in non-linear interaction [11]. However, several factors, for example, Coulomb collisions, play a role in order for this local ionization to support a high-quality pre-plasma [12] [13].…”

Section: Ech Powermentioning

confidence: 99%

“…Usually, a higher ECH power can achieve quicker ionization [13], corresponding to a quick ramp-up of hydrogen alpha after ECH power is applied. However, we also find a couple of shots different from the others.…”

Section: Ech Pulse-widthmentioning

confidence: 99%

Electron cyclotron heating assisted start-up experiments in J-TEXT

Zhang

Vries²,

Nagasaki³

et al. 2023

EPJ Web Conf.

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Electron cyclotron heating (ECH) breakdown and burn-through assist has been adopted to make the ITER start-up, which uses a low toroidal electric field of about 0.3 V/m, more robust. Related studies have been carried out on many fusion machines such as DIII-D, JT-60U, Tore Supra and FTU. However, the required ECH power to ensure an effective breakdown assist at ITER is not yet clear. Experiment was also performed in J-TEXT to determine the minimum ECH power requirements to assist breakdown and develop a better physics description of the process. The breakdown loop voltage for a successful shot was reduced from 34 to 3.7 V (corresponding to 0.56 V/m) by 300 kW X2-mode ECH power. The critical ECH power to assist the successful formation of a tokamak discharge is about 200kW, and below this value, ionization caused by ECH power is observed, however, the tokamak discharge start-up does not succeed. The effect of different loop voltages and ECH pulse-width on start-up was also studied. As injecting ECH power at a proper time, low ECH power can achieve a similar pre-ionization result as the case with high power. Extremely low breakdown voltage leads to a higher toroidal field later when putting a capacity to continue discharging in J-TEXT. The earlier shutdown of ECH power caused a failed discharge.

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

Cited by 10 publications

References 20 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

Electron cyclotron heating assisted start-up experiments in J-TEXT

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