Runaway electron generation during tokamak start-up

Hoppe, Mathias; I., Ekmark,; E., Berger,; Fülöp, Tünde

doi:10.1017/s002237782200054x

Cited by 12 publications

(16 citation statements)

References 32 publications

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“…In figure 7(c), it can be seen that the pressure range for significant REs to be produced expands to approximately 1 mPa if one assumes early density pumping after breakdown using Y BD = 0.9. Because of the decrease in electron density and increase in electron temperature, the E/E D ratio increases, resulting in significant RE production [30,33]. Simultaneously, we find that such density pumping extends the burn-through regime to high pressure, which is consistent with the observations in JET-ILW [44].…”

Section: Key Factors Affecting the Burn-through Limitsupporting

confidence: 86%

“…Because the onset of radiation collapse is sensitive to small changes in the heating power, a decrease in the heating power owing to the conversion from the ohmic current to the RE current plays a role in causing the burnthrough problem. This possibility was suggested in a previous study [33]. A finding here is that radiative collapse leads to strong RE generation when it occurs at a relatively high T e with substantial RE seed currents, which can even drive the formation of a full RE discharge.…”

Section: Full Runaway Electron Discharge Simulationsupporting

confidence: 67%

“…The last term of equation ( 3) represents the confinement loss of the generated REs after the magnetic flux surfaces form and a physical basis to set τ RE needs to be considered. Although a number of models have been proposed for the 0D burn-through analyses [30,33], the confinement of REs is governed by the drift orbit topologies in self-consistent 2D magnetic equilibrium, or 3D magnetic fields including the error fields and eddy currents, as well as the spatial position of the RE beam populations, whose quantitative treatments are out of the capability of the 0D model. For this reason, we consider only the lowest-order analysis that compares the cases that enable or disable the RE loss term in equation (3).…”

Section: Runaway Electron Modellingmentioning

confidence: 99%

“…As illustrated in figure 3(c), the confinement time of a 10 MeV electron released from the LFS at I p = 100 kA is about 0.5 ms (see the figure caption), which is regarded as characterising the confinement loss when the magnetic flux surface formation is marginal. However, τ RE = 0.5 s can be an underestimation compared to the modelling [30,33] and the experiments [69], the results will be compared also with the case of disabling the RE loss term (τ RE = ∞).…”

Section: Runaway Electron Modellingmentioning

confidence: 99%

“…To date, RE generation during startup has long been studied [18,19,29], and different RE models have recently been incorporated into the 0D plasma burn-through codes, such as SCENPLINT/TRANSMAK [7,[30][31][32], STREAM [33], and DYON [34]. In a similar approach, the possibility of RE discharges occurring at JT-60SA is studied numerically using the RE generation model [35].…”

Section: Introductionmentioning

confidence: 99%

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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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Section: Key Factors Affecting the Burn-through Limitsupporting

confidence: 86%

Section: Full Runaway Electron Discharge Simulationsupporting

confidence: 67%

Section: Runaway Electron Modellingmentioning

confidence: 99%

Section: Runaway Electron Modellingmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

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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Measurement of electromagnetic waves from runaway electrons

Bin

Buratti

Cardinali

et al. 2022

Review of Scientific Instruments

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Electromagnetic waves emitted during a tokamak discharge can be partially ascribed to coupling with plasma waves. In particular, in the presence of runaway electrons, the electromagnetic waves deliver information, otherwise inaccessible, about kinetic instabilities excited by the fast particles. Experiments aimed at studying radio frequency emissions from runaway electron scenarios during different stages of plasma discharge have been carried out at the Frascati Tokamak Upgrade. Frequencies in the range of lower hybrid and whistler waves have been explored, in the presence of relativistic electrons with different energies, ranging from a few to tens of MeV. A pronounced sensitivity of the radio frequency measurements in detecting driven instabilities is observed, providing the possibility to exploit this kind of technique as a monitor of the instability processes and for studies of the fast electron activity. In particular, in this work, we propose a simplified analysis of the frequency scaling of a specific family of kinetic instabilities arising at the lower hybrid frequency range during the current ramp-up stage. The study is performed with respect to the density profile and the wave vector coupling conditions and is aimed at obtaining a rough estimate of the most likely radial location of the interaction between the runaway electron beam and plasma waves at the emission times of the observed signals.

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Electromagnetic emission from plasma with counter-streaming electron beams in the regime of oblique instability dominance

Annenkov,

Volchok,

Timofeev

2024

Physics of Plasmas

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In this study, we investigate the generation of electromagnetic emission near the second harmonic of the plasma frequency induced by pairs of counter-propagating electron beams. Such systems can naturally occur in cosmic plasmas when particle acceleration regions are closely spaced, and they can also be implemented in a laboratory device. We specifically focus on the regime where the oblique beam–plasma instability dominates. The emission mechanism relies on the coalescence of counter-propagating plasma waves with different transverse structures. It has been demonstrated that the parameters of the system necessary for efficient radiation generation can be determined using the exact linear theory of beam–plasma instability. Through particle-in-cell numerical simulations, we show that a high beam-to-radiation conversion efficiency can be achieved when the beams excite small-scale oblique plasma oscillations. Importantly, we find that the efficiency and spectral characteristics of the radiation are not dependent on the thickness of the beams. We explore two scenarios involving pairs of symmetric beams: one with relativistic beams having a directed velocity of vb=0.9c and another with sub-relativistic beams at vb=0.7c. Additionally, we consider the injection of two beams with different velocities. In all cases considered, the beam-to-radiation power conversion efficiency reaches a level of a few percent, a sufficiently high value for beam–plasma systems.

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Runaway electron generation during tokamak start-up

Cited by 12 publications

References 32 publications

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

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

Measurement of electromagnetic waves from runaway electrons

Electromagnetic emission from plasma with counter-streaming electron beams in the regime of oblique instability dominance

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