The Impact of Collisionality on the Runaway Electron Avalanche during a Tokamak Disruption
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arXiv
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| Format: | Preprint |
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2024
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| author | Arnaud, Jonathan McDevitt, Christopher |
| author_facet | Arnaud, Jonathan McDevitt, Christopher |
| contents | The exponential growth (avalanching) of runaway electrons (REs) during a tokamak disruption continues to be a large uncertainty in RE modeling. The present work investigates the impact of tokamak geometry on the efficiency of the avalanche mechanism across a broad range of disruption scenarios. It is found that the parameter $ν_{*, crit}$ describing the collisionality at the critical energy to run away delineates how toroidal geometry impacts RE formation. In particular, utilizing a reduced but self-consistent description of plasma power balance, it is shown that for a high-density deuterium-dominated plasma, $ν_{*, crit}$ is robustly less than one, resulting in a substantial decrease in the efficiency of the RE avalanche compared to predictions from slab geometry. In contrast, for plasmas containing a substantial quantity of neon or argon, $ν_{*, crit} \gtrsim 1$, no reduction of the avalanche is observed due to toroidal geometry. This sharp contrast in the impact of low-versus high-Z material results primarily from the relatively strong radiative cooling from high-Z impurities, enabling the plasma to be radiatively pinned at low temperatures and thus large electric fields, even for modest quantities of high-Z material. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2401_11291 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | The Impact of Collisionality on the Runaway Electron Avalanche during a Tokamak Disruption Arnaud, Jonathan McDevitt, Christopher Plasma Physics The exponential growth (avalanching) of runaway electrons (REs) during a tokamak disruption continues to be a large uncertainty in RE modeling. The present work investigates the impact of tokamak geometry on the efficiency of the avalanche mechanism across a broad range of disruption scenarios. It is found that the parameter $ν_{*, crit}$ describing the collisionality at the critical energy to run away delineates how toroidal geometry impacts RE formation. In particular, utilizing a reduced but self-consistent description of plasma power balance, it is shown that for a high-density deuterium-dominated plasma, $ν_{*, crit}$ is robustly less than one, resulting in a substantial decrease in the efficiency of the RE avalanche compared to predictions from slab geometry. In contrast, for plasmas containing a substantial quantity of neon or argon, $ν_{*, crit} \gtrsim 1$, no reduction of the avalanche is observed due to toroidal geometry. This sharp contrast in the impact of low-versus high-Z material results primarily from the relatively strong radiative cooling from high-Z impurities, enabling the plasma to be radiatively pinned at low temperatures and thus large electric fields, even for modest quantities of high-Z material. |
| title | The Impact of Collisionality on the Runaway Electron Avalanche during a Tokamak Disruption |
| topic | Plasma Physics |
| url | https://arxiv.org/abs/2401.11291 |