Runaway electron seed formation at reactor-relevant temperature. (16th April 2020)

• Record Type: Journal Article
• Authors: Paz-Soldan, C.; Aleynikov, P.; Hollmann, E.M.; Lvovskiy, A.; Bykov, I.; Du, X.; Eidietis, N.W.; Shiraki, D.
• Is Part Of: Nuclear fusion. Volume 60:Number 5(2020)
• Publication Date: 2020-04-16
• Subjects: tokamak -- runaway electron -- seed generation; Nuclear fusion -- Periodicals; 621.48405
• DOI: 10.1088/1741-4326/ab7fe1
• Languages: English
• ISSNs: 0029-5515

Abstract

Abstract: Systematic variation of the pre-disruption core electron temperature ( T e ) from 1 to 12 keV using an internal transport barrier scenario reveals a dramatic increase in the production of 'seed' runaway electrons (REs), ultimately accessing near-complete conversion of the pre-disruption current into sub-MeV RE current. Injected Ar pellets are observed to ablate more intensely and promptly as T e rises. At high T e, the observed ablation exceeds predictions from published thermal ablation models. Simultaneously, the thermal quench (TQ) is observed to significantly shorten with increasing T e —a surprising result. While the reason for the shorter TQ is not yet understood, candidate mechanisms include: insufficiently accurate thermal ablation models, enhanced ablation driven by the seed RE population, or significant parallel heat transport along stochastic fields. Kinetic modeling that self-consistently treats the plasma cooling via radiation, the induced electric field, and the formation of the seed RE is performed. Including the combined effect of the inherent dependence of hot-tail RE seeding on T e together with the shortened TQ, modeling recovers the progression towards near-complete conversion of the pre-disruption current to RE current as T e rises. Measurement of the HXR spectrum during the early current quench (CQ) reveals a trend of decreasing energy with pre-disruption T e . At the very highest T e (≍ 12 keV), ≍ 100% conversion of the thermal current to runaway current is found. The energy of this peculiar RE beam is inferred to be sub-MeV as it emits vanishingly few MeV hard x-rays (HXRs). These measurements demonstrate novel TQ dynamics as T e is varied and illustrate the limitations of treating the RE seed formation problem without considering the inter-related dependencies of the pellet ablation, radiative energy loss, and resultant variations of the TQ duration. If the observed shortening of the TQ with increasing T e extends to fusion-grade plasmas, than their propensity to form large quantities of RE seeds at high T e may be far worse than previously thought. Positively, the high T e scenario in DIII-D produces REs so prodigiously that it can serve as a meaningful new platform for demonstrating RE avoidance techniques.