DEMO physics challenges beyond ITER

Abstract

For electricity producing tokamak fusion reactors like EU-DEMO, it is prudent to choose a plasma scenario close to the ITER baseline, where the largest amount of experimental evidence is available. Nevertheless, there are some aspects in which ITER and EU-DEMO have to differ, as the simple exercise of up-scaling from ITER to a larger device is constrained both by physical nonlinearities and by technological limits. In this work, relevant differences between ITER and the current EU-DEMO baseline in terms of plasma scenario are discussed. Firstly, EU-DEMO is assumed to operate with a very large amount of radiative power originating both from the scrape-off layer and, markedly, from the core. This radiation level is obtained by means of seeded impurities, whose presence significantly affects many aspects of the scenario itself, especially in terms of transient control. Secondly, because of the need of breeding tritium, the EU-DEMO wall is less robust than the ITER one. This implies that every offnormal interruption of the plasma discharge, for example in presence of a divertor reattachment, cannot rely on fastshutdown procedures finally triggering a loss of plasma control at high current, but other strategies need to be developed. Thirdly, the ITER method for the control of the so-called sawteeth (ST) has been shown to be too expensive in terms of auxiliary power requirements, thus other solutions have to be explored. Finally, the problem of actively mitigating, or suppressing, the Edge Localised Modes (ELMs) has recently increased the interest on naturally ELM-free regimes (like QH-mode, I-mode, and also negative triangularity) for EU-DEMO, thus increasing the needs for ELM mitigation or suppression with respect to the approach adopted in ITER.

EU DEMO EC equatorial launcher pre-conceptual performance studies

“…Besides several theoretical studies are done on e.g. steady-state options like Flexi-DEMO [4] or even negative triangularity, just to mention some [5]. The reference scenario for the EU DEMO is a 2 h pulsed machine with a duty cycle pulse/dwell ratio of 2 hours / 10 min and ~18 MA of plasma current.…”

Basic design considerations for a frequency step-tunable electron cyclotron wave system to suppress NTMs in DEMO

“…The current baseline parameters of the European DEMO are summarized in [16]. The NTMs with (m, n) = (3, 2) and (2, 1) are critical, where m and n are poloidal and toroidal mode number, respectively.…”

“…In this part of the present study, TORBEAM [42,43] is used to calculate the achievable CD profiles, steering range and steering resolution of a frequency tunable setup in the nominal plasma scenario [16]. The current NTM launcher parameters [14] are taken as reference in the following way.…”

“…Characteristic parameters will be assumed based on the current technological developments. Section 3 shows the considerations on the tuning of deposition profiles in the reference plasma scenario [16]. To consider the assumptions and parameters altogether, simulation cases for the envisaged NTM stabilization strategy are presented in section 4.…”

Targeting a Versatile Actuator for EU-DEMO: Xenon Doping of Fueling Pellets

“…The plasma scenario currently favored for the EU-DEMO fusion reactor intends for the presence of xenon (Xe) in the edge and pedestal region of the plasma for the purpose of radiative cooling in order to avoid dissipating a too-large heating power (i.e., about 450 MW originating from fusion α's and auxiliary control) in the scrape off layer (SOL) and divertor volume by redistributing around 65% of the exhaust power by radiation already inside the confined region. 1,2 In cases where the Xe is supplied via gas puffing, a very low fueling efficiency is expected, i.e., a rather high Xe flux has to be applied in order to reach the targeted value in the plasma. This is mainly due to a low fraction of gas penetrating through the SOL and the pedestal and a strong pedestal outward transport.…”