The role of plasma–wall interactions in thermal instabilities at the tokamak edge

Abstract

Plasma-wall interaction leads to the release of impurities and neutrals of the working gas, which contribute significantly to the energy losses from the plasma edge, and therefore, crucially affects the development of thermal instabilities in fusion devices. An analytical model for impurity radiation is proposed, which takes into account the erosion mechanisms of wall material and the motion of impurity particles across magnetic surfaces. The temperature dependence of radiation losses is found to be very different from that predicted by the coronal approximation often used in considering thermal instabilities. The consequences for the development of poloidally symmetric detachment and multi-faceted asymmetric radiation from the edge ͑MARFE͒ are analyzed. It is demonstrated that the MARFE threshold principally depends on the mechanism by which working gas neutrals are released from the wall and on the neutral's properties, e.g., their ionization rate.

Physics of ultimate detachment of a tokamak divertor plasma

“…From Tokar & Kelly (2003), we find the estimate E imp ion ∼ 3 keV for the low-Z impurities (although this value can depend on the magnetic topology and the geometrical configuration of the divertor). Then, taking E eff ion ∼ 40 eV and M H /M imp ∼ 0.1, from (4.11) we find (4.12) which is in the same ballpark with the recent experimental data from AUG (see (1) from Kallenbach et al 2015).…”

“…Therefore, the neutral flux into the ionization region, Γ H , which balances the plasma flux Γ W , is simply proportional to P H √ 1/T W M H , where P H is the neutral hydrogen pressure in the divertor and M H the hydrogen atom mass. Taking into account the finite lifetime of the impurity ion, Q imp can also be expressed in terms of the effective 'ionization cost', E imp ion , that describes the plasma energy loss per an impurity ion during the time between its origination from ionization in the SOL plasma and complete volumetric recombination or surface neutralization (Tokar & Kelly 2003). So, Q imp = E imp ion Γ imp , where Γ imp is the impurity flux into the ionization region, which is proportional to P imp 1/T W M imp .…”

Influence of the Dynamic Ergodic Divertor on the Density Limit in TEXTOR

“…Over the years, results from both experimental observations and theoretical models show that the interaction between plasma, wall, and recycling neutrals of the working gas plays an important role in the thermal instabilities leading to MARFE formation [3][4][5][6]. In the tokamak TEXTOR, the density limit has been significantly extended with fresh boronization or siliconization [3] and exceeded by a factor of 1.7 the Greenwald-density limit, n GW e m ÿ3 1 10 17 kA ÿ1 m ÿ1 I p =a 2 , with the total plasma current I p and minor radius a measured in kA and m, respectively [7].…”

Theoretical Study of the Effect of the Dynamic Ergodic Divertor on MARFE Onset

“…In our previous analysis [5], we demonstrated the importance of plasma-wall interactions, i.e. the role of sources of impurity and hydrogen neutrals at the machine walls, on edge thermal instabilities.…”