Effect of magnetic geometry on the energy partition between ions and electrons in the scrape-off layer of magnetic fusion devices

Abstract

A universal energy partition mechanism between ions and electrons has been confirmed to exist in the scrape-off layer of both the WEST tokamak and the W7-X stellarator. A peaked plasma density structure induced by an infinite magnetic connection length structure is observed to destroy this dependence and enhance the local ion and electron temperature ratio in the stellarator. A theoretical analysis reveals that the ratio of ion and electron parallel heat conduction is predominant in determining this universality, while electrons would further be cooled in the density-peaked region.

Characteristics of the SOL ion-to-electron temperature ratio on the J-TEXT tokamak with different plasma configurations

“…The retarding field analyzer (RFA) probe has been developed and widely used for the measurement of the edge ion temperature profile in many fusion devices such as JET [3,4], Tore Supra [1], Alcator C-mod [5], ISTTOK [6], STOR-M [7,8], EAST [9], LHD [10], W7-X [11] and J-TEXT [12]. A strong dependence of the edge ion-to-electron temperature ratio, τ i/e , on the SOL ion self-collisionality, , has been observed in several tokamak experiments, which is consistent with the simulation results of two-dimensional (2D) edge transport codes, EDGE2D and onion-skin [13]. As increases, τ i/e decreases gradually, and the energy exchange between ions and electrons is more efficient when is higher and τ i/e is closer to 1.…”

“…Previous experiments and simulation results have shown that τ i/e decreases with an increase of the SOL ion self-collisionality, [13]. In the complex collision process between ions and electrons, it is generally assumed that the electron-ion collision time is approximately equal to the electron-electron collision time [31].…”

“…In the complex collision process between ions and electrons, it is generally assumed that the electron-ion collision time is approximately equal to the electron-electron collision time [31]. Since the electron selfcollisionality lengths λ ee can be described as 10 16 T 2 e /n = λ ii /(T i /T e ) 2 , it is generally not distinguished whether λ ee or λ ii is the normalization parameter for the definition of the SOL collisionality [13]. In order to better compare with previous research results, we chose the SOL ion self-collisionality as the control parameter to study the τ i/e in this paper.…”

“…The is defined as , and it is roughly varied between 1 and 20. Here, the ion self-collisionality lengths [13]. The L c is the field-line connection length calculated with a field-line tracing code in the near-SOL region, as shown in figure 6 , the τ i/e measured in the near-SOL region in the HFS-MSN poloidal divertor plasmas decreases as the distance between the X-point and the divertor target increases.…”

Effect of edge ion temperature on the divertor tungsten sputtering in WEST

“…The effect of the ion temperature is barely considered in the theoretical simulation [17]. While in some experiments, the upstream ion temperature T iu has been measured to be much larger than the upstream electron temperature T eu , and their ratio τ u = T iu /T eu is strongly dependent on the plasma density and collisionality [18][19][20][21]. This implies that the effect of upstream ion temperature may play indirectly an important role on the W erosion in the downstream area of the divertor.…”