“Snowflake” H Mode in a Tokamak Plasma

Abstract

An edge-localized mode (ELM) H-mode regime, supported by electron cyclotron heating, has been successfully established in a ''snowflake'' (second-order null) divertor configuration for the first time in the TCV tokamak. This regime exhibits 2 to 3 times lower ELM frequency and 20%-30% increased normalized ELM energy (ÁW ELM =W p ) compared to an identically shaped, conventional single-null diverted H mode. Enhanced stability of mid-to high-toroidal-mode-number ideal modes is consistent with the different snowflake ELM phenomenology. The capability of the snowflake to redistribute the edge power on the additional strike points has been confirmed experimentally. In the dominant tokamak approach to magneticconfinement nuclear fusion, the divertor configuration has emerged as the preeminent solution to managing power exhaust and core impurity content. In the presence of a magnetic X point, where the poloidal field vanishes, energy and particle losses are channeled primarily into a divertor region that is separated from the confined plasma region. The drawback of this approach is the high power flux impinging on the divertor plates, which may consequently overheat and undergo destructive erosion. This is particularly true during edge localized modes (ELMs) [1][2][3][4][5] occurring in the high-confinement H mode [6], which cause periodic, violent, and potentially highly damaging ejections of energy and particles onto the divertor surfaces [7,8]. Heat flux management in the tokamak divertor and, in particular, the control of ELMs [9-11], remains therefore a primary goal of magnetic fusion research.A ''snowflake'' (SF) divertor configuration [12,13] has recently been proposed to reduce the plasma-wall interaction by changing the divertor's poloidal magnetic field topology. The feasibility of this configuration was demonstrated for the first time in the TCV tokamak [14,15]. The X point of a SF configuration features a second-order null point, i.e., both the poloidal magnetic field and its spatial first derivatives vanish. By perturbing this configuration, a SF þ configuration or SF À configuration is obtained with two contiguous X points that, for the SF À configuration, are both located at the separatrix [12]. The distance between the X points normalized to the plasma minor radius () parametrizes the proximity to an ideal SF configuration. SF plasmas feature a longer scrape-off layer (SOL) connection length and higher flux expansion in the null-point region compared with conventional singlenull (SN) configuration [14]. Additionally, the increased magnetic shear in the edge region is predicted to influence ELM activity [16,17]. This Letter reports the first H-mode discharges obtained with a SF þ divertor. Stationary ELM H modes are obtained and a significant change in the ELM frequency is observed, compared to an equivalent SN divertor plasma, demonstrating the potential of SF plasmas to lead to a new type of highconfinement scenario with better edge properties. In this Letter, we (1) examine the stability and power thr...

Advanced divertor configurations with large flux expansion

“…These affects are being studied in the SF configurations in NSTX and TCV. In TCV, recent measurements using high spatial density Langmuir probe arrays [35] have confirmed the initial infrared thermography measurements [23,24] of the heat and particle flux spreading into the additional strike points during Type I ELMs. The power partitioning between the SF separatrix branches was also observed in L-mode discharges, albeit at small values of σ [36].…”

“…The observed reduction was attributed to the reduction of carbon physical sputtering fluxes in the SF divertor (due to very low divertor T e ), and to the particle expulsion effect from ELMs that appeared in the SF phase [20]. In the standard divertor Hmode discharge, lithium coatings on lower divertor PFCs reduced recycling and led to modified edge plasma pressure and current profiles and low-n peeling-ballooning mode stabilization [26,27], as the pedestal stability op- close to the kink-balooning stability boundary, and the SF-plus configuration was consistent with improved kink-ballooning stability [23,24,28]. Equilibria reconstructions also confirmed a higher magnetic shear inside the separatrix in the SF-plus phase.…”

“…Up to 16 poloidal field coils are available for plasma shape control, making it an ideal test bed for SF studies [22]. The SF experiments were conducted in 0.3 MA L-and H-mode discharges with up to 2 MW of electron cyclotron heating (ECH) [22,23,24].…”

Overview of recent and current research on the TCV tokamak

“…In Hmode, the frequency of type-I ELMs is found to increase with σ, with a less-thanproportional decrease in fractional energy release, resulting in a favorable [16].) OV/4-4 scaling of average heat flux for the SF configuration [19,21]. The lowest σ values in this scan, however, were accompanied by a transition to type-III ELMs, with higher frequency and degraded confinement.…”

“…The snowflake (SF) divertor, a configuration predicated on an X-point with vanishing poloidal field gradients [3], is now well recognized as a viable operating configuration thanks to its successful demonstration on TCV [18][19]. Its ability to reduce the energy and particle flux to an appropriately designed divertor, due to the doubling of the number of divertor legs and to the increased flux expansion, is well documented [18,20].…”