On the breakdown modes and parameter space of ohmic tokamak start-up

Abstract

Tokamak start-up is strongly dependent on the state of the initial plasma formed during plasma breakdown. We have investigated through numerical simulations the effects that the pre-filling pressure and induced electric field have on pure ohmic heating during the breakdown process. Three breakdown modes during the discharge are found, as a function of different initial parameters: no breakdown mode, successful breakdown mode and runaway mode. No breakdown mode often occurs with low electric field or high pre-filling pressure, while runaway electrons are usually easy to generate at high electric field or low pre-filling pressure (${<}1.33\times 10^{-4}$ Pa). The plasma behaviours and the physical mechanisms under the three breakdown modes are discussed. We have identified the electric field and pressure values at which the different modes occur. In particular, when the electric field is $0.3~\text{V}~\text{m}^{-1}$ (the value at which ITER operates), the pressure range for possible breakdown becomes narrow, which is consistent with Lloyd’s theoretical prediction. In addition, for $0.3~\text{V}~\text{m}^{-1}$, the optimal pre-filling pressure range obtained from our simulations is $1.33\times 10^{-3}\sim 2.66\times 10^{-3}$ Pa, in good agreement with ITER’s design. Besides, we also find that the Townsend discharge model does not appropriately describe the plasma behaviour during tokamak breakdown due to the presence of a toroidal field. Furthermore, we suggest three possible operation mechanisms for general start-up scenarios which could better control the breakdown phase.

Simulation study of the influences of beryllium on the tokamak start-up process

“…2016; Peng et al. 2018), we have found that the breakdown can be divided into three phases: fast avalanche, transition and slow avalanche. From this figure, three phases can be figured out clearly, showing in figure 2( b – d ), respectively.…”

“…In this work a PIC-MCC method (Lapenta, Brackbill & Ricci 2006) is used. This method which was developed previously has been applied to different plasma areas, such as microdischarges in the low-temperature plasma , magnetized capacitively coupled plasma (Yang et al 2017a,b;) and fusion plasma (Jiang et al 2016;Peng et al 2018).…”

“…The cross-section data used in this paper are referred from Phelps & Petrovic (1999) and Zatsarinny & Bartschat (2004). Since the dissociated hydrogen is widely used in the simulation (Lloyd et al 1996 2013c; Peng et al 2018), here we also adopt this assumption. The ionization cross-sections of the hydrogen atom and beryllium with electrons are shown in figure 1.…”

“…2013 c ; Peng et al. 2018), here we also adopt this assumption. The ionization cross-sections of the hydrogen atom and beryllium with electrons are shown in figure 1.…”

“…Experimental studies (Sand, Waelbroeck & Waidmann 1973;Holly et al 1981;Lloyd & Edlington 1986;Yoshino & Seki 1997;Kajiwara et al 2005) revealed that the electric field can be smaller when using the assist heating or preionization methods. In recent decades, many further studies have been developed on plasma breakdown (Jiang et al 2016;Yoo et al 2017;Peng et al 2018;Yoo et al 2018). It has been found that the self-electric field produced by charge separation plays a large role in plasma breakdown.…”