Application of Townsend avalanche theory to tokamak startup by coaxial helicity injection

Hammond, K. C.; Raman, R.; Volpe, F.

doi:10.1088/1741-4326/aa8fa4

Cited by 18 publications

(19 citation statements)

References 32 publications

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“…Also, the transport loss is dominant only after the electron temperature becomes high (which happens during the later burnthrough phase and not during breakdown), which is also the case for the drift loss and error field loss (Hada et al 2015). Even during tokamak start-up by coaxial helicity injection (CHI) the average parallel velocity ( v ) driven by E is much larger than the E × B drift motion (Hammond, Raman & Volpe 2017). Perpendicular transport could be included by several methods even in a one-dimensional model.…”

Section: Simulation Resultsmentioning

confidence: 99%

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

et al. 2018

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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.

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Section: Simulation Resultsmentioning

confidence: 99%

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

et al. 2018

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“…4 of Ref. [19]. However, due to the smaller vessel size and lower toroidal field employed, the connection lengths in the main NSTX vessel were at least one order of magnitude lower, resulting in correspondingly higher values of p min .…”

Section: Chi Breakdown In Reactor-scale Devicesmentioning

confidence: 88%

“…Hence, we will evaluate the breakdown for these configurations using the spatially resolved approach developed in Ref. [19], which applies the Townsend avalanche theory to individual flux tubes linking the two electrodes.…”

Section: Chi Breakdown In Reactor-scale Devicesmentioning

confidence: 99%

“…This model has been applied to Ohmic heating in tokamaks by interpreting V bd as the integral of the parallel electric field E induced by the central solenoid along the length of the flux tube [21]. Good agreement between the model and experiment for Ohmic start-up has been found for γ = 0.58 [21,22,23,19]. Note that, with γ fixed at 0.58, the second term in the denominator of Eq.…”

Section: Chi Breakdown In Reactor-scale Devicesmentioning

confidence: 99%

“…It was postulated in Ref. [19] that the assumption γ = 0.58 works well for regions in the main vessel where flux tubes make many toroidal revolutions, whereas a lower estimate may be necessary for the divertor area where flux tubes may make less than one revolution. Considering that γ ≈ 0.05 has been observed for deuterium discharges between parallel plates [25], we will make the conservative assumption that γ = 0.01.…”

Section: Breakdown Requirementsmentioning

confidence: 99%

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Application of transient CHI plasma startup to future ST and AT devices

2019

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Employment of non-inductive plasma start-up techniques would considerably simplify the design of a spherical tokamak fusion reactor. Transient coaxial helicity injection (CHI) is a promising method, expected to scale favorably to next-step reactors. However, the implications of reactor-relevant parameters on the initial breakdown phase for CHI have not yet been considered. Here, we evaluate CHI breakdown in reactor-like configurations using an extension of the Townsend avalanche theory. We find that a CHI electrode concept in which the outer vessel wall is biased to achieve breakdown, while previously successful on NSTX and HIT-II, may exhibit a severe weakness when scaled up to a reactor. On the other hand, concepts which employ localized biasing electrodes such as those used in QUEST would avoid this issue. Assuming that breakdown can be successfully attained, we then apply scaling relationships to predict plasma parameters attainable in the transient CHI discharge. Assuming the use of 1 Wb of injector flux, we find that plasma currents of 1 MA should be achievable. Furthermore, these plasmas are expected to Ohmically self-heat with more than 1 MW of power as they decay, facilitating efficient hand-off to steady-state heating sources. These optimistic scalings are supported by TSC simulations.

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