First demonstration of full ELM suppression in low input torque plasmas to support ITER research plan using n = 4 RMP in EAST

Abstract

Full suppression of type-I edge localized modes (ELMs) using n = 4 resonant magnetic perturbations (RMPs) as planned for ITER has been demonstrated for the first time (n is the toroidal mode number of the applied RMP). This is achieved in EAST plasmas with low input torque and tungsten divertor, and the target plasma for these experiments in EAST is chosen to be relevant to the ITER Q = 10 operational scenario, thus also addressing significant scenario issues for ITER. In these experiments the lowest neutral beam injection (NBI) input torque is around T NBI ∼ 0.44 Nm, which extrapolates to around 14 Nm in ITER (compared to a total torque input of 35 Nm when 33 MW of NBI are used for heating). The q 95 is around 3.6 and normalized plasma beta β N ∼ 1.5–1.8, similar to that in the ITER Q = 10 scenario. Suppression windows in both q 95 and plasma density are observed; in addition, lower plasma rotation is found to be favourabe to access ELM suppression. ELM suppression is maintained with line averaged density up to 60%n GW (Greenwald density limit) by feedforward gas fuelling after suppression is achieved. It is interesting to note that in addition to an upper density, a low density threshold for ELM suppression of 40%n GW is also observed. In these conditions energy confinement does not significantly drop (<10%) during ELM suppression when compared to the ELMy H-mode conditions, which is much better than previous results using low n (n = 1 and 2) RMPs in higher q 95 regimes. In addition, the core plasma tungsten concentration is clearly reduced during ELM suppression demonstrating an effective impurity exhaust. MHD response modelling using the MARS-F code shows that edge magnetic field stochasticity has a peak at q 95 ∼ 3.65 for the odd parity configuration, which is consistent to the observed suppression window around 3.6–3.75. These results expand the physical understanding of ELM suppression and demonstrate the effectiveness of n = 4 RMPs for reliable control ELMs in future ITER high Q plasma scenarios with minimum detrimental effects on plasma confinement.

Bifurcation behaviour of resonant magnetic perturbation control of edge localized modes in tokamaks: nonlinear simulation results

“…In principle, therefore, our two-fluid CUTIE model predicts both the transition, the near-complete mitigation of the type-I ELMs and the existence of a ‘point/discrete spectrum (coherent modes)’ after transitions, which may manifest as observable oscillations near , near the plasma edge. These findings bear a close qualitative resemblance to recent experimental observations [ 21 ] and may have important implications for RMP suppression of ELMs on ITER [ 27 ]. We next turn to the question of how the transitions in the turbulence (both waveform amplitudes and spectral characteristics) affect the global properties such as ‘Density pump out’ and ) post-transition.…”

“…Given structure of the RMP coils, K and K∗ (for any specified n) can be calculated by electrodynamics. In EAST [21], the authors quote 16kAturns in their RMP coils that lead to a few Gauss (i.e. δBrfalse(rmpfalse)/B0≃10−4).…”

“…Thus, we can seamlessly continue from previously calculated fields and turn on (or off) any source/equations and investigate the effects on the evolution; there is no further need for any ‘initial’ data. In the present work, the influence of RMPs with and various field strengths is explored in some detail with a view to explaining several phenomena, including bifurcations observed in recent RMP experiments [ 13 , 21 ]. In the absence of an RMP, the fluctuating part of .…”

“…Issues relating to amplitudes, repetition rates and mode spectra of ELMs remain to be elucidated, at least partly due to numerical limitations of codes operating with realistic conditions. Recent works [10][11][12][13][14][15][16] have tackled the challenge of explaining the mechanism of resonant magnetic perturbation (RMP) control (by mitigation/suppression) of type-I ELMs observed in several experiments [17][18][19][20][21]. The simulations presented in this article were carried out using the Culham Transporter of Ions and Electrons (CUTIE) [8,12,[22][23][24][25] -a two-fluid electromagnetic nonlinear global code.…”

“…From the definitions, it can be verified that ψ rmp has dimensions of length (when K * is a purely numerical parameter), and since the fluctuating fields δB/B 0 are scaled by B 0 (the 'toroidal' field at the magnetic axis), the fluctuating poloidal flux function has dimensions of length. Hence, the boundary value ψ rmp yields the radial RMP field perturbation (at r/a = 1) upon differentiating with θ, the poloidal In EAST [21], the authors quote 16kAturns in their RMP coils that lead to a few Gauss (i.e. δB r (rmp)/B 0 10 −4 ).…”