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• W2247649757 abstract "The maximum beta which can be sustained for a long pulse in ITER-shaped plasmas in DIII–D with q95 >~ 3, ELMs, and sawteeth is found to be limited by resistive tearing modes, particularly m/n = 3/2 and 2/1. At low collisionality comparable to that which will occur in ITER, the beta limit is a factor of two below the usually expected n = ∞ ballooning and n = 1 kink ideal limits. Successful steady-state tokamak operation requires operating at the highest possible beta while avoiding both ideal and resistive MHD instabilities which reduce confinement and induce disruption. Experimental results from a large number of tokamaks indicate that the high beta operational envelope of the tokamak is well defined by ideal magnetohydrodynamic (MHD) theory [1] and is given by β (%) <~ 4liI/aB MA/m/T for a large range of conditions. The maximum beta values experimentally obtained, consistent with the ideal limit, are more than sufficient for the goals of long pulse burning experiments, such as ITER. The highest beta values achieved have historically been obtained in fairly short pulse discharges, often <1–2 sawteeth periods and <1–2 energy replacement times. In these discharges, the current profile is not fully relaxed. It is well recognized that the ideal limit depends on the details of the current density profile and pressure profile and it is expected that the bootstrap current from the pressure gradient at high beta can lead to lower MHD stability limits. Furthermore, in some previous experiments, the instabilities limiting the achievable beta are very clearly pressure driven resistive modes and significantly below the threshold predicted for ideal instabilities [2]. It is of interest to determine the maximum beta in discharges of sufficient length to have fully penetrated profiles and for opportunities for resistive modes to play a role. The maximum operational beta in single-null divertor (SND), long-pulse discharges in DIII–D with a cross-sectional shape similar to the proposed ITER tokamak (Fig. 1) is found to be limited significantly below the threshold for ideal instabilities by the onset of resistive MHD instabilities. [A hard disruptive beta limit is usually considered to be due to ideal MHD instabilities, either the n=1 kink or the n=∞ ballooning mode where n is the toroidal mode number.] The temporal evolution of a typical discharge is shown in Fig. 2; the beam power is increased gradually. There is a “soft” beta limit due to the onset of an m/n = 3/2 rotating tearing mode which saturates at an amplitude that decreases energy confinement by ∆τE/τE ≈ –20% [Fig. 2(b,c)] and a “hard” beta limit at slightly higher beta due to the onset of an m/n = 2/1 rotating tearing mode which grows to an amplitude that destroys the confinement and induces a disruption [Fig. 2(b,d)]. (Plasmas are neutral beam heated ELMing H–mode with sawteeth; the safety factor q95 is just above 3.) Higher stable beta in these long-pulse discharges is successfully run by operating at either higher density n and/or lower field and thus higher collisionality which suppresses both the 3/2 and 2/1 mode onsets. By long pulse, we mean that beta is evolving on a time scale long compared to the ELM and sawteeth periods and the energy replacement time τE. At fixed field, density n is the control parameter varied PRACTICAL BETA LIMIT IN ITER-SHAPED DISCHARGES IN DIII–D AND ITS INCREASE BY HIGHER COLLISIONALITY R.J. La Haye, et al. 2 GENERAL ATOMICS REPORT GA–A22427 –0.5" @default.
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• W2247649757 title "PRACTICAL BETA LIMIT IN ITER-SHAPED DISCHARGES IN DIII-D AND ITS INCREASE BY HIGHER COLLISIONALITY" @default.
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