Abstract
In electron (cyclotron) heated plasmas, in both ASDEX Upgrade (L-mode) and Wendelstein 7-X, clamping of the ion temperature occurs at T i ∼ 1.5 keV independent of magnetic configuration. The ions in such plasmas are heated through the energy exchange power as , which offers a broad ion heating profile, similar to that offered by alpha heating in future thermonuclear fusion reactors. However, the predominant electron heating may put an additional constraint on the ion heat transport, as the ratio T e/T i > 1 can exacerbates ITG/TEM core turbulence. Therefore, in practical terms the strongly ‘stiff’ core transport translates into T i-clamping in electron heated plasmas. Due to this clamping, electron heated L-mode scenarios, with standard gas fueling, in either tokamaks or stellarators may struggle to reach high normalized ion temperature gradients required in a compact fusion reactor. The comparison shows that core heat transport in neoclassically optimized stellarators is driven by the same mechanisms as in tokamaks. The absence of a strong H-mode temperature edge pedestal in stellarators, sofar (which, like in tokamaks, could lift the clamped temperature-gradients in the core), puts a strong requirement on reliable and sustainable core turbulence suppression techniques in stellarators.
Highlights
The alpha particles released in the fusion process will predominantly heat the plasmas in a fusion reactor
Plasmas with electron cyclotron resonance heating (ECRH) feature ion temperature clamping where the central ion temperature does not rise above at Ti ∼ 1.5 keV [1]. This clamping is the result of a combination of several effects: (a) the broad ion heating from power transfer from electrons in ECRH heated plasmas depends on plasma density ne and electron to ion temperature difference as pex
In conditions where (a/Ln ∼ a/LTi), both ion temperature gradient (ITG) and trapped electron mode (TEM) turbulence can be suppressed in W7-X, and be replaced by so-called iTEM turbulence, which drives less transport and may lead to enhance core confinement [5,6,7,8], where the neoclassical transport becomes a more dominant transport mechanism
Summary
The alpha particles released in the fusion process will predominantly heat the plasmas in a fusion reactor. In conditions where (a/Ln ∼ a/LTi), both ITG and TEM turbulence can be suppressed in W7-X, and be replaced by so-called iTEM turbulence, which drives less transport and may lead to enhance core confinement [5,6,7,8], where the (ion) neoclassical transport becomes a more dominant transport mechanism These conditions were achieved in W7-X after a train of hydrogen ice pellets produced a peaked density profile. We investigate: (1) the broad indirect ion heating by the power exchange profile, (2) the strong ion temperature dependence of the microturbulence related gyroBohm transport and (3) the exacerbation of the (ITG and TEM) turbulent transport due to an increased ratio of Te/Ti in ECRH plasmas. Mitigation the effect of the enhanced turbulent transport is discussed and is found either by providing an H-mode edge barrier or by core turbulence mitigating techniques
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