Reduced core transport in T-10 and TEXTOR discharges at rational surfaces with low magnetic shear

Abstract

It has been observed in the T-10 tokamak that immediately after off-axis electron cyclotron resonance heating (ECRH) switch-off, the core electron temperature stays constant for some time, which can be as long as several tens of milliseconds, i.e. several energy confinement times (τ E), before it starts to decrease. Whether or not the effect is observed depends critically on the local magnetic shear in the vicinity of the q = 1 rational surface, which should be close to zero. It is hypothesized that a small shear can induce the formation of an internal transport barrier. Measurements of density fluctuations in the transport barrier with a correlation reflectometer show immediately after the ECRH switch-off a clear reduction in the fluctuation level, corroborating the above results. The delayed temperature decrease has also been observed in similar discharges in the TEXTOR tokamak; however, the delay is restricted to ∼1×τ E .

Explanation of Experimentally Observed Phenomena in Hot Tokamak Plasmas from the Nonequilibrium Thermodynamics Position

“…The safety factor q characterizes a helicity of magnetic field lines, / q m n = . This result was found in experiments on many tokamaks: TCV [17], T-10 [18][19][20], and JET [21][22][23].…”

Observation of turbulence suppression after electron-cyclotron-resonance-heating switch-off on the HL-2A tokamak

“…If one fully understands the role of inverse cascade of energy associated with this process, one can exert some control of this process (such as ECCD or high-power central ECRH plus offaxis ECRH profile control) to direct the energy transition for the best effects or a steady-state barrier rather than a transient one as discussed here. Although there are several plausible mechanisms relating to the observed phenomena, such as the central low-frequency zonal flow, nonlocal transport, plasma rotation, or magnetic shear [5][6][7]18,19], the underlying physics responsible for the formations of these structures are still not clear. More detailed studies on the three-dimensional structure of the observed low-frequency mode and its nonlinear coupling with high-frequency turbulence are necessary and will be the subject of further investigations.…”

“…Many experiments have been performed on various devices to clarify the formation of the transport barrier with reduced transport triggered by localized heating and fueling, such as pellet injection, supersonic molecular beam injection (SMBI), high-Z impurity injection, and electron cyclotron resonance heating (ECRH) [1][2][3][4][5][6][7]. There are two types of explanations relating to these phenomena [5][6][7]. One is relevant to the current profile, the magnetic shear or plasma rotation.…”

“…The other one is relevant to the so-called nonlocal effects, for which theories have not been well developed so far. In the off-axis ECRH experiments, observations in T-10 and TEXTOR suggest that the necessary condition to reduce the core transport after ECRH switch-off is relevant to the appearance of the low value of dq/dρ near rational surfaces [4,5]. A large increase on the central electron temperature is induced by off-axis ECRH in DIII-D, which was modeled in terms of a significant heat pinch and suppression of heat diffusivity [3].…”

“…A large increase on the central electron temperature is induced by off-axis ECRH in DIII-D, which was modeled in terms of a significant heat pinch and suppression of heat diffusivity [3]. However, previous off-axis ECRH experiments were mostly carried out around r/a ∼0.4 [4][5][6], and the core turbulence study is not sufficient. The links among the improved confinement, the ECRH power deposition, and the core turbulence are still unclear.…”