Experimental confirmation of efficient island divertor operation and successful neoclassical transport optimization in Wendelstein 7-X

Abstract

We present recent highlights from the most recent operation phases of Wendelstein 7-X, the most advanced stellarator in the world. Stable detachment with good particle exhaust, low impurity content, and energy confinement times exceeding 100 ms, have been maintained for tens of seconds. Pellet fueling allows for plasma phases with reduced ITG turbulence, and during such phases, the overall confinement is so good (energy confinement times often exceeding 200 ms) that the attained density and temperature profiles would not have been possible in less optimized devices, since they would have had neoclassical transport losses exceeding the heating applied in W7-X. This provides proof that the reduction of neoclassical transport through magnetic field optimization is successful. W7-X plasmas generally show good impurity screening and high plasma purity, but there is evidence of longer impurity confinement times during turbulence-suppressed phases.

Existence of global symmetries of divergence-free fields with first integrals

“…It is not known whether symmetry and first integrals are essential for good confinement; chaotic fields are in some scenarios desirable for the exhaust of heat and deleterious impurities [29]. In the face of Grad's conjecture, if symmetry and first integrals are mathematically impossible beyond Euclidean symmetry, the flavour of the question changes to how "good" or "close" fields are to "useful" or "approximate" symmetry.…”

D-Mag — a laboratory for studying plasma physics and diagnostics in strong magnetic fields

“…Electrical access to the magnet is provided on the cryostat service neck, where two LHe level probes and a carbon resistance temperature sensor (Allen-Bradley resistor) are mounted. The latter is monitored by a digital multimeter (Agilent 34410 A 6 1 2 ) and the LHe and LN levels are read out by two level meters (OXFORD ILM 211). The MAGNABOND [10,11] system makes the magnet physically and cryogenically stable under the considerable Lorentz force generated during operation.…”

Validation of theory-based models for the control of plasma currents in W7-X divertor plasmas

“…The device is being upgraded with actively cooled plasma-facing components to sustain 10 MW-class heating power for up to 30 min. Ultimately, the forthcoming campaigns aim at a power throughput of 18 GWs [7].…”

“…As indicated in figure 2, electron-root conditions show up for even wider ranges when the magnetic configuration is altered to the high-iota divertor configuration (ι -= 5/4), or if a higher toroidal mirror is introduced (high-mirror data) or by scanning the rotational transform and in a scan from ι -= 5/5 → 5/4 [22,23]. On the other hand, steady-state compatible detached plasmas have been attained [7,11,24] already but are operating at fairly high collisionalities. And even the best plasma performance (red triangle in figure 2) in W7-X [25,26] is far from the performance targets.…”

“…The gap to reactor conditions did not come unexpectedly: W7-X is not yet equipped with the heating power to obtain the required increase of β at low ν * and T e ≈ T i at high density. Heating upgrades are underway [7].…”