Initial results from boron powder injection experiments in WEST lower single null L-mode plasmas

Abstract

Using a recently installed impurity powder dropper (IPD), boron powder (< 150 μm) was injected into lower single null (LSN) L-mode discharges in WEST. IPDs possibly enable real-time wall conditioning of the plasma-facing components and may help to facilitate H-mode access in the full-tungsten environment of WEST. The discharges in this experiment featured Ip = 0.5 MA, BT = 3.7 T, q95 = 4.3, tpulse = 12–30 s, ne,0 ~ 4×1019 m-2, and PLHCD ~ 4.5 MW. Estimates of the deuterium and impurity particle fluxes, derived from a combination of visible spectroscopy measurements and their corresponding S/XB coefficients, showed decreases of ~ 50% in O+, N+, and C+ populations during powder injection and a moderate reduction of these low-Z impurities (~ 50%) and W (~ 10%) in the discharges that followed powder injection. Along with the improved wall conditions, WEST discharges with B powder injection observed improved confinement, as the stored energy WMHD, neutron rate, and electron temperature Te increased significantly (10–25% for WMHD and 60–200% for the neutron rate) at constant input power. These increases in confinement scale up with the powder drop rate and are likely due to the suppression of ion temperature gradient (ITG) turbulence from changes in Zeff and/or modifications to the electron density profile.

A reduced-turbulence regime in the Large Helical Device upon injection of low-Z materials powders

“…The most probable cause for the improvement was judged to be the suppression of ion temperature gradient (ITG) turbulence driven by the modification of the plasma density profile and the increase of the effective charge Z eff . Indeed, this mechanism has been proposed to explain the confinement improvement previously observed in tokamaks upon impurity injection [4][5][6][7][8], and recently obtained even using impurity powder injection [9]. A similar improvement was also obtained transiently with pulsed powder injection in the W7-X stellarator [10].…”

“…Most possibly the same mechanism is at play, and steady-state improvement of confinement would be possible in W7-X as well with a continuous powder flow like the one provided by the IPD. Also, improvement of electron temperature upon B powder injection with the IPD has been observed in the WEST tokamak L-mode plasma as well [9], possibly caused by the same mechanism as in LHD.…”

“…Also, no toxic gas such as diborane B 2 H 6 is used. This technique has been tested in several tokamaks [9,[12][13][14][15] where in general positive wall conditioning effects have been shown upon injection of B or BN (boron nitride) powder.…”

Interpretive modelling of boron transport in the boundary plasma of WEST experiments with the impurity powder dropper

“…In every simulation, B injection resulted in a 2-10 times decrease of the peak T OSP e , which was not consistent with the experimental results. In the experiment, B powder injection had no effect on Ts, as shown in [6] and in figure 12. Additionally, n e at the lower divertor increased in the simulations while the experimental measurements slightly decreased [6], further suggesting the reduction in recycling observed in experiment as a probable cause for the discrepancy.…”

“…A device named the IPD was developed by the Princeton Plasma Physics Laboratory [5] to test this real-time wall conditioning technique. Results of the first deployment of the IPD on WEST are detailed in [6]. Time traces of line-averaged n e , input LH heating power P total LH , total radiated power P total rad and O-II visible line spectroscopy measurements are given in figure 1.…”