Operational space for Lower Hybrid scenarios in the full tungsten environment of WEST

WEST (tungsten –W– Environment in Steady-state Tokamak) is a device specialized for long pulse operation in a tungsten environment. Its main purpose is to develop reactor compatible long pulse scenarios and to study plasma-wall interactions in a metallic environment [1, 2, 3]. In WEST, we use routinely Lower Hybrid Current Drive (LHCD) systems for plasma heating and current drive. During plasma operation, we need to consider three main limits to achieve robust steady-state LHCD scenarios: (i) the power reflected back to the antennas needs to be below a specified threshold, (ii) fast electron ripple losses must be limited, (iii) power density needs to be large enough to increase central electron temperature (T_e0) above 3 keV and avoid radiative collapses [4]. This value is required because at this temperature we are beyond the tungsten radiation peak (located at 1.5 keV). Above 3 keV the tungsten-cooling factor is lower and decreases slowly with T_e.

To determine the operational space for LHCD scenarios we use data from an entire WEST campaign. Using statistical analyses, we define three boundaries corresponding to the three limits listed above. The boundaries are defined for controllable parameters; mainly LHCD injected power and central line integrated density. Moreover, we focus on the LHCD power ramp-up because this is a critical phase where T_e0 needs to increase rapidly to cross the tungsten radiation peak. To achieve successfully this phase we find that LHCD power over density and plasma-antenna distance play a major role. The operational space identified and the method used in this study give important guidelines that will help to improve LHCD operation in tungsten environment tokamaks.

[1] J. Bucalossi et al. 2014, Fusion Eng. Des. 89 907

[2] J. Bucalossi et al. 2021, Nucl. Fusion 62 042007

[3] C. Bourdelle et al. 2015, Nucl. Fusion 55 063017

[4] V. Ostuni et al. 2022, submitted to Nucl. Fusion