Description
The WEST tokamak provides key insights into long pulse operation in a full tungsten environment, anticipating challenges for ITER-like reactors. WEST Lower Hybrid Current Drive (LHCD) enables fully non-inductive regimes and has sustained plasmas for up to 22 minutes. However, its off-axis power and current deposition constrain the operational space in plasma current, density, and injected power. In particular, insufficient central heating can prevent tungsten burn-through and increase the risk of core radiative collapses when radiation losses exceed absorbed power [1] and reduced on-axis current drive may lead to safety factor reversal and degraded MHD stability [2].
A new ECRH system [3] is currently under commissioning to extend this operational domain, with the objective of reaching up to 3 MW for 10 s. The impact of ECRH/ECCD on WEST operation is assessed [4], using the validated integrated modeling IMAS-compatible HFPS framework [5], now including self-consistent EC wave predictions with TORBEAM [6], and complemented by standalone radiofrequency heating, turbulent transport and MHD analyses.
Simulations show that even at modest injected power ($P_{ECRH}=0.3$ MW), on-axis ECRH increases the accessible density range up to $n_{el}=7\cdot10^{19}$ $\mathrm{m^{-3}}$, ensuring central heating exceeds core W radiative losses (within $\rho<0.3$, $P_{rad}/P_{RF} < 0.3$). Central ECCD shifts LHCD profiles on-axis, while off-axis ECCD drives them further outward, modifying the total current and power density profiles. For comparable total power, adding on-axis ECCD to LHCD-only scenario results in a ~10% increase in energy confinement time, with similar overall current-drive efficiency. At the same time, ECCD substantially improves MHD robustness with a monotonous q-profile down to $q_{axis} = 1$. Conversely, off-axis ECCD enables accessing reversed shear configurations with reduced core turbulence but reduced stability margins. A potentially robust configuration is identified for $q_\min > 2$, although it requires precise control of the q-profile, from the ramp-up up to steady-state phase. These results provide quantitative guidance for upcoming combined LHCD/ECRH long pulse experiments on WEST.
[1] V. Ostuni et al., Nucl. Fusion, 62, 106034 (2022).
[2] P. Maget et al., Nucl. Fusion, 45, 69 (2005).
[3] L. Delpech et al., Fusion Engineering and Design, 186, 113360 (2023).
[4] T. Fonghetti, et al., Submitted to Nucl. Fusion (2025).
[5] T. Fonghetti et al., Nucl. Fusion, 65, 056018 (2025).
[6] E. Poli et al., Comp. Phys. Comm., 225, 36 (2018).