Speaker
Description
Most of high-performance discharges based on the advanced scenario have shown the active generation of the Alfvén eigenmodes (AE), driven by enhanced fast-ion pressure gradient and broad current density profile in the core region$^{1,2}$.
Among various AE control tools, it has been found that the ECCD and ECH are able to mitigate or suppress the toroidal Alfvén eigenmodes (TAE) and the reversed-shear Alfvén eigenmodes (RSAE) in DIII-D$^{2-4}$, ASDEX-Upgrade$^{5,6}$ and KSTAR tokamaks$^{7}$ as well as helical devices such as TJ-II$^{8}$ and LHD$^{9}$. ECCD tailors the current density profile near or inside the location of the TAEs so that continuum damping is enhanced effectively. This research shows the feasibility of the active control of TAE using the electron cyclotron current drive (ECCD) and the heating (ECH) in the high-performance discharges.
Previous work has shown the co-current directional ECCD mitigates and suppresses the TAEs in the elevated $q_0$ discharge$^{7}$, leading to the increase in $\beta_N$, $\beta_P$ and the reduction of neutron deficit. In addition to co-ECCD applications, the effect of counter-current directional ECCD was investigated. Experiments show that both co- and counter-current directional ECCD are able to mitigate and suppress the TAE activities while the ECCD deposition location stays in the specific range (–12 cm < $Z_{EC}$ < +3 cm), as shown in figures 1 and 2. Moreover, co-ECCD shows TAE mitigation effects in a wider deposition range than the counter-ECCD. It is found that the co-ECCD is more useful to enhance the overall performance, associated with TAE mitigations. Plasma performance in the co-ECCD application is higher than in the counter-ECCD (co-ECCD: $\beta_N$ ~ 2.4, $\beta_P$ ~ 2.1, neutron ~ 920 (a.u.); counter-ECCD: $\beta_N$ ~ 2.1, $\beta_P$ ~ 1.8, neutron ~ 830 (a.u.)). Maximum plasma performance in the TAE mitigation stage seems to be associated with the elevated plasma pressure (or $\beta_N$, $\beta_P$), change in internal inductance ($l_i$) and $q_0$, which affect the effective ECCD deposition window. Radial locations of both co- and counter-ECCD deposition affect the major damping channels such as the continuum damping (magnetic shear)$^{10}$ and the thermal ion Landau damping (central electron and ion temperatures). In particular, NOVA-k$^{11}$ modeling indicates that continuum damping is enhanced more in case of co-ECCD by increasing core magnetic shear. Total damping rate in the co-ECCD case is higher than the case of counter-ECCD, hence the TAE amplitude in the initial stage of counter-ECCD is higher than in the co-ECCD application. Radiative damping seems to be negligible because the observed TAEs are the low-n modes (n < 6). Another stabilizing effect by enhancing the plasma pressure is expected in both co- and counter-ECCD, which is beneficial to mitigate TAEs by making the normalized pressure gradient larger than the critical level$^{12,13}$.
To extend the operational space of the ECCD-assisted TAE control experiment, empirical scanning in the parameter space, represented by the central safety factor ($q_0$)$^{14}$, core $T_e$ (or fast-ion slowing-down time) and the internal inductance ($l_i$), has been carried out. Based on the parameter scan, experimental condition with high $q_0$, low $l_i$ and high $T_e$ is prone to excite the TAEs in the core due to reduction of continuum and beam-ion damping rates and the enhanced fast-ion pressure gradient (drive). From this parametric study, mild off-axis deposition of ECCD ($\rho_{pol,ECCD}$ ~ 0.15 – 0.2 for high $\beta_{P}$ scenario, $\rho_{pol,ECCD}$ ~ 0.25 – 0.35 for high $q_{min}$ scenario) is quite beneficial to reduce or suppress the core TAE activities, hence the plasma performance is elevated significantly in the advanced scenarios. Experimental database provides the best option for ECCD/ECH-assisted active TAE control in the variety of advanced scenarios. Co-ECCD has shown TAE mitigation and suppression with elevated performance, however, additional consideration for keeping high $q_{min}$ in the TAE control phase should be focused on the optimization of counter-ECCD application as demonstrated in shot# 22937 (figure 2), which shows suppression of n = 3, 4 TAEs under the conditions of high $q_0$ ~ 1.8 and m / n = 2/1 tearing-mode avoidance.
References
$^{1}$ Holcomb C.T. et al., 2015 Phys. Plasmas 22 055904
$^{2}$ Heidbrink W.W. et al., 2014 Plasma Phys. Control. Fusion 56 095030
$^{3}$ Van Zeeland M.A. et al., 2009 Nucl. Fusion 49 065003
$^{4}$ Kramer G.J. et al., 2017 Nucl. Fusion 57 056024
$^{5}$ Sharapov S.E. et al., 2018 Plasma Phys. Control. Fusion 60 014026
$^{6}$ Garcia-Munoz M. et al., 2019 Plasma Phys. Control. Fusion 61 054007
$^{7}$ Kim J. et al., 2019, 16th IAEA Technical Meeting on Energetic Particle Physics (Shizuoka-City, Japan), ID 49.
$^{8}$ Cappa A. et al., 2019, 16th IAEA Technical Meeting on Energetic Particle Physics (Shizuoka-City, Japan), ID 58.
$^{9}$ Nagaoka K. et al., 2013 Nucl. Fusion 53 072004
$^{10}$ Zonca F. and Chen L. 1993 Phys. Fluids B 5 3668
$^{11}$ Cheng C.Z. 1992 Phys. Rep. 211 1
$^{12}$ Fu G.Y. 1995 Phys. Plasmas 2 1029
$^{13}$ Sharapov S.E. et al., 1999 Nucl. Fusion 39 373
$^{14}$ Strait E. et al., 1993 Nucl. Fusion 33 1849
| Affiliation | National Fusion Research Institute |
|---|---|
| Country or International Organization | Korea, Republic of |
