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The DIII-D tokamak has developed a new regime for high-beta hybrid plasmas where the broad current profile is achieved with strong off-axis electron cyclotron current drive (ECCD) rather than anomalous poloidal magnetic flux pumping. The high-beta hybrid regime with $q_{min}$ slightly above 1 and without sawteeth is a candidate for the $Q=5$ steady-state scenario on ITER$^{1-3}$, but the anomalous flux pumping mechanism that maintains $q_{min}>1$ despite strong central current drive is not yet understood$^4$. Experiments on DIII-D have found that high performance with $\beta_N=3.7$ and $H_{98y2}=1.6$ is maintained (Fig. 1) in high-density hybrids when 3.4 MW of ECCD is moved from $\rho<0.2$ to $\rho\sim 0.5$. The good agreement between the experimental $q_{min}$ evolution and TRANSP simulations (Fig. 2) differs from the usual hybrid situation where the simulation predicts $q_{min}<1$ but experimentally $q_{min}>1$, showing there is no evidence for anomalous flux pumping in this new hybrid regime with off-axis ECCD. Transport analysis finds higher density leads to weaker Alfven eigenmode (AE) activity (Fig. 3) and lowers the electron thermal diffusion, but there is little change to thermal transport in moving ECCD from on-axis to off-axis.
A reproducible high-beta hybrid regime has been developed on DIII-D$^{1-3}$ with stationary performance as high as $\beta_N=3.7$ and $H_{98y2}=1.6$ using 11.2 MW of NBI and 3.4 MW of co-ECCD (Fig. 1), with a total injected energy of up to 56 MJ. Stationary hybrid plasmas always exhibit a $m=3/n=2$ or higher order mode. While these discharges are fully non-inductive at moderate densities ($\sim4\times 10^{19}\mbox{ m}^{-3}$), at higher densities (up to $6.1\times 10^{19}\mbox{ m}^{-3}$) this long-duration regime is well suited for radiative divertor studies with the ECCD aimed at $\rho\sim 0.5$ to avoid the right hand density cutoff$^5$. Off-axis ECCD results in a broader current profile than on-axis ECCD, as measured by motional Stark effect (MSE) polarimetry. In these experiments the six gyrotrons use identical aiming, giving very localized ECCD profiles. Perhaps because of this, deleterious $m=2/n=1$ modes are destabilized when the ECCD location is between $\rho=0.25–0.40$, which corresponds to the region of the $q=2$ surface. Furthermore, for off-axis deposition, radial ECH injection is found to be more stable to $m=3/n=1$ mode onset than current drive injection.
While hybrids with strong off-axis ECCD have a broad current profile with $q_{min}>1$, this behavior is not anomalous and there is no evidence of poloidal magnetic flux pumping. It is well established that hybrid plasmas with on-axis current drive have anomalously broad current profiles with a measured $q_{min}$ well above simulations of the current profile evolution from transport codes like TRANSP$^{2-3}$. When the ECCD is moved off-axis to $\rho\sim 0.5$, the measured $q_{min}$ value increases (as expected for off-axis current drive) and TRANSP simulations of the expected $q_{min}$ evolution are in reasonable agreement with the minimum safety factor from MSE-constrained equilibrium reconstructions (Fig. 2). The measured and simulated loop voltage profiles are also in good agreement, with central values around 50 mV and edge values near zero; the peaked loop voltage profile shows that the current profile is continuing to broaden with time. (While the loop voltage profile rests at zero for hybrids with $n_e\sim 4\times 10^{19}\mbox{ m}^{-3}$ and on-axis ECCD, these hybrids with $n_e\sim 6\times 10^{19}\mbox{ m}^{-3}$ and lower off-axis ECCD efficiency retain positive loop voltage.) While steady-state hybrids with on-axis ECCD and $q_{min}\sim 1.1$ generate strong fishbones, the absence of the fishbone instability for hybrids with off-axis ECCD confirms the higher $q_{min}$ values ($\sim 1.5$).
An additional advantage of off-axis ECCD in hybrids is that higher density plasmas can be investigated without encountering the right hand density cutoff, which increases the confinement time and allows higher $\beta_N$ to be achieved due to lower electron thermal transport and reduced AE activity. Moving the ECCD deposition from on-axis to off-axis by itself has little measurable effect on local thermal transport, although the smaller electron temperature gradient inside the ECCD deposition zone does decrease the global confinement time. By raising the density, however, high confinement and high $\beta_N$ can be recovered for off-axis ECCD. About half of the confinement improvement at higher density is due to ~30% lower electron thermal transport. The remaining confinement improvement at higher density is due to reduced beam ion transport. In TRANSP simulations that adjusted an anomalous beam ion diffusion coefficient to match the experimental neutron rate, $D_{beam}$ decreased from $\sim 1.5\mbox{ m}^2/\mbox{s}$ to less than $1.0\mbox{ m}^2/\mbox{s}$ with higher density (Fig. 3). While high $D_{beam}$ does not directly affect thermal transport, it reduces the neutral beam heating effectiveness and thus lowers the heat flux and temperature gradients. The lower $D_{beam}$ at higher density is consistent with the weaker AE activity observed on the density interferometer. It is also found in these hybrid plasmas that particle transport, but not thermal transport, is strongly affected by the onset of a $m=3/n=1$ mode, such that higher and broader density profiles can be achieved only in the absence of the $m=3/n=1$ mode.
Since the mechanism of poloidal magnetic flux pumping in hybrid plasmas is still under investigation, there is interest in developing an ITER scenario with comparable performance that does not rely on anomalous flux pumping to maintain the target safety factor profile. The 'classical' behavior of the current profile with off-axis ECCD reported here demonstrates the existence of a new hybrid regime that does not rely on anomalous flux pumping to maintain $q_{min}>1$. It is interesting to note that this regime has similar characteristics to experiments in the "high $q_{min}$" regime with broad off-axis ECCD between $\rho\approx 0.3–0.6$ in which $q_{min}$ dropped to $\approx 1.4$, although the $m=3/n=2$ mode that seems important in hybrids was not present in these "high $q_{min}$" plasmas$^6$. This indicates that different paths (starting from low or high $q_{min}$) can be taken to access this regime.
This material is based upon work supported by the Department of Energy under Award Number(s) DE-FC02-04ER54698, DE-FG02-04ER54761, and DE-AC52-07NA27344.
$^1$F. Turco, et al., Phys. Plasmas 22 (2015) 056113.
$^2$C.C. Petty, et al., Nucl. Fusion 56 (2016) 016016.
$^3$C.C. Petty, et al., Nucl. Fusion 57 (2017) 116057.
$^4$C.C. Petty, et al., Phys. Rev. Lett. 102 (2009) 045005.
$^5$T.W. Petrie, et al., Nucl. Fusion 57 (2017) 086004.
$^6$T.C. Holcomb, et al., Nucl. Fusion 54 (2014) 093009.
| Affiliation | General Atomics |
|---|---|
| Country or International Organization | United States |
