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Recent experiments in the DIII-D tokamak have shown that a broadened fast-ion pressure profile enables better control of Alfvén Eigenmodes (AEs), improves fast-ion confinement, and allows access to new regimes. New discharges reach 15% higher normalized plasma beta ($\beta_N$) than previously achieved in steady-state scenarios with negative central shear and $q_{min}>2$ at high field ($B_T=2.0~T$) and $q_{95}=6.0$. Reverse shear, $q_{min}>2$ scenarios are attractive candidates for fully non-inductive tokamak operation and are among those envisioned for compact fusion power plants, with high normalized beta limits, elevated confinement, and avoidance of low-order tearing modes. One potential drawback of these scenarios, however, is their susceptibility to AE-induced fast-ion transport which has been shown to significantly reduce performance, resulting in measured neutron rates that are typically half of the classically expected values {1}. Understanding regimes in which AE control strategies work and charting a path to improved fast-ion confinement is important for ITER and essential for optimization of Advanced Tokamak relevant scenarios.
Experiments show that Reversed Shear Alfvén Eigenmodes (RSAEs) can be reduced using DIII-D’s recently upgraded off-axis beams, which have the effect of both reducing the fast-ion pressure gradient at $q_{min}$ and altering the q-profile while maintaining $q_{min}>2$. In the current ramp (t<2.5 sec), replacing on-axis beams with new off-axis beams at equivalent total power resulted in fewer AEs, with ~24% higher ratio of measured neutrons to calculated classical neutrons (Fig. 1c, blue), and ~8% higher neutron ratio in the flattop. The neutron fraction in the current ramp was further improved using off-axis beams along with Electron Cyclotron Current Drive (ECCD) aimed on-axis (instead of mid radius), with ~36% higher neutron ratio than the reference shot (Fig. 1c, red).
Analysis suggests RSAEs were reduced in these discharges because ECCD moved the $q_{min}$ location ($\rho_{qmin}$) inward to a location of reduced beam pressure gradient and higher plasma pressure (Fig. 2). In general, AEs can be suppressed by manipulating equilibrium profiles to alter or remove eigenmodes, increase mode damping by changing background plasma parameters and profiles, or decreasing the fast-ion drive by reducing geometric or velocity-space gradients. The reduced beam pressure gradient reduces drive for the modes, and increased thermal pressure/gradient has been associated with increased RSAE continuum damping and even removing the eigenmode altogether {2}.
![Neutron ratio from Fig. [1] shows improvement (green to red) when $\rho_{qmin}$ was located at lower beam pressure gradient (reduces RSAE drive) and increased pressure (increases RSAE damping).](https://fusion.gat.com/conference/event/104/attachments/161/1581/Collins.Cami.IAEA2020.Fig2.png)
The fast ion confinement improved as the maximum in the classical beam pressure gradient was decreased (Fig. 3a). In previous experiments with predominantly on-axis beam power, AE activity was altered by transiently increasing the density, broadening the density profile with injected deuterium pellets, radially scanning electron cyclotron current drive or heating, or reducing neutral beam voltage. Some of these techniques resulted in decreased performance as well as inability to maintain steady-state. Recent experiments repeated these AE control methods but increased the off-axis beam power fraction from 30% to 70% using DIII-D’s upgraded beams, enabling operation at decreased beam pressure gradient and ~25% increase in fast ion confinement while maintaining the high-qmin steady-state scenario.
Record parameters were achieved for this scenario at high-field ($B_T=2.0~T$) and $q_{95}=6.0$ (Fig. 3b). The highest performing discharges were produced at relatively higher flattop density using pre-programmed, early beam and ECCD heating and density feedback control in order to raise the plasma temperature early and slow the inward diffusion of poloidal flux to maintain elevated $q_{min}$. Improvements to fast-ion confinement in the current ramp made beam heating more efficient and allowed access to higher flattop density while still maintaining $q_{min}>2$ and reverse shear.
These experiments mark significant progress in advancing the high-qmin scenario and increasing understanding of AE control. This advances our ability to determine how proper plasma parameters and current profile in alpha-dominated plasmas can avoid AE-induced fast-ion redistribution, loss, reduced heating efficiency, and limits to the achievable $β_N$. This is important to enable ITER to reach its peak performance goals, as well as for optimization of the Advanced Tokamak approach to fusion energy.
This work was supported in part by the US Department of Energy under DE-FC02-04ER54698, DE-FG03-94ER54271, and DE-AC02-09CH11466.
{1} C.T. Holcomb, et.al., Physics of Plasmas 23, 062511 (2016)
{2} M.A. Van Zeeland, et al., Nucl. Fusion 56, (2016) 112007.
| Affiliation | General Atomics |
|---|---|
| Country or International Organization | United States |
