25aA06
Resistive Wall Mode Control of High Beta Plasmas in the DIII-D Device*
Michio Okabayashi for DIII-D RWM study group
Princeton Plasma Physics laboratory, Princeton University, Princeton NJ, USAA steady-state power plant with a high fusion power density, utilizing a high fraction ofthe bootstrap current, is an important goal of tokamak fusion research. This self-sustaining
tokamak configuration requires the suppression of long wavelength ideal external kink. It wasexperimentally demonstrated that the vulnerability to external kink can be overcome by plasma
rotation and/or magnetic feedback with non-axisymmetric external coils assisted by the presenceof a nearby conducting wall with low resistivity which converts the external kinks into theresistive wall mode (RWM) branch. It was also shown that the external coils stabilize theseexternal kinks by a combination of sustained high plasma rotation frequency (Ω > Ωcrit) withdynamic error field correction and direct feedback process.
New internal control coils (I-coils) have been installed inside the vacuum vessel locatedabove and below the midplane. These I-coils will allow us to compare the performance andimportance of internal coils relative to the original external coils and to eventually develop arobust and economical control scheme for the RWM aimed at the practical application tocommercially-oriented reactors.
Advantages of the I-coils are their proximity to the plasma surface and the more flexible
poloidal field structure — providing a better coupling to the RWM. Feedback using I-coils
largely eliminates the time delay due to the finite impedance of the resistive wall, resulting inoverall faster feedback time response. According to the VALEN code, feedback utilizing theinternal coils can stabilize the plasma β_N up to C_β ≈ 0.9 in the absence of plasma rotation,
almost to the ideal wall beta limit [C_ β is a measure of achievable beta above no-wall limit
defined as (β - β_no-wall) /( β_ideal wall _ β_no-wall)], whereas for the case of feedback withthe external coils, the achievable beta value is predicted to be limited to C_ β ≈ 0.5. Feedback
with the I-coils is found to be indeed more efficient compared to feedback with the external coils.
Similar conclusion is also affirmed by the MARS code without plasma rotation.
In the experiments near the critical velocity, without feedback a discharge was terminatedaround C_β ≈ 0.2 while increasing the plasma pressure. With a proper choice of feedback gains
using internal coils, the plasma β_N is raised to C_β ≈ 0.8. Feedback using external coils also
allows the plasma to reach higher pressure, however, up to the reduced range of C_β ≈ 0.4-0.5. Itshould be cautioned that the feedback reduces the error field component and increase the plasma
rotation, providing an additional stabilization effect in these discharges. Thus, this improvement
is to be considered due to the combination of rotational stabilization and direct feedback. By
using the experimental technique of resonant magnetic braking, the plasma rotation has been
reduced to nearly zero outside the q=2 region. Nonetheless, feedback with the internal coils cansustain the plasma to C_β ≈ 0.4 above the no_wall limit for more than 100 ms (which is morethan 20 times the wall time constant of 5 ms). We plan to upgrade the speed of the amplifiers andcontrol system, which should allow us to get closer to the theoretical prediction
The implication for RWM stabilization in ITER has been studied using the DIII-D
experimental rotational profiles applied to the ITER-AT equilibrium. The critical rotation
velocity Ωcrit /ΩA is 5% at the magnetic axis, suggesting that the high beta operation regime
could be severely limited without the use of an auxiliary momentum input system or direct
magnetic feedback stabilization.
*Supported by the U.S. Department of Energy under Contract Nos. DE-AC03-99ER54463, DE-AC02-76CH03073, W-7405-ENG-48, DE-AC05-00OR22725, andGrant No. DE-FG02-89ER52397.