Session BO4: NSTX and Spherical Torus
9:30 AM–12:30 PM, Monday, November 14, 2011
Room: Ballroom E
Chair: Ted Biewer, Oak Ridge National Laboratory
Abstract ID: BAPS.2011.DPP.BO4.10
Abstract: BO4.00010 : NSTX operation with reduced gas fueling
11:18 AM–11:30 AM
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Abstract 
Author:
Dennis Mueller
(Princeton University)
The National Spherical Torus Experiment, NSTX, has successfully produced shaped plasmas that transition to H-Mode early and which have long plasma current, I$_{p}$, flattop durations. These plasmas have low internal inductance, l$_{i}$, and the density rises throughout the discharge. Presently, the successful discharges in NSTX have used gas puffing or a high-recycling wall to obtain good, stable, plasmas free from deleterious MHD. For the NSTX upgrade, it is important to have lower-collisionality plasmas in order to maximize the non-inductive current drive. This talk will describe the efforts to achieve stable, lower-density, high-performance plasmas by reducing the high gas fueling rate that has, so far, been required to routinely produce high-performance plasmas in NSTX. A concern is that locked modes are more common in low-density plasmas and will limit low-density operation. A range of plasma start-up and ramp-up scenarios will be explored using minimal gas fueling. These scenarios include low-voltage ($<$1.5 V/turn) start-up, slow I$_{p}$ ramp-up rate, higher-voltage ($>$4 V/turn) initiation at full aperture, and the use of low or zero solenoid pre-charge. The latter will help clarify if motion of the toroidal-field bundle inside the solenoid winding during the pre-charge is responsible for error-field induced mode locking.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2011.DPP.BO4.10