Progress in Levitation of the Levitated Dipole Experiment
Author: Darren T. Garnier
Submitted: 2005-12-21 16:02:00
Co-authors: A.Boxer, J.Ellsworth, A.K.Hansen, J.Kesner, I.Karim, M.E.Mauel, P.C.Michael, E.E.Ortiz
175 Albany St, NW17-209
Cambridge, MA 02176
The Levitated Dipole Experiment (LDX) is currently carrying out a series of experiments in a campaign to achieve its first levitated dipole plasmas. Results from supported operation of LDX in its first year of operation indicate that confinement of the core plasma plays an important role in the stability of the plasma. Heated by electron cyclotron resonance heating, the plasma is composed of a low beta background plasma and a high beta, fast electron population. When insufficient background plasma density exists, the plasma becomes unstable to the hot electron interchange mode . While stable, a significant plasma loss channel is the supports; thus eliminating the supports by levitation of the dipole coil should lead to higher background plasma density and higher stability limits.
Current work on LDX focuses on integrating the levitation system. The floating dipole coil (F-coil) is levitated magnetically by a levitation coil (L-coil) situated on top of the LDX vacuum vessel, roughly 1.5 m away. In this geometry, the system is unstable only to vertical motion of the F-coil. The system is stabilized by feedback control of the L-coil current. The L-coil is a high-temperature superconducting coil, which was chosen to reduce the operating power and cooling requirements. It presents a unique additional requirement on the control problem; the AC losses induced in the coil from the feedback signal must be smaller than the cooling capacity of 20 W. Ensuring this requirement is met means controlling the noise in the position measurement system.
The levitation control system is a fully digital system and is composed of two control processors. A slow PLC system handles L-coil cryostat monitoring, interlocks, and the pneumatic launcher/catcher system. A fast real-time computer, with a 10 kHz control cycle, is used to control the current feedback system, L-coil quench detection, and shaping coils for plasma operations. Detection of the F-coil position is done using a system of 8 commercially available laser position detectors.
A series of control experiments are planned prior to our first studies of plasma confined by a levitated dipole. First, a L-coil only test will be performed to verify operation of the control system and confirm our model of eddy current effects in the installed system. Next, a plasma experimental run will be performed with the supported dipole partially supported by the L-coil. This experiment will directly measure the L-F coil dynamical response and evaluate changes in plasma operations that result from the weak shaping of the L-coil field. Third, a full test of the integrated levitation system will be made with physical safety restraints but without plasma. This full safety test of the levitation system will be followed by plasma operations with a levitated coil.
 Garnier, D.T. et al. Submitted to Phys. Plasmas, 2005.