LDX Status and Research Report
April 13, 2005
FLOATING COIL EVALUATION AND CRYOGENIC UPGRADES
The bottom vacuum port of the floating coil was repaired to
permit the opening of the port within the large vacuum chamber
of LDX while it is under vacuum. This permits the vacuum space
of the floating coil vacuum to be pumped by the LDX vacuum
system. Our measurements indicated that the pressure within the
floating-coil had increased during previous runs due to the
outgassing of the fiberglass shield. In the preparations of the
March campaign the floating-coil vacuum port was opened for
several days to establish a good vacuum, below 2.5e-6 Torr.
During the three day experimental campaign described below
(March 16-18) the floating-coil cryogenics
achieved record performance. The
floating-coil was charged to the same current level used previously
(approximately 0.9 MA-turns) and the time of superconducting
operation between the interruption of the helium cooling flow
and the quench reached 2 h 33 min. (During previous experiments
the maximum operation time of floating-coil without cooling was
less than 2 h 20 min which occurred for an approximate charging
of 0.75 MA-turns.) The temperature charts of floating-coil
warming process indicated a substantially slower warming of the helium
vessel compare with previous temperature charts. These results
are the evidences of a better performance of the insulating system
of the floating-coil helium vessel due to a better vacuum in the
floating coil vacuum space.
Operation during the March experimental campaign confirmed that
the December incident had not had a negative influence on the
floating-coil cryogenic and superconducting performance.
Additionally, we identified the importance of restoration vacuum
in the floating-coil vacuum. This can now be accomplished
before each plasma experimental run using floating-coil pump-out
The load cells, which are installed in the charging station and
measure the forces on the coil during charging, were upgraded.
Analysis of the load cells signals during floating-coil charging
showed that the maximum downward load was about 200 kg when the
charging-coil was discharged from 300 A. We have decided to lift
the charging-coil with respect to the charging station to
decrease this force before next tests in which the charging-coil
will be charged to a higher current.
The experiments were conducted to investigate the transition
between the low density and the high beta modes of operation
previously observed in LDX. The experiments included a gas puff
fueling and a ECRH power sequence aimed to maximize plasma beta,
as measured by diamagnetism, and plasma stability.
Experiments included: (1) a deuterium gas fueling scan with
puffs of varying gas amounts injected at different times, (2) a
Langmuir probe position scan performed on an optimal discharge
at high beta plasma in which we began the 6.4 GHz heating 2 s
before the heating at 2.45 GHz, (3) a heating power and
modulation scan including an investigation of single and
multiple frequency ECRH. During the heating power modulation
experiments, one source was run for 8 s and the second source
was pulsed on and off. This heating sequence allows us to
observe several heating conditions in a single discharge and the
ability of multiple-frequency ECRH to adjust the plasma pressure
profile. Additionally, high-speed digitizers were used to detect
high-frequency fluctuations within the plasma.
Heating modulation on discharge 5-03-18-010 illustrating the use
of multiple-frequency ECRH to adjust the plasma pressure
The ECRH power scans provided interesting data on the
diamagnetic buildup. It was found that the 2.45 GHz heating was
more effective in producing the highest beta plasmas when it was
delayed (~2 s) with respect to the 6.4 GHz heating. Ratios of
diamagnetic signals clearly indicated that the plasma moves
outwards when heated at a lower frequency, consistent with the
motion of the ECRF resonance zone. This observation is supported
by the results of the plasma filament code (which estimates the
plasma diamagnetic current profile by finding the currents that
flow in a series of "plasma filaments" that best match external
magnetic measurements.) This analysis indicates that the 2.45
GHz heating applied to a plasma formed with 6.4 GHz heating will
move the current centroid outwards by ~15 cm. Analyses of the
data are ongoing.
The operation of the newly upgraded, and now more automated,
launcher controls was successful.
Preliminary design of the levitation catcher system was
completed. The linear area beam laser position detectors for
the floating-coil position control feedback system were ordered
and tested. These are relatively low-cost and highly-reliable
commercial laser detectors made from Keyence Corporation.
J. Kesner, "The Levitated Dipole Concept: First Experimental Results", seminar presented at the University of Wisconsin, Madison, 4/4/05.
D.T. Garnier "First Plasma Results in the Levitated Dipole Experiment", seminar presented at the MIT Plasma Science and Fusion Center, Cambridge, Ma 4/8/05
A.K. Hansen, "Initial Results of Multi-Frequency Electron Cyclotron Frequency Heating in the Levitated Dipole Experiment", Poster and Conference Paper presented at the 16th Topical Conference on Radiofrequency Power in Plasmas, Park City, UT 4/11/05.
M.E. Mauel, "The Levitated Dipole Experiment for Plasma Confinement", Invited talk presented at XII Conference of the Physical Society of Mexico, 4/15/05.
D.T. Garnier "First Plasma Results in the Levitated Dipole Experiment", Invited talk presented at the APS spring meeting, Tampa, Fl, 4/17/2005.