LDX Project Status

May 2006

LEVITATION COIL TESTS

Integrated tests of the levitation coil were successfully completed
during a series of tests carried out in March and April. The
levitation coil (L-coil) is a high temperature superconducting
coil that supplies the force to levitate the 560 kg
superconducting dipole coil. Active digital feedback is used to
provide vertical stability of the levitated dipole by modulating
the L-coil current.

There were two main objectives of the tests. First, the operation
of the L-coil at the nominal operation point of 105 Amp DC with
+/- 1 Amp amplitude of 1 Hz excitation, the upper limit of
simulated feedback excitation. This test confirmed that the coil
could operate at the required operational point for over 2 hours
without thermal instability. These tests also confirmed the
operation of a continuous high speed digital control system.

Previous tests had indicated a desire to reduce heating of the
superconducting leads to the L-coil. The lead heating was reduced
using a new nitrogen shield subcooling system which pumped the
shield to a sub-atmospheric pressure thereby lowering the
temperature of the liquid nitrogen reservoir.

The second objective of the test was to gather information about
the response function of the L-coil (at the position of the
floating coil). Several excitation voltages and frequencies were
used and numerous diagnostic systems, including the LDX plasma
magnetic diagnostic system, recorded the results. From these
experiments we now have a complete picture of the major mode
response of the entire system (control system, power supply,
L-coil, vacuum vessel eddy currents). The response function will
be used in tuning the levitation feedback algorithm.


FLOATING COIL CONTROL

The location of the floating dipole (F-coil) is determined by
occlusion of laser beams by a laser-alignment ring, which is
mounted on the F-coil. Stripes on the surface of the ring serve to
determine the speed of rotation of the F-coil during levitation.
The side surface of the laser-alignment ring has been polished,
silver plated, and then sand blasted with one inch pitch to
provide high (silver plated) and low (sand blasted) reflection
stripes.


CATCHER/LAUNCHER

The catcher/launcher system consists of two parts: an upper
catcher and a lower launcher.

The upper catcher serves to limit uncontrolled upward motion of
the floating coil and provides a nearby inner surface for mounting
diagnostics. All the items for the upper catcher (limiter) have
been received. They were pre-assembled outside of the vacuum
vessel and showed good manufacturing quality. The upper catcher is
now being installed into the vacuum vessel.

The lower launcher lifts the F-coil from the bottom of the
charging station where it is cooled and charged with current into
place at the center of the vacuum chamber. The launcher is also
designed to safely catch the F-coil in case of an uncontrolled
fall. The design of lower launcher-catcher has been finished and
the set of drawings was sent to several venders for cost
quotations. We selected Hollis Line Machine Co. (Hollis, NH) as
vendor.


RECENT PUBLICATION

D.T. Garnier, et al., "Production and study of high-beta plasma
confined by a superconducting dipole magnet", Phys. Plasmas 13,
(2006), 056111/1-8.