LDX Progress Report
March 9, 2007


FIRST FLIGHT AND DAMAGED L-COIL

The first plasma physics experiments conducted during levitation of
the large LDX floating coil (F-coil) took place on Feb 9, 2007.

The F-coil is a Nb3Sn superconducting dipole (1.2 MAT, 1200 lbs)
having a cryostat that allows repetitive, quasi-steady plasma
experiments for periods lasting more than one hour between re-cooling
with liquid helium. The coil is levitated by attraction to a coil that
sits above the vacuum chamber, the L-coil. In this configuration it is
vertically unstable and a real-time, computerized feedback control
system maintains the coil within less than 1/2 mm of the programmed
position. Eight occluded lasers track the F-coil position. If the
F-coil is out of the range of the lasers the L-coil is discharged and
the F-coil will drop onto a “launcher-catcher” that gently breaks the
fall of the F-coil. During the past several months, the L-coil, the
catcher, the detection lasers, and the digital feedback control system
were installed and carefully tested.

Although the LDX Project Plan originally called for a normal,
water-cooled copper L-coil, the PSFC magnet group received an SBIR to
build a first-of-a-kind HTc (BSCCO) coil to gain experience with these
first-generation high-temperature superconductors at high field and
current. The LDX Team enthusiastically welcomed this opportunity to
have a fully superconducting experiment, and this high-Tc coil became
the LDX L-coil.

The manufacture and operation of the L-coil gave many opportunities to
understand in practice HTc magnets, and several publications and
invited presentations resulted. A problem arose when the coil was
improperly vacuum impregnated and afterwards shorts were measured in
the coil. It was decided that in spite of the shorts the coil might
be utilizable. L-coil tests with the F-coil weight not fully supported
confirmed the coil’s operation. Although a super conducting coil does
not have voltages during steady state operation, inductive voltages
arise when the coil current changes. These voltages can drive current
through the shorts and cause some ohmic heating and possibly cause a
magnet quench.

On Feb 9, 2007 preparations were in place and the first levitation
experiments were performed. In these experiments the launcher lifted
the floating coil into place and the L-coil and control system was
turned on. As the weight of the coil was gradually reduced and finally
completely taken up by the upward force of the L-coil the
launcher-catcher was separated from the F-coil by 2 cm. (In future
plasma experiments, the launcher-catcher will be separated by 20 cm so
as to be clear of the confined plasma flux bundle.) The control
system worked very well. The F-coil levitated for 40 minutes, and
plasma physics and control experiments were conducted.

During levitation, control experiments were performed to test the
positioning of the F-coil within the range allowed by the laser
occlusion system. During one test, the F-coil was raised above a
programmed allowable, which triggered a shut-down of the L-coil, as
designed. The L-coil was discharged through a dump resistor, and the
F-coil gently dropped onto the launcher-catcher as planned. However,
it appears that the inductive voltage generated during the current decay
caused a localized heating of the L-coil which lead to a coil quench
that damaged the L-coil.

L-COIL REPAIRS

After examining the L-coil, it has been determined that a repair would
be too costly (and operation would remain questionable). Therefore we
decided to replace this coil by a normal water-cooled Cu coil. This
coil has been designed, and vendors have been contacted. At this time
it appears that the coil will take 10-12 weeks to manufacture and will
cost 50-60k$. We expect to place an order within the next 2 weeks.


ICC MEETING: College Park, Md, 2/12/2007 - 2/14/2007

J. L. Ellsworth et al, “Overview of Results from Supported Mode
Operation of the Levitated Dipole Experiment”, Invited talk.

D.T. Garnier et al., “Levitation in LDX”, poster.

A.C. Boxer et al, “Density Profile Measurements in the Levitated
Dipole Experiment”, poster.

J. Kesner et al, “Observation of Low Frequency Modes in LDX”, poster.


DOCTORAL DISSERTATION

A. Kouznetsov successfully defended a theoretical thesis entitled
“Theoretical prediction of and in a high-aspect ratio LDX”.
Advisors were J.P. Freidberg and J. Kesner. Related articles
submitted for publication include “Theoretical prediction of and
in a hardcore Z-pinch”, “The effect of sheared axial flow on the
stability of the interchange mode in a hard-core Z-pinch”, and
“Quasilinear Ideal MHD transport model for an Axisymmetric Closed
Field Line Magnetic Configuration”.