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The Levitated
Dipole Experiment (LDX) is a newly completed research facility developed
as a collaborative project of the Plasma Physics Laboratory of Columbia
University and the Plasma Science and Fusion Center (PSFC) of the
Massachusetts Institute of Technology to test whether fusion can
benefit from nature’s way to confine high-temperature plasma.
The goal of the program is to understand the equilibrium, stability
and confinement properties for plasma that is confined in the field
of a levitated dipole. LDX experiments are yielding new data on
confinement and stability of high-beta plasmas in a dipole magnetic
field, control of particle circulation and control of adiabatic
heating.
An important objective of the LDX research program is to create
an innovative partnership between plasma scientists and magnet technology
experts. By incorporating state-of-the-art engineering and design
in its three superconducting magnets, LDX has gained a unique and
world-class research facility for long-pulse plasma physics research.
The LDX device is shown in Figs. 1 and 2. LDX consists of three
circular and coaxial superconducting magnets, the floating coil,
the charging coil, and the levitation coil, a large cylindrical
vacuum chamber, basic plasma diagnostics, two microwave ECRH heating
systems, and two low-current shaping coils with independent power
supplies. The dipole geometry gives the experiment a remarkably
wide diagnostic access to large volume plasmas and enables scientific
experiments that have never before been possible.
Superconducting Floating Coil: The floating coil (F-coil) is a superconducting
magnet comprised of a single 1.5 km length of conductor carrying
up to 1.5 MA turns in a persistent mode. The design of this novel
conductor, coil and its cryostat include:
- High critical current density, low loss, high stability
Nb3Sn (cable-in-copper-channel) conductor,
- Inductive charging arrangement with one very low
resistance joint,
- Very low heat loss cryostat design with high load,
low heat leak laminated crash supports,
- Indirect cooling by a flow heat exchanger with
removable helium transfer ports.
Optimal combination of these technologies has allowed for up to
a 2 hour levitation time.
High Temperature Superconducting Levitation Coil:
The floating coil is supported by a levitation coil (L-coil) which
is located on the top of the vacuum vessel. In addition to providing
the magnetic force to levitate the 550 kg floating coil, the L-coil
must also be modulated with a feedback signal to provide vertical
stability. This coil uses a high temperature superconducting (HTS)
tape (BSCCO-2223) and is the first HTS coil to be used in a US fusion
program.
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| Fig. 1 Schematic cross-section of the LDX. |
Superconducting Charging Coil: The NbTi C-coil serves to inductively
charge/discharge the floating superconducting magnet to/from 1.2
MA·T when it is resting in the charging station at the bottom
of the LDX vacuum vessel. The charging coil (C-coil) was designed
and fabricated by the Efremov Institute-Sintez (St. Petersburg,
Russia).
The LDX experiment began operation in August 2004. Using multifrequency
electron cyclotron resonance heating (ECRH), high-beta plasmas have
been created, sustained for many seconds, and studied while the
high-field dipole magnet was mechanically supported by three, thin
(1 cm dia.) support rods. These experiments were the first stage
of a carefully planned operational program that allows for the safe
and reliable operation of the LDX superconducting magnets and the
coordinated installation and test of diagnostics and research tools.
During our first stage operation, significant programmatic and scientific
results were achieved including: (i) demonstration of reliable operation
of the high-field superconducting magnets, (ii) long-pulse, quasi-steady-state
(> 10 s) plasma formation using electron-cyclotron resonance
heating (ECRH), (iii) achievement of peak equatorial plasma beta
near 10%, (iv) operation of all base diagnostics and data acquisition
systems, (v) identification and parameterization of three discharge
“regimes” having unique physics properties, (vi) preliminary
study of ECRH profile control using multiple-frequency heating,
(vii) identification of beta-limiting instability driven by a large
fractional density of energetic trapped electrons, (viii) preliminary
study of plasma shape effects, (ix) preliminary study of plasma
density profile using movable edge probes and a single-cord microwave
interferometer, and (x) preliminary study of X-ray emissivity using
an array of X-ray detectors.
These first-stage experiments using a supported dipole are nearly
complete, and they have already met, or exceeded, all first-stage
program objectives. During the next stage of the project modifications
are being made to enable fully levitated operation.
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Fig 2.a) View inside the vacuum chamber of the supported dipole
magnet.
Fig2.b) Overview of the LDX. |
- Department of Applied Physics, Columbia University
New York, NY 10027
Email: mauel@columbia.edu
- MIT Plasma Science and Fusion Center
Cambridge, MA 02139
Email: kesner@psfc.mit.edu
or minervini@psfc.mit.edu
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