A. Zhukovsky

The Levitated Dipole Experiment (LDX) magnet system

In the Levitated Dipole Experiment (LDX), a hot plasma is formed about a levitating superconducting dipole magnet in the center of a 5 m diameter vacuum vessel. The levitated magnet is suspended magnetically during an eight hour experimental run, then lowered and recooled overnight. The floating F-coil magnet consists of a layer-wound magnet with 4 sections, designed to wrap flux lines closely about the outside of the levitated cryostat. The conductor is a niobium-tin Rutherford cable, with enough stabilizer to permit passive quench protection. Lead strips are used as thermal capacitors to slow coil heating. An optimized system of bumpers and cold-mass supports reduces heat leak into the helium vessel. Airbags catch the floating coil on quenches and faults, preventing collision with the vacuum vessel.

Design, fabrication and test of the react and wind, Nb3Sn, LDX floating coil

The Levitated Dipole Experiment (LDX) is an innovative approach to explore the magnetic confinement of fusion plasma. A superconducting solenoid (floating coil) is magnetically levitated for up to 8 hours in the center of a 5-meter diameter vacuum vessel. The floating coil maximum field is 5.3 T, and a react-and-wind Nb/sub 3/Sn conductor was selected to enable continued field production as the coil warms from 5 K during the experiment up to a final temperature of about 10 K. The coil is wound using an 18-strand Rutherford cable soldered into a half-hard copper channel, and is self protected during quench. The coil is insulated during winding and then vacuum impregnated with epoxy. The impregnated coil is tested with 2 kA operating current at 4.2 K, and then a single, low resistance joint is formed at the outer diameter of the coil before the coil is enclosed in its toroidal helium vessel. This paper presents details of the coil design and manufacturing procedures, with special attention to the techniques used to protect the coil from excessive strain damage throughout the manufacturing process.

Charging magnet for the floating coil of LDX

The charging coil (C-coil) for the joint Columbia University/MIT Levitated Dipole Experiment (LDX) is under development jointly by MIT and the Efremov Institute. The NbTi superconducting C-coil serves to charge/discharge inductively the floating superconducting magnet to/from 2277 A when it is resting in the charging port at the bottom of the LDX vacuum vessel. The C-coil is designed for 3200 charge-discharge cycles. The solenoid magnet is installed in a low heat leak liquid helium cryostat with a warm bore of more than 1 m. The magnet protection system has an external dump resistor, which dissipates most of the 12 MJ stored during a quench.

High temperature superconducting levitation coil for the Levitated Dipole Experiment (LDX)

The Levitated Dipole Experiment (LDX) is an innovative approach to explore the magnetic confinement of fusion plasmas. A superconducting solenoid (floating coil) is magnetically levitated for up to 8 hours in the center of a 5-meter diameter vacuum vessel. This coil is supported by a levitating coil (L-Coil) on top of the vacuum vessel. In the initial machine design, this levitating coil was a water-cooled copper solenoid, and was the experiments single largest load on the available water system. The main benefit of using a high temperature superconducting coil is the ability to apply more auxiliary heating power to the plasma. However, this coil will also be the first high temperature superconducting coil to be used in a US fusion program experiment. The high temperature superconducting L-Coil is a solenoid, using a two-in-hand winding of a commercially available 0.17 mm/spl times/3.1 mm tape by American Superconductor Corporation with a critical current of 62 A at 77 K and self-field. The L-Coil will be operated at 0.9 T and 20 K. The L-Coil has a protection circuit that not only protects it against overheating in the event of quench, but also against F-Coil collision in the event of a control failure.

750 MHz NMR magnet development

The authors consider the development of a magnet for a high-resolution 750-MHz proton spectrometer. The magnet operates in persistent mode at a central field of 17.63 T. The tenth-order coil system consists of seven niobium-tin and five niobium-titanium sections. Rectangular cross-section conductors are used for high radial homogeneity and for high filling factor. The details of design, construction, and quench analysis are described, and results of some preliminary tests are presented. >

Design and fabrication of the cryostat for the floating coil of the Levitated Dipole Experiment (LDX)

The Levitated Dipole Experiment (LDX) is a new, innovative magnetic confinement fusion experiment being designed and installed in collaboration with Columbia University at the Massachusetts Institute of Technology (MIT). The primary objective of the experiment is to investigate the possibility of steady-state, high-beta plasma confinement with near classical transport. The main component of the experiment is a levitated cryostat with a 5.7 T Nb/sub 3/Sn superconducting magnet, housed in an Inconel high pressure helium vessel. The pressure vessel is surrounded by a large thermal mass radiation shield and an outer vacuum shell, all of which are magnetically levitated inside a much larger vacuum chamber. The cryostat, now under construction is described in this paper. The cryostat keeps the magnet temperature between 5 and 10 K during 8 hours of levitated operation.

PTF, a new facility for pulse field testing of large scale superconducting cables and joints

A magnetic Pulse Test Facility (PTF), in which samples of CICC electrical joints from each ITER home team will be tested, has been fabricated at the MIT Plasma Fusion Center under an ITER task agreement. Construction of this facility has recently been completed, and an initial test phase on the first CICC joint sample has begun. PTF includes capabilities for sample currents up to 50 kA from a superconducting transformer developed by the University of Twente, magnetic fields up to 6.6 T with ramp rates to +1.5 T/s and -20 T/s, and a cryogenic interface, supplying supercritical helium with flow rates to 20 g/s through each CICC leg at controlled temperatures to 10 K and pressures to 10 atmospheres. A sophisticated, multiple-channel data acquisition system is provided to processed, digitally recorded sensor signals from both the sample and the facility. The facility is totally remote-controlled from a control room through a fiber optic link, and qualified users worldwide are afforded secured access to test data on a 24-hour basis via the Internet. The facility has successfully exercised the first joint sample over the ITER test spectrum with positive results.

NMR magnet technology at MIT

Two key issues in the construction of high field NMR (nuclear magnetic resonance) magnets are discussed: field drift due to the index of the superconductor and the shimming of large high order gradients. It is noted that the index of the conductor as measured at the critical current gives a conservative guide to the drift rate of the magnet. With the assumption of a Gaussian distribution of critical current the effective index increases as the current decreases. The use of ferromagnetic shimming to improve the quality of the field generated by a nonuniform winding is a relatively simple procedure, particularly if linear programming is used to calculate the shim array. A commercial device maps the field along a helical path. Those field measurements are then used for a least squares fit to the harmonics. From that a shim array is designed by linear programming. >

Operation of the Levitated Dipole Experiment Floating Coil

The Levitated Dipole Experiment (LDX) is an innovative facility to study plasma confinement in a dipole magnetic field, created by a superconducting solenoid (floating coil), which is magnetically levitated in the center of a 5 m diameter by 3 m tall vacuum chamber. This persistent mode, floating coil (F-coil) consists of a Nb3Sn magnet installed inside a high-pressure vessel filled with 12.5 MPa helium gas at room temperature. It is surrounded by a high heat capacity fiberglass-lead composite radiation shield and by a toroidal vacuum shell. The built-in tube heat exchanger serves to cool the magnet, the helium vessel, and the thermal shield. When positioned at the lower part of the vacuum chamber the F-coil is cooled by retractable cryogenic transfer lines to about 4.5 K and it is charged inductively by the charging coil installed outside of the vacuum chamber. Then the helium flow is interrupted, the heat exchanger is pumped out, retractable lines are disengaged from F-coil ports, the ports are plugged, and the F-coil is lifted to the middle of the chamber to initiate and study plasmas. After several hours and just before the F-coil warms up to about 10 K it is lowered down for the next re-cooling or for discharging. This paper describes the F-coil cooling system and inductive charging system operation and performance

Performance of the Conduction‐Cooled LDX Levitation Coil

The Levitated Dipole Experiment (LDX) was developed to study plasma confinement in a dipole magnetic field. Plasma is confined in the magnetic field of a 680‐kg Nb3Sn Floating Coil (F‐coil) that is electromagnetically supported at the center of a 5‐m diameter by 3‐m tall vacuum chamber. The Levitation Coil (L‐coil) is a 2800‐turn, double pancake winding that supports the weight of the F‐coil and controls its vertical position within the vacuum chamber. The use of high‐temperature superconductor (HTS) Bi‐2223 for the L‐coil minimizes the electrical and cooling power needed for levitation. The L‐coil winding pack and support plate are suspended within the L‐coil cryostat and cooled by conduction to a single‐stage cryocooler rated for 25‐W heat load at approximately 20 K. The coil current leads consist of conduction‐cooled copper running from room temperature to 80 K and a pair of commercially‐available, 150‐A HTS leads. An automatically filled liquid‐nitrogen reservoir provides cooling for the coil’s radiat...