R. Benjegerdes

Magnetic field decay in model SSC dipoles

LBL-25139 Lawrence Berkeley Laboratory UNIVERSITY OF CALIFORNIA Accelerator & Fusion Research Division Presented at the 1988 Applied Superconductivity Conference, San Francisco, CA, August 21-25, 1988 Magnetic Field Decay in Model sse Dipoles W.S. Gilbert, R.F. Althaus, PJ. Barale, R.W. Benjegerdes, M.A. Green, M.l Green, and R.M. Scanlan August 1988 r Prepared for tbe U.S. Department of Energy under Contract Number DE-AC03-76SF00098.

Quadrupole magnets for the SSC

The authors have designed, constructed, and tested four short (1-m) models and six full-size (5-m) models of the Superconducting Super Collider (SSC) main-ring 5-m focusing quadrupole magnet (211 T/m). The results of this program are summarized. The magnet construction, test procedures, and test results are discussed. Except for modest training above the SSC operating point, the magnets performed very well and proved to be self-protecting. Some design flaws, e.g., inadequate end-clamping and pole shimming, were identified and corrected sufficiently so that the later magnets exceeded specifications.<<ETX>>

The Effect of Flux Creep on the Magnetization Field in the SSC Dipole Magnets

The sextupole fields of model SSC dipole magnets have been observed to change with time when the magnets are held at constant current under conditions similar to injection into the SSC accelerator. The changes in the sextupole component have close to a linear log time dependence, and is felt to be caused by flux creep decay of the magnetization currents in the superconductor filaments. Measurements of this decay have been made under various conditions. The conditions include various central field inductions and changes of field prior to when the decay was measured. The measured field decay in the dipole’s sextupole is proportional to the magnitude and sign of the sextupole due to magnetization which was measured at the start of the decay. This suggests that the decay is a bulk superconductivity flux creep. Proximity coupling appears to play only a minor role in the flux creep according to recent LBL measurements with a stable power supply.

Design and testing of the 2 MV heavy ion injector for the Fusion Energy Research Program

The Fusion Energy Research Group at the Lawrence Berkeley Laboratory has constructed and tested a pulsed 2 MV injector that produces a driver size beam of potassium ions. This paper will describe the engineering aspects of this development which were generated in a closely coupled effort with the physics staff. The details of the ion source and beam transport physics are covered in another paper at this conference. This paper will discuss the design details of the pulse generator, the ion source, the extractor, the diode column, and the electrostatic quadrupole column. Included will be the test results and operating experience of the complete injector.

New measurements of magnetic field decay in 1 meter SSC-type dipoles

Previous studies of magnetic field decay in model SSC (Superconducting Super Collider) dipoles due to changes in magnetization currents caused by flux creep have used the assumed SSC injection energy of 1 TeV, or an 0.33-T central dipole field, and an excitation to the storage field of 6.6 T. More recently, it has been decided to inject at 2 TeV, or 0.66 T and so more recent tests have been carried out at the new injection field, or at both the new and old fields. Additionally, the effects of temperature changes and excitation cycles on the field decay have been studied. The roughly log linear decay of magnetization current multipole fields has been demonstrated in several new dipoles. A suggested mechanism of thermally induced activation in the additional 0.5-K range has been effectively countered by temperature measurements that show temperature fluctuation ten to thirty times lower, over a 90-min time period.

Training of LBL-SSC model dipole magnets at 1.8 K

The 1.8 K training behavior of SSC (Superconducting Supercollider) magnets, several of which have reached a peak current of 9400 A is presented. For the SSC Project, more than 30 one-meter-long dipole magnets have been built and tested. Magnet operation, primarily reaching design field without premature training, is expected to be superior in superfluid helium at 1.8 K as compared with helium at 4.3 K. Not only is the critical current increased at the lower temperature, but the heat transfer is much improved. LBL (Lawrence Berkeley Laboratory) has had an operating helium II facility for nine years and the standard test sequence has been to check for training in helium I at 4.3 K and then cool the system down to 1.8 K and train the magnet to its new, high limit. Because the mechanical forces are much greater at the higher currents and fields achieved at the lower temperature, information on the adequacy of the mechanical design has been obtained. Even for those magnets in which training quenches occurred in the inner layer at 4.3 K, many of the quenches at 1.8 K occurred in the outer layer.<<ETX>>

Quench antenna and fast-motion investigations during training of a 7 T dipole

Equipment was installed to detect fast conductor motion and quench propagation in a 1 meter long superconducting dipole magnet. (1) The fast-motion antenna, centered within the bore of the magnet, used three long dipole coils, mounted end-to-end to span the magnet length. Coil signals were nulled against a neighbor to produce low-ripple signals that were sensitive to local flux changes. A low microphonic signal was used as an event trigger. (2) Nulling improvements were made for the magnets coil-imbalance signals for improved cross-correlation information. (3) A quench-propagation antenna was installed to observe current redistribution during quench propagation. It consisted of quadrupole/sextupole coil sets distributed at three axial locations within the bore of the magnet. Signals were interpreted in terms of the radius, angle, orientation, and rate of change of an equivalent dipole. The magnet was cooled to 1.8 K to maximize the number of events. Twenty-four fast-motion events occurred before the first quench. The signals were correlated with the magnet-coil imbalance signals. The quench-propagation antenna was installed for all subsequent quenches. Ramp-rate triggered quenches produced adequate signals for analysis, but pole-turn quenches yielded such small signals that angular localization of a quench was not precise.<<ETX>>

Correction of Magnetization Sextupole in One-Meter Long Dipole Magnets Using Passing Superconductor

The generation of higher multipoles due to the magnetization of the superconductor in the dipoles of the SSC is a problem during injection of the beam into the machine. The use of passive superconductor was proposed some years ago to correct the magnetization sextupole in the dipole magnet. This paper presents the LBL test results in which the magnetization sextupole was greatly reduced in two one-meter long dipole magnets by the use of passive superconductor mounted on the magnet bore tube. The magnetization sextupole was reduced a factor of five on one magnet and a factor of eight on the other magnet using this technique. Magnetization decapole was also reduced by the passive superconductor. The passive superconductor method of correction also reduced the temperature dependence of the magnetization multipoles. In addition, the drift in the magnetization sextupole due to flux creep was also reduced. Passive superconductor correction appears to be a promising method of correcting out the effects of superconductor magnetization in SSC dipoles and quadrupoles.