Magnet Design 

Magnets  are used both to steer the heavy-ion beams and to focus them during acceleration and compression. Fusion drivers will rely primarily on cryogenic superconducting dipole and quadrupole electromagnets, because these magnets use the least amount of energy at high field strengths.  However, other types of magnets, such as permanent magnets or "normal" (non-superconducting) electromagnets, may prove to be more feasible than superconducting magnets in certain sections of a driver. For example, both permanent magnets and pulsed electromagnets can be made very compact, and pulsed magnets are capable of generating extremely high fields. Permanent magnets use no energy at all, but they are limited in field strength, are difficult to tune, and the material is still very expensive. These types of magnets may be preferable in space-imited areas or perhaps in high-radiation environments. 

The HIF-VNL laboratories are pursuing a number of magnet research areas to develop superconducting and pulsed quadrupoles.  Superconducting magnet development is carried out by a collaboration of the VNL (LBNL, LLNL) with  MIT and Advanced Maget Laboratory Inc (AML).  This effort comprises design studies of quadrupole arrays for long term applications as well as the development of single-bore prototypes for near term experiments. In particular, the High-Current Experiment (HCX).   provides an opportunity to address key design issues like maximum achievable gradient, design simplicity and cost-effectiveness, optimization of the conductor parameters, field quality in compact geometries, modularity, and compact cryostat designs. Two magnet design approaches were proposed in 2000 by LLNL and AML. Following fabrication and test of two prototypes of each design, the LLNL design was selected for further optimization having demonstrated higher gradient and better training performance. The LLNL design uses double-pancake coils surrounded by iron yoke and a structural shell. A prototype quadrupole with optimized parameters is presently being fabricated. At the same time, the design of a cryostat for a focusing doublet compatible with the HCX lattice, with features to accommodate induction acceleration modules, has been completed and a first prototype is being fabricated. 

Compared with equivalent constant-field normal or superconducting designs, pulsed electromagnets are inexpensive to manufacture and easy to operate, making them well suited for near-term experiments that do not require the 5-10 Hz pulse rate of a driver. A large-bore pulsed quadrupole is being designed and built for the Neutralized Transport Experiment (NTX).  Pulsed octupoles to correct certain fundamental field aberrations are also being designed for NTX and will be placed coaxially with the main quadrupoles.

Multiple-beam quadrupole arrays present specific issues that require a dedicated research and development effort. Such issues include designing edge coils to adjust the field in boundary cells and terminate the flux, minimizing the number of joints, achieving precise and simultaneous alignment of all channels, and attaining high vacuum to minimize beam loss. and the associated activation and energy deposition in the coils. Several studies have already been carried out to address general array-design issues, and a pulsed quadrupole-array prototype was built and successfully tested in 2000 . The development of a superconducting prototype array has also started. 

For comments or questions contact or  Work described here was supported by the Office of Fusion Energy at the US Department of Energy under contracts  DE-AC03-76SF00098 and W-7405-ENG-48.  This document was last revised June, 2002.