G.A. Deis

A long electromagnetic wiggler for the PALADIN free-electron laser experiments

A report is presented of the design, fabrication, and testing of a 25.6-m-long wiggler for a free-electron-laser (FEL) experiment. It is a DC iron-core electromagnetic wiggler that incorporates a number of important and unique features: permanent magnets are used to suppress saturation in the iron and extend the linear operating range; steering-free excitation allows real-time adjustment of the field taper without causing beam steering; and wiggle-plane focusing is produced by curved pole tips. The magnitude of random pole-to-pole field errors is minimized by a mechanical design concept that reduces tolerance stackup in critical locations. Three five-meter sections of this wiggler design were tested individually, and when connected together the measurements show exceptionally low levels of random errors. Detailed analysis of the error distribution has shown that the errors are not truly random, and the net effect on steering is much less than would be expected from random errors of the same magnitude. Analysis is still in progress to identify the cause of this beneficial effect. >

Design concept for the GEM detector magnet

The magnet has two symmetric and independent halves, each containing a cold mass assembly operating nominally at 4.5 K, a set of vapor cooled leads, a cold mass support system, a liquid nitrogen shield system, and a vacuum vessel. Also included in each half is a forward field shaper which provides a component of magnetic induction normal to the path of low angle muons in the forward region, thereby improving their resolution. The unique features of this magnet are the conductor design itself and the large coil diameter, which demands an on-site winding and assembly operation. The use of a natural convection thermosiphon loop for thermal radiation cooling eliminates plumbing complications. Locating the aluminium sheath outside the conduit for quench protection permits optimizing the copper-to-superconductor ratio inside the conduit for stability alone. The conceptual design for the magnet, including the design for the detector dependent magnetics, the superconducting coils and coil structure (cold mass), the coil winding process, the vacuum vessel and liquid nitrogen shields, the cold mass supports, and the magnet assembly procedure, are described.<<ETX>>

Development of a laced electromagnetic wiggler

The construction of a one-period wiggler system called a laced wiggler is presented along with test results from a prototype design. The laced electromagnetic wiggler is being developed to attain higher magnetic fields, shorter wavelengths, and larger gaps for the induction-linear accelerator, free-electron-laser (FEL) program. In the laced wiggler design, permanent magnets are located (laced) between the electromagnetic coils to increase the reverse-bias flux in the iron pole beyond that possible with only pole-edge (side) permanent magnets. This increase in reverse-bias flux allows wiggler operation at midplane magnetic-field intensities comparable to those of a hybrid permanent magnet/steel wiggler, but with field adjustability over a specified range. The maximum field intensity and tuning range are selected, within limits, for specific design requirements. The test results show good agreement with the analytical predictions and confirm the ability of the laced wiggler to attain the desired midplane magnetic flux density and tuning range. Both the nominal wiggle field along the wiggler axis and the focusing field variation are within the acceptable limits of design requirements. >

Electromagnetic wiggler technology development at the Lawrence Livermore National Laboratory

As a part of the program in induction-linac free-electron laser research, the authors summarize the Laboratorys work in a variety of activities addressing the requirements imposed on wiggler systems. The development of improved designs is reported for DC iron-core electromagnetic wigglers to attain higher peak fields, greater tunability, and lower random-error levels. Specialized control systems are noted (such as magnetic-field and beam-position controllers) that can relax requirements on the wiggler itself. Basic studies to establish the effect of radiation on permanent magnets are also discussed briefly. >

Development of the strong electromagnet wiggler

A description is given of the design approach used for a permanent magnet-assisted electromagnet under development as part of the Induction Linac Free-Electron-Laser program. A single-period schematic of the wiggler is described. This device is an iron-core, DC electromagnet in which poles of identical magnetic scalar potential are attached to large iron plates. These scalar potential busses are bridged by iron plates around which the large primary coils are wound. Smaller trim coils that provide the magnetic-field taper are placed at the base of the poles. Permanent-magnet material, magnetized in a direction opposite to that of the field created by coils, is placed between the poles. The magnetic scalar potential of the poles can be elevated to the desired point while delaying the onset of magnetic saturation in the iron. Data are presented from a seven-period prototype wiggler that has been designed, fabricated and tested. >

GEM detector conductor manufacturing experience

Feasibility studies and manufacturing experience on the GEM Magnet conductor are presented, including all components-NbTi strand, cable, conduit manufacture, cable pulling, and aluminum sheath application.<<ETX>>

CONDUCTOR DESIGN FOR THE GEM DETECTOR MAGNET

The Gammas, Electrons, Muons (GEM) Detector, one of the two large detectors planned to be built at the SSC, features high muon momentum resolution. This is achieved by magnetization (by a huge magnet, about 20 m in diameter and 31 m long) of roughly 10,000 m3 of space within the muon chambers. The GEM Detector Magnet1 should be designed to operate with highest possible reliability level to ensure maximum availability of the Detector Systems. That means that the magnet and the conductor should be as stable as practically possible. The conductor should be reliably protected against overheating and electrical breakdown in the case of a quench or fast discharge. For reliability reasons, the magnet dump voltage is relatively low, 500 V to ground, which implies low current density in the conductor. Table 1 lists general requirements for the conductor.

Plans for building the largest thin solenoid ever

The superconducting solenoid magnet for the GEM detector poses unusual fabrication and handling challenges because of its extraordinary size. It will be more than 30% larger in diameter than the largest existing particle detector coils. Each of the two coil elements that compose the air-core solenoid, will be about 19 meters in diameter and 15 meters long. Major components weighing as much as 1500 Mg must be transported and manipulated at the Interaction Region 5 (IR5) fabrication site of the SSC Laboratory as the magnets are fabricated. Because of their large size, the magnets will be fabricated, assembled and tested at special purpose facilities at the IR5 site. The site-use plan must accommodate the fabrication of other detector components and the assembly of large flux shaping iron structures in a timely manner to allow subsequent testing and defector assembly. Each cold mass will be composed of twelve 45-Mg coil windings that are joined prior to assembly into the 19-m diam annular cryostat. >

The superconducting solenoid magnet system for the GEM detector at the SSC

The design of the magnet for the GEM detector at the SSC is described. It is an 18 m inner diameter, 30 m long superconducting solenoid, with a magnetic field of 0.8 T. The basic solenoidal field is shaped by large ferromagnetic cones, to improve detector performance in the ends of the solenoid. Because of the systems large size and mass, field-fabrication on-site at the SSC is required. The challenges in this process, together with the large stored energy of the system 2.5 GJ, have lead to novel design choices in several areas, including the conductor. The design of the conductor, cold mass, vacuum vessel, cold mass supports, thermal shields, forward field shapers, and auxiliary systems are described. >

A Liquid Helium Cryogenic System Design for the Gem Magnet

The Superconducting Super Collider (SSC) Gammas, Electrons, Muons (GEM) magnet is a large superconducting solenoid with a total mass of 1.05×106 kg and a stored energy of 2.5 GJ. A cryogenic system to cool and to maintain the GEM magnet to liquid helium temperature is described. The system is designed to operate effectively under a variety of operating conditions, including cooldown/warm-up, steady state operations, and quench. Primary cooling during steady-state operation is based on natural circulation thermosiphon flow through cooling tubes in the solenoid support bobbin. Additional cooling loops are included for lead and joint cooling and conductor stabilization. A helium refrigerator/liquefier rated at 2 kW and 20 gls will be specified to meet the refrigeration requirements. Cooldown of the magnet from 300 K to liquid nitrogen temperatures is accomplished using a counterflow helium-to- liquid-nitrogen heat exchanger independent of the helium refrigerator. The system incorporates provisions for maintenance access during accelerator beam operation.