Superconducting final focus for the slac linear collider

Abstract

Triplets composed of superconducting quadrupoles have beer, built and installed as the final focusing element for the high-energy positron and electron beams of the SLAC Linear Collider. Special features include independent alignment to 100-micron tolerance inside a common cryostat; non-magnetic materials to allow operation inside the detector's solenoid field; a continuous-flow helium-only system using 50-meter-long flexible transfer lines; and complete operation of the system before installation. The mechanical design and cryogenic operation experience are presented. INTRODUCT,_ONThe Superconducting Final Focus was designed to solve several novel and difficult requirements posed by the experimental program at the SLAC Linear Collider. Machine optics require high-gradient (120 T/m ) precisely aligned magnetic quadrupoles to focus the 50 GeV/c electron and positron beams to few-microndiameter spols [1 ]. The new physics experiment for this facility, the SLC Large Detector (SLD), requires focusing elements that can work in the presence of its 6-Tesla solenoidal field and with as small an encumbrance of the detector area as possible. Finally, this new focusing system had to be made ultra reliable with ali controls remoted, be completely tested and aligned in final form offline, and then installed with as few changes as possible.These criteria guided the design of the focusing system as follows:.The high-gradient and extemal-field conditions were met by using superconducting quadrupoles with nonmagnetic materials. Precision alignment was achieved by pinning the magnet coils and laminations into stiff cylinders which formed part of the cryostat. These cylinders were adjusted from the outside through lowloss gimbals and supports while monitoring the field position through a probe in a warm bore on the magnet axis.The reliability, testing, and installation issues were solved by using simple pool-boiling helium-only cryostats continuously fed by flexible, low-loss transfer lines.A schematic overview of the system is shown in Figure 1. lt has been successfully operating at the SLC since early 1991. MAGNETS/CRYOSTATSEach of the three quadrupole magnets, which were designed and fabricated at Fermi National Laboratory [2], is precisely located, pinned and welded inside, close-toleranced, mechanically stiff, machined 304L stainless steel tubes (600/1200-mm long, 170-mm OD × 7.6-mm wall), which also form , part of the helium vessel. The three vessels/magnets are interconnected by 180-mm OD stainless steel i bellows which allow independent movement. The 44-mm OD x 1.5-mm cold bore is constructed similarly.Each helium vessel is fixed at its center by an epoxy composite gimbals, which locates and retains each vessel/magnet along the magnetic axis while allowing sufficient transverse freedom, (Fig. 2a). ._av a-)