Wire breakage in SLC wire profile monitors

Wire-scanning beam profile monitors are used at the Stanford Linear Collider (SLC) for emittance preservation control and beam optics optimization. Twenty such scanners have proven most useful for this purpose and have performed a total of 1.5 million scans in the 4 to 6 years since their installation. Most of the essential scanners are equipped with 20 to 40 μm tungsten wires. SLC bunch intensities and sizes often exceed 2〈107 particles/μm2 (3C/m2). We believe that this has caused a number of tungsten wire failures that appear at the ends of the wire, near the wire support points, after a few hundred scans are accumulated. Carbon fibers, also widely used at SLAC (1), have been substituted in several scanners and have performed well. In this paper, we present theories for the wire failure mechanism and techniques learned in reducing the failures. SCANNER OPERATION Wire-scanning beam profile monitors (or wire scanners) are used throughout the SLC for beam size monitoring and optimization (2). A typical scan takes about 1 second, with the wire actually within the beam envelope for about 20% of the pulses that occur during that time. Two million scans have been done during the roughly 40 months of SLC operating time elapsed since most of the scanners were installed, averaging about 1 scan/minute. Approximately 0.3% of all SLC pulses have intercepted a wire, illustrating the utility of such phase space monitors in the linear collider. The SLC is a prototype linear collider and our ability to control emittance propagation has developed as both the beam size monitors and the tools to effectively use them have developed. Four groups of wire scanners have proven most useful: 1) at the exit of the damping ring, 2) following the bunch length compressor at the entrance of the linac (RTL - S2), 3) near the end of the linac (S28), and 4) at the entrance to the final focus (FF), following the SLC arcs about 100 m from the IP. The 10 FF wires, installed as part of an FF upgrade (3) in 1994, are different from the others since they