Design and wakefield performance of the new SLC collimators

The very small transverse beam sizes of the flat SLC bunches are 100-170 μm in the horizontal and 30-50 μm in the vertical near the end of the SLAC linac. Unexpectedly large transverse Wakefield kicks were observed from the collimators in this region during 1995. Upon inspection, it was found that the 20 μm gold plating had melted and formed a line of spherules along the beam path. To refurbish the collimators, an improved design was required. The challenging task was to find a surface material with better conductivity than the titanium core to reduce resistive wakefields. The material must also be able to sustain the mechanical stress and heating from beam losses without damage. Vanadium was first chosen for ease of coating, but later TiN was used because it is more chemically inert. Recent beam tests measured expected values for geometric Wakefield kicks, but the resistive wall Wakefield kicks were four times larger than calculated