A test of NLC-type beam loading in the SLAC linac

The linac at the Stanford Linear Accelerator (SLAC) runs routinely with a beam loading of around 12% for the fixed target experiment E-158. Typical energy spread and energy jitter are 0.1% and 0.05%. To explore the conditions for the Next Linear Collider (NLC) the linac was operated with 20% beam loading. This was attained by increasing the beam charge from 5·10¹¹ to 9·10¹¹ particles and increasing the pulse length from 250 ns to 320 ns. Although the beam loading compensation was more difficult to achieve, a reliable operating point was found with a similar energy spread and energy jitter as at the lower loading. Furthermore, using the subharmonic buncher (SHB), the beam was bunched at 178.5 MHz instead of the nominal 2.8 GHz so that the charge from 16 adjacent buckets was combined into one. Increased transverse instability and beam losses along the linac were observed indicating the possible onset of beam break-up.