The Next Linear Collider Machine Protection System

The Next Linear Collider (NLC) electron and positron beams are capable of damaging the linac accelerating structure and beamline vacuum chambers during an individual aberrant accelerator pulse. Machine protection system (MPS) considerations, outlined in this paper for the 1 TeV NLC design, have an impact on the engineering and design of most machine components downstream of the damping ring injector complex. The MPS consists of two functional levels. The first level provides a benign, single bunch, low intensity, high emittance pilot beam that will be used for commissioning and also whenever the integrity or the settings of the downstream components are in doubt. This level also provides for the smooth transition back and forth between high power operation and the benign diagnostic pilot bunch operation. The pilot bunch parameters in the main linac are estimated on the basis of the expected stress in the accelerator structure copper. Beam tests have been done at the SLAC linac to examine the behavior of the copper at the damage stress threshold. Typical pilot beam parameters (compared with nominal) are: 10 times reduced intensity, 10 times increased horizontal emittance and 1000 times increased vertical emittance, resulting in a reduction in charge density of 105. The second level is the primary protection against a single aberrant pulse. Its goal is to reduce the possibility that a substantial transverse field changes the trajectory of the high power beam from one pulse to the next. All devices that could produce such a field are 1) monitored by a fast response network and where possible have 2) deliberately slowed response times. A ‘maximum allowable interpulse difference ’ is evaluated for each such device as well as the beam trajectory monitors in each interpulse period.