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Abstract:

The small vertical spot size at the International Linear Collider (ILC) will make the luminosity very sensitive to ground motion and facilities noise. Three generations of prototype feedback system have been developed and tested to correct for the relative beam misalignment caused by this. These systems were based on fast analogue beam position monitor (BPM) processors aimed at correcting the beam within the short (~ 270 ns) bunch crossing of the warm (X-band) machine. Future generations of feedback prototype will be aimed towards the cold (superconducting) ILC design, where a much longer bunch train of ~ 1 ms would allow the use of a digital processor to employ more sophisticated feedback algorithms. Details and results from the first three prototype systems are discussed and the development of a digital feedback processor for future tests is described. Plans to irradiate a BPM with background particles, of similar flux to that expected in the ILC interaction region, are also discussed.

Abstract:

We report the first comprehensive study of large amplitude Langmuir waves in a plasma of nanometer-scale clusters. Using an oblique angle single-shot frequency domain holography diagnostic, the shape of these wakefields is captured for the first time. The wavefronts are observed to curve backwards, in contrast to the forwards curvature of wakefields in uniform plasma. Due to the expansion of the clusters, the first wakefield period is longer than those trailing it. The features of the data are well described by fully relativistic two-dimensional particle-in-cell simulations and by a quasianalytic solution for a one-dimensional, nonlinear wakefield in a cluster plasma.