Loss Factor of Tapered Structures for Short Bunches
Using the electromagnetic simulation code ECHO, we have found a simple phenomenological formula that accurately describes the loss factor for short bunches traversing an axisymmetric tapered collimator. In this paper, we consider tapered collimators with rectangular cross-section and use the GdfidL code to calculate the loss factor dependence on the geometric parameters for short bunches. The results for both axisymmetric and rectangular collimators are discussed. The behaviour of the impedance of tapered structures for very short bunches in the optical regime has been determined in refs. [10,11]. Here, for the loss factors for two particular geometries, we have studied the departure from the optical regime behaviour as bunch length is increased. In both cases, the ratio of the loss factor for the tapered collimator to the loss factor in the optical regime is a function only of the scaling parameter {sigma}L/d{sup 2}. The fact that the bunch length a and the taper length L appear as a product is consistent with the recent scaling derived by Stupakov in ref. [12], since there is only a weak dependence on g. One noteworthy fact that is not a priori expected is that only the larger radius or vertical half-aperture d appears. The reduction factor is independent of b. Moreover, it is striking that the specific form involving the arctan given in Eq. (5) holds for both geometries, with only the coefficient {mu} differing by a factor of {approx}2 for flat vs round. This suggests that there may be a useful phenomenological form for more general geometries which may follow from natural extensions of Eq. (5). This possibility is presently being investigated.
- Research Organization:
- Brookhaven National Lab. (BNL), Upton, NY (United States). National Synchrotron Light Source
- Sponsoring Organization:
- DOE - Office Of Science
- DOE Contract Number:
- DE-AC02-98CH10886
- OSTI ID:
- 1012577
- Report Number(s):
- BNL-94855-2011-CP; 39KC02000; TRN: US1102319
- Resource Relation:
- Conference: PAC 2011; New York, NY; 20110328 through 20110401
- Country of Publication:
- United States
- Language:
- English
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