Obtaining the Bunch Shape in a Linac from Beam Spectrum Measurements
In linacs with high single-bunch charge, and tight tolerances for energy spread and emittance growth, controlling the short-range wakefield effects becomes extremely important. The effects of the wakefields, in turn, depend on the bunch length and also on the bunch shape. It was shown in the linac of the Stanford Linear Collider (SLC), for example, that by shaping the bunch, the final rms energy spread could be greatly reduced, compared to for the standard Gaussian bunch shape[1]. Therefore, in machines with high single-bunch charge, a method of measuring bunch shape can be an important beam diagnostic. In a linac with low single-bunch charge, the longitudinal bunch shape can be obtained relatively easily from a single measurement of the beam's final energy spectrum, provided that the final to initial energy ratio is large. One merely shifts the average phase of the beam, so that it rides off-crest sufficiently to induce an energy variation that is monotonic with longitudinal position. Then, by knowing the initial and final energies, the rf wave number, and the average beam phase, one can directly map the spectrum into the bunch shape. In a linac with high single-bunch charge, however, due to the effect of the longitudinal wakefield, this method either does not work at all, or it requires such a large shift in beam phase as to become impractical. In earlier work[2],[3] it was shown that, even when wakefields are important, if one measures the final beam spectrum for two different (properly chosen) values of beam phase, then one can again obtain the bunch shape, and--as a by-product--also the form of the wakefield induced voltage; this method was then illustrated using data from the linac of the SLC. These SLC measurements, however, had been performed with the machine in a special configuration, where the current was low; in addition, the noise the data was low and the measured spectra were smooth distributions. Under normal SLC conditions, however, the currents were higher, and it was difficult to get the required separation in phase for the two measurements (the required separation increases with current); and the measured spectra were not smooth functions. Under such conditions, the above method works poorly or fails. If we know the Green function wake of the linac, however, we can still obtain the bunch shape from beam spectrum measurements. In this report, we present two such methods. One requires one spectrum measurement and involves the solution of a Volterra integral equation. The other requires a knowledge of upstream beam and transport properties and involves a least squares minimization to simulated spectra. We then apply these methods to data from the SLC.
- Research Organization:
- SLAC National Accelerator Lab., Menlo Park, CA (United States)
- Sponsoring Organization:
- USDOE Office of Energy Research (ER) (US)
- DOE Contract Number:
- AC03-76SF00515
- OSTI ID:
- 10202
- Report Number(s):
- SLAC-PUB-8118; TRN: US0103379
- Resource Relation:
- Other Information: PBD: 14 Apr 1999
- Country of Publication:
- United States
- Language:
- English
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