New multiGauss distribution

Summary

In the cathode at the Argonne Wakefield Accelerator they have the ability to produce microbunched electron bunches, thanks to a laser beam which has a train of Gaussian pulses. Temporal Laser Pulse Shaping for RF Photocathode Guns

At the moment there is no OPAL distribution to reproduce this.

Here is an example with 4 Gaussian microbunches separated (peak-to-peak) by 1.26 mm:

Dist: DISTRIBUTION, TYPE = MULTIGAUSS,                                                                                                                 
      SIGMAPX = 1e-2, SIGMAPY = 1e-2, SIGMAPZ = 1e-2,  // In units of betaGamma                                                                        
      CUTOFFPX = 4.0, CUTOFFPY = 4.0, CUTOFFPZ = 4.0,  // In units of SIGMAP                                                                           
      SIGMAR = 340e-6,                                                                                                                                 
      SIGMAZ = .9e-3 / 2.355, // FWHM  = 2.355 * sigma                                                                                          
      CUTOFFLONG = 4.0,  // In units of SIGZ                                                                                                           
      SEPPEAKS = 1.26e-3,                                                                                                
      NPEAKS = 4,                                                                                                                                      
      EMITTED = FALSE;

This is what it looks like:

In the case where the bunch is emitted, SIGMAPX/Y/Z and CUTOFFPX/Y/Z are omitted, and SIGMAZ and SEPPEAKS are to be given in seconds.