Velocity Bunching Experiment at the Neptune Laboratory

In this paper we describe the ballistic bunching compression experiment at the Neptune photoinjector at UCLA. We have compressed the beam by chirping the beam energy spectrum in a short S-band high gradient standing wave RF cavity and then letting the electrons undergo velocity compression in the subsequent rectilinear drift. Using a standard Martin Puplett interferometer for coherent transition radiation measurement, we have observed bunch length as short as 0.4 ps with compression ratio in excess of 10 for an electron beam of 7 MeV and charge up to 0.3 nC. We also measured slice transverse emittance via quad scan technique. The observed emittance growth agrees with the predictions and the simulations. Extension of this scheme to a future advanced accelerator injector system where solenoidal magnetic field can compensate the emittance growth is studied. INTRODUCTION In recent years electron beam users have increased their demands for high brightness beam in short sub-ps pulses [1-3]. Applications in the advanced accelerator community like the injection into short wavelength advanced accelerators, or driving a plasma wakefield experiment, and in the light source community like driving a short wavelength SASE Free Electron Laser or Thompson-scattering generation of short Xray pulses, demand high brightness very short electron beam. Recent designs of such systems include the use of conventional photoinjectors in conjunction with magnetic compressors [4]. While the magnetic compression scheme has been proved successful in increasing the beam current, the impact on the beam phase space has been shown to be quite relevant: performing the compression at low energy [5], space charge forces are still very significant and their emittance-damaging effect becomes especially important in bending trajectories, in the case of compression at higher energy [6], one has to deal with the deleterious effects on the longitudinal as well as the transverse phase space of Coherent Synchrotron Radiation. Phase space filamentation and in general emittance growth jeopardize the goal of achieving the high brightness. An alternative scheme that could preserve the phase space quality has been recently proposed to supply electron beams with the brightness required by the applications. In the context of an injector for X-ray Free Electron Laser, Serafini and Ferrario [7] proposed to use the old idea of RF rectilinear compression. More generally, in every application in which compression at low energy is required, it seems that velocity bunching is an efficient alternative to magnetic compression. The idea is based on the CP647, Advanced Accelerator Concepts: Tenth Workshop, edited by C. E. Clayton and P. Muggli © 2002 American Institute of Physics 0-7354-0102-0/02/$19.00 858 weak synchrotron motion that the beam undergoes at moderate energies in the RF wave of the linac accelerating structure. The compression happens in a rectilinear section so that the damage suffered by going through bending trajectories is avoided. A main ingredient of this recipe to produce high brightness sub-ps electron beam is to integrate this compression section in the emittance compensation scheme, by keeping the transverse beam size under control through solenoidal magnetic field in the region where the bunch is compressing and the electron density is increasing. Another possibility is a thin lens version of velocity bunching. Here the synchrotron motion inside the RF structure is very limited. There is almost no phase advance inside the longitudinal lens and all the bunching happens in the drift following the linac. In this paper we experimentally studied this configuration. At the Neptune photoinjector at UCLA there is a 1.6 cell gun and a PWT standing wave linac that could be used to test this idea. In the next section we draw the schematics of the experiment, and show the results. We measured the bunch length by using the Coherent Transition Radiation technique. After observing a good longitudinal compression we turn our attention to the transverse dynamics. The big energy spread on the beam makes it impossible to measure projected emittance so that we had to concentrate on slice emittance. A 45 degrees dispersing dipole is used to select the central slice of the beam and as the beam compresses it is clear that the emittance grows. Simulations agree with this observation. It is important to note that the beamline at the Neptune photoinjector is not optimized for this experiment, in the sense that no solenoidal magnetic fields are present to match the increasing spacecharge forces and there is no post acceleration to remove the induced energy spread. We also studied a system optimized for the ballistic bunching compression, the proposed injector for the Orion Research facility [8]. Here the solenoids wrapped around the accelerator should keep the beam under control and the simulations show the high brightness of the output beam. NEPTUNE EXPERIMENT The Neptune facility at UCLA currently operates as an injector for a plasma beatwave advanced accelerator experiment. At the same time the Neptune photoinjector is being used for pure high brightness beam dynamics studies like emittance growth in bends [5] and negative R56 compressors [9]. The accelerator can be tune up for ballistic compression. A 266 nm 12 ps FWHM long laser pulse hits a single crystal copper cathode inside a 1.6 cell BNL-SLAC-UCLA RF gun. The photoelectrons generated are then accelerated by the RF fields and go through the emittance compensation solenoid. At this point the beam can be energy chirped inside a 6+2 Vi cell S-band PWT RF cavity. There is the capability of controlling independently the phases of the two accelerating structures allowing us to test the ballistic bunching idea. Downstream of the linac an aluminum foil can be inserted and the transition radiation generated is collected by a parabolic mirror and reflected to a Martin Puplett autocorrelator for pulse length diagnostic. There are also 4 chicane dipoles along the beamline and they can be turned on in the 45 degrees dipole mode in order to select a slice of the beam of which