Initial Observations of Micropulse Elongation of Electron Beams in a Scrf Accelerator*

Commissioning at the SCRF accelerator at the Fermilab Accelerator Science and Technology (FAST) Facility has included the implementation of a versatile bunch-length monitor located after the 4-dipole chicane bunch compressor for electron beam energies of 20-50 MeV and integrated charges in excess of 10 nC. The team has initially used a Hamamatsu C5680 synchroscan streak camera to assess the effects of space charge on the electron beam bunch lengths. An Al-coated Si screen was used to generate optical transition radiation (OTR) resulting from the beam’s interaction with the screen. The chicane bypass beamline allowed the measurements of the bunch length without the compression stage at the downstream beamline location using OTR and the streak camera. We have observed electron beam bunch lengths from 5 to 16 ps (sigma) for micropulse charges of 60 pC to 800 pC, respectively. We also report a compressed subps micropulse case. INTRODUCTION One of the more obvious effects of space-charge forces acting within micropulses in photoeinjectors and SCRF linacs is the elongation of the electron bunch compared to the initial drive laser bunch length [1,2]. During the initial 20-MeV commissioning run of the Fermilab Accelerator Science and Technology (FAST) facility [3], we took advantage of a 1.5-m drift between the photoinjector rf gun and the initial accelerating capture cavity CC2. Recently, another cavity, CC1, has been installed in this drift space so that the beam is accelerated to higher energies (~50 MeV) immediately following the photoelectric injector and gun diagnostics station [4]. We now have a direct comparison available of the observed electron beam bunch lengths for different micropulse charges and the two accelerator configurations: both with the full 1.5 m drift and with the much shorter drift following installation of the additional ~1 m of accelerating structure. The chicane bypass beamline allowed the measurements of the bunch length without the compression stage at the downstream beamline location using OTR and the streak camera. The UV component of the drive laser had previously been characterized with a Gaussian fit sigma of 3.5-3.7 ps. However, the uncompressed electron beam was observed to elongate as expected due to space-charge forces in the 1.5-m drift from the gun to the first SCRF accelerator cavity in this initial configuration. We also report our results with the CC1 cavity installed. A preliminary ASTRA-Elegant prediction is noted. Finally, we report generation of sub-ps micropulses at FAST for the first time using the 4-dipole bunch compressor (chicane, see Fig. 1). EXPERIMENTAL ASPECTS Two main aspects of the experiment are the injector as the source of the electrons in a 3-MHz pulse train and the Hamamatsu C5680 streak camera configured with the 81.25 MHz synchroscan plugin unit. The beam generates the OTR at the X121 converter screen These topics will be discussed in this section. The Injector Linac The high-power electron beams for the FAST facility [3] are generated in a photoelectric injector (PEI) based on a UV drive laser and the L-band rf photocathode (PC) gun cavity. The PEI drive laser is comprised of multiple stages including a Calmar Yb fiber oscillator and amplifier, several YLF-based amplification stages, a final Northrup Grumman IR amplification stage, and two frequency-doubling stages that result in a UV component at 263 nm with a nominal 3-MHz micropulse bunch structure [5]. The UV component is transported from the laser lab through the UV transport line to the photocathode of the gun for generation of the photoelectron beams for use in the SC rf accelerator. The low-energy section of the facility and part of the first cryomodule are schematically shown in Fig. 1. After the L-band rf PC gun, the beam is accelerated through two L-band superconducting cavities resulting in a beam energy of up to 50 MeV, though initially this was limited to 20 MeV due to the absence of CC1. We will report the bunch length elongation observed downstream for both configurations. The Streak Camera System The linac streak camera consists of a Hamamatsu C5680 mainframe with S20 PC streak tube and can accommodate a vertical sweep plugin unit and either a horizontal sweep unit or blanking unit. The UV-visible input optics allow the assessment of broadband OTR. A M5675 synchroscan unit with its resonant circuit tuned to 81.25 MHz from the Master Oscillator (MO) and a M5679 horizontal sweep unit were used for these studies. The low-level rf is amplified in the camera to provide a sine wave deflection voltage for the vertical plates that results in low jitter (~1ps) of the streak camera images and allows for synchronous summing of a pulse train of OTR. The temporal resolution is about 2.0 ps (FWHM), or 0.8 ps (sigma), for NIR photons at 800 nm. When combined with the C6878 phase locked loop (PLL) delay box we can track phase effects at the ps level over several ___________________________________________ * This work was supported by the DOE contract No.DEAC0207CH11359 to the Fermi Research Alliance LLC. # [email protected] This manuscript has been authored by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the U.S. Department of Energy, Office of Science, Office of High Energy Physics. FERMILAB-CONF-16-718-AD