Specification and design for full energy beam exploitation of the compact linear accelerator for research and applications

The compact linear accelerator for research and applications (CLARA) is a 250 MeV ultrabright electron beam test facility at STFC Daresbury Laboratory. A user beamline has been designed to maximize the exploitation of CLARA in a variety of fields, including novel acceleration and new modalities of radiotherapy. In this paper, we present the specification and design of this beamline for full energy beam exploitation. We outline the key elements that will provide users access to ultrashort, low emittance electron bunches in two large experiment chambers. The results of start-to-end simulations are reported that verify the expected beam parameters delivered to these chambers. Key technical systems are detailed, including those which facilitate a combination of electron bunches with high-power laser pulses.

Figure 1

Schematic of the CLARA linear accelerator test facility, including the FEBE beamline, shielded FEBE hutch, 100 TW laser system, and space reserved for potential future applications (shaded blue area).

Figure 2

Schematic of the new beamline for full energy beam exploitation (FEBE), including arc (connecting to the upstream CLARA main beamline, Fig. 1), experiment hutch, and posthutch beam dump with energy and emittance diagnostics.

Figure 3

Left: photograph of the mask to be installed in FEBE for variable longitudinal shaping. Right: simulated 250 pC longitudinal profile at the FEBE FEC IP using the bottom hole of the mask, including triangular drive bunch ($\sim 1\mathrm{ps}\mathrm{rms}$) and four trailing bunches ($\sim 100\mathrm{fs}\mathrm{rms}$ duration with 1 ps separation).

Figure 4

Twiss parameters for the full FEBE beamline starting from the first dipole in the FEBE arc.

Figure 5

Charge densities and cumulative probability function for an optimized 250 pC solution given at the FEBE FEC1 IP for (a) the transverse horizontal, (b) vertical, and (c) longitudinal planes. Beam parameters extracted from the density functions are presented.

Figure 6

Semianalytic model predictions of the (a) microbunching gain and (b) energy modulation amplitude as a function of initial modulation wavelength (${\lambda }_0$) after compression in both the VBC and the FEBE arc, with and without the energy spread added by intrabeam scattering (IBS).

Figure 7

Engineering diagram of FEBE beamline, highlighting areas with diagnostic systems undergoing research and development. These areas are supported by standard beam diagnostics including Ce:YAG screens, beam position monitors, and integrated current transformers.

Figure 8

Plot of the predicted trajectories of electrons in the full energy range through the 1-m-long five-section spectrometer. $z=0$ is located at the spectrometer midpoint, $y=0$ is the beam entry height. The region of the magnetic flux is shown in blue between $\pm 500\mathrm{mm}$; trajectories for 2 GeV (solid blue), 600 MeV (orange dashed), 250 MeV (green dotted), and 50 MeV (red dash-dot) are plotted.

Figure 9

Simulated average resolution of the DLW streaker at 250 pC for varying bunch length ${\sigma }_t$ for Gaussian (red lines) and flat top (black) profiles, where flat top has a total length of $4{\sigma }_t$. The improvement in resolution from using a pair of orthogonally oriented structures (solid lines) as compared to a single structure (dashed) is shown.

Figure 10

Laser transport within the FEBE hutch, with electrons from CLARA traveling FMBOX1-FEC-FMBOX2 (left to right). The laser is brought to a common focus with the electron beam at IP1. OAP: off-axis parabola; IP: interaction point; DM: deformable mirror; WFS: wavefront sensor; and LD: laser dump.

Figure 11

(a) Gas pressure profile for hydrogen and argon and (b) normalized vertical emittance and vertical beta functions $\pm 10\mathrm{m}$ from the FEC1 IP (located at $s=0\mathrm{m}$). Base gas pressure of 0.01 mbar at the IP is assumed, with a pressure reduction to ${10}^{-6}\mathrm{mbar}$ at either end of the vacuum line.

Figure 12

Demonstration of solutions to deliver arbitrary photoinjector laser temporal profiles via spectral phase manipulation, comparing target (red dashed lines), and predicted (black solid) profiles. Results for different profiles are compared, including: (a) double ramp, (b) triple flat top, (c) cut sine wave, and (d) triangular.