Design, specifications, and first beam measurements of the compact linear accelerator for research and applications front end

The compact linear accelerator for research and applications (CLARA) is an ultrabright electron beam test facility being developed at STFC Daresbury Laboratory. The ultimate aim of CLARA is to test advanced free electron laser (FEL) schemes that can later be implemented on existing and future short-wavelength FELs. In addition, CLARA is a unique facility to provide a high-quality electron beam to test novel concepts and ideas in a wide range of disciplines and to function as a technology demonstrator for a future United Kingdom x-ray FEL facility. CLARA is being built in three phases; the first phase, or front end (FE), comprises an S-band rf photoinjector, a linac, and an S-bend merging with the existing versatile electron linear accelerator beam line; the second phase will complete the acceleration to full beam energy of 250 MeV and also incorporate a separate beam line for use of electrons at 250 MeV; and the third phase will include the FEL section. The CLARA FE was commissioned during 2018, and the facility was later made available for user experiments. Significant advancements have been made in developing high-level software and a simulation framework for start-to-end simulations. The high-level software has been successfully used for unmanned rf conditioning and for characterization of the electron beam. This paper describes the design of the CLARA FE, performance of technical systems, high-level software developments, preliminary results of measured beam parameters, and plans for improvements and upgrades.

This article appears in the following collection:

Figure 1

Schematic of the full CLARA facility showing different phases in which CLARA is being built and the low-energy beam line (merging with VELA) as well as high-energy (FEBE) beam line. The total length of the facility is $\approx 90\mathrm{m}$.

Figure 2

Schematic of the CLARA FE and VELA. The S bend transports the beam with a momentum up to $50\mathrm{MeV}/\mathrm{c}$ from the CLARA FE to be delivered to beam areas at the end of the VELA line.

Figure 3

Schematic of BA1.

Figure 4

Velocity bunching as a function of distance (s) from the cathode with a $7.5\mathrm{MV}/\mathrm{m}$ buncher cavity of length 1 (red line), 2 (green line), and 3 m (blue line).

Figure 5

Transverse Twiss parameters for the CLARA to BA1 beam line, showing the horizontal and vertical Beta function along with the horizontal dispersion. The BA1 interaction point is at $\approx 26\mathrm{m}$ in this plot.

Figure 6

Transverse normalized emittances and longitudinal momentum spread in the CLARA to BA1 beam line. These parameters have been calculated using elegant with an off-crest linac phase of $-10°$ (maximum compression) and including longitudinal space-charge and CSR effects. The horizontal emittance has been truncated at $5\mu \mathrm{m}\mathrm{rad}$.

Figure 7

Comparison of the simulated longitudinal phase space at the BA1 interaction point between astra, elegant, and gpt for various linac 1 off-crest phases and with space-charge forces included but no CSR effects. Maximum compression was experimentally measured around $-12°$ off crest in linac 1. In all simulations, we utilize the same astra-based photoinjector model.

Figure 8

Comparison of the simulated longitudinal phase space at the BA1 interaction point between astra, elegant, and gpt for various linac 1 off-crest phases, with both space-charge forces and CSR effects included in elegant and gpt. The astra model does not feature CSR.

Figure 9

Summary of the simulated beam properties at the CLARA FE straight on for a 70 pC bunch in two parameter regimes.

Figure 10

Multiparameter plot at the exit of the linac comparing various transverse and longitudinal beam parameters as a function of both gun solenoid settings and linac phases. The block color coding indicates scans at a constant linac phase but varying gun solenoid strength. At all solenoid settings, the bucking solenoid is set to cancel the magnetic field at the cathode.

Figure 11

rf design and cst [32] simulation of the 2.5-cell 10 Hz photoinjector.

Figure 12

The 2.5-cell photoinjector cavity before installation. Inset: The diamond turned copper photocathode used during operation at CLARA.

Figure 13

(a) Design of the cavity and focusing solenoids of the 10 Hz photoinjector upgrade. (b) Design of the photocathode socket of the 10 Hz photoinjector upgrade.

Figure 14

Cavity design of the 400 Hz photoinjector.

Figure 15

Schematic of the photocathode transport system.

Figure 16

Simulated temperature profile at the photoinjector cavity surface.

Figure 17

Frequency of the 400 Hz photoinjector at each tuning step before brazing.

Figure 18

Field flatness in the 400 Hz photoinjector after brazing. Blue dots represent the measurement; the red line is a fit that removes the effect of image charges of the bead on the cathode.

Figure 19

400 Hz photoinjector installed on the VELA beam line before the waveguide, cathode exchange system, and solenoids were installed.

Figure 20

A section of 400 Hz photoinjector conditioning data showing scaled gradient and cumulative breakdowns.

Figure 21

SEM image of a diamond occlusion in the copper photocathode surface.

Figure 22

(a) Picture of a used copper photocathode showing laser-induced damage in the active area at the center. (b) Optical microscopy image of the central damaged region.

Figure 23

Measured phase at the linac probe and low-level rf (LLRF) output phase with phase feed-forward on (left) and off (right). The fast noise in the probe signal phase is the rf phase jitter, which is discussed in Sec. 5i.

Figure 24

Normalized on-axis fields through the cavity of the components that contribute to the beam deflection—$\mathrm{Re}[E_y]$ (red curve) and $\mathrm{Im}[H_x]$ (blue curve).

Figure 25

The effect of the metal bead on the square root of the normalized frequency error in the reflected low-power rf signal as it travels through the cavity. As there is a zero on-axis E field in the middle of the cell, this quantity is a sufficient analog of the H field to determine the field flatness.

Figure 26

Cross-calibration measurement of the bunch charge between the CLARA WCM, the straight-on FC (S02-FC), and the VELA spectrometer FC (SP1-FC) as a function of the photoinjector laser energy.

Figure 27

C2V BPM and camera position, and summed pixel intensity, as a function of the dipole current. The reliable region for the BPMs is reached above a dipole current of 66 A; below this value, the BPM readings are unreliable due to a saturated horizontal pickup.

Figure 28

Charge readings from three CLARA BPMs and two charge diagnostic devices as a function of the PI laser pulse energy. Below a bunch charge of $\approx 25\mathrm{pC}$, the BPM readings are no longer reliable.

Figure 29

Schematic of the connections between the physical and virtual accelerators, the low-, mid-, and high-level control interfaces, and the machine’s data stores.

Figure 30

Bunch charge (pC) as a function of the $x$, $y$ laser position on the photocathode. The region displayed is the central field region of the photocathode.

Figure 31

Electron beam steering on first screen (gray tone) as a function of the laser $x$, $y$ position (mm) on the virtual cathode. Darker tone indicates smaller steering. The radial laser beam size (shaped by aperture) was 0.25 mm (rms).

Figure 32

A comparison of plots of on-crest momentum versus power for the 2.5-cell gun in both (a)VELA and (b) CLARA FE positions.

Figure 33

Momentum from the 10 Hz photoinjector as a function of the rf phase relative to crest for the gun in the CLARA position.

Figure 34

Momentum spread versus phase (from crest) for the gun in the CLARA position with the linac switched on at a constant phase and power level.

Figure 35

Measured momentum as a function of the linac power.

Figure 36

Momentum spread versus rf phase for various bunch charges.

Figure 37

Results from phase space tomography in two d.o.f. in the CLARA FE. Each plot shows a different projection of the charge density in four-dimensional phase space, using normalized phase space variables. Coupling in the beam is evident in the tilt of the charge distribution in the cases that the axes refer to different d.o.f. The black ellipses shown in the horizontal and vertical phase spaces show the rms of a Gaussian distribution fitted to the projections of the four-dimensional distribution onto these phase spaces: The emittances and Courant-Snyder parameters (neglecting coupling) can be estimated from the sizes and shapes of these ellipses.

Figure 38

Validation of results from phase space tomography in two d.o.f. in the CLARA FE. The plot at the top right shows the beam image observed directly on YAG-02; the plot at the bottom left shows the charge distribution in coordinate space reconstructed from phase space tomography in two d.o.f. (a projection of the four-dimensional phase space onto the coordinate plane). The plots at the top left and bottom right show the charge density projected onto the horizontal and vertical axes, respectively: Black lines are from the tomography analysis, and red lines are from the directly observed image.

Figure 39

FWHM energy spread as a function of the linac off-crest phase: Red dots, after the linac as measured in the S-bend spectrometer (C2V); black dots, as measured in the BA1 spectrometer. The legend refers to YAG screens located after the dipole magnets at these two locations.

Figure 40

CTR signal dependence on the linac off-crest phase. Measurements were made in BA1.

Figure 41

Data and fit from a typical linac phase scan.

Figure 42

Energy modulation imposed on the beam energy spectrum with DLW structure (red line) and without (black line) at two linac off-crest phases: $-15°$ (a) and $-22°$ (b), respectively.

Figure 43

Bunch length in BA1 measured at 10% of the peak level as a function of the linac off-crest phase: black dots, by using the energy modulation technique; green dots, by CTR diagnostic; red dots, elegant simulations shifted toward the crest by 2°.

Figure 44

Variation in output rf power, phase, and beam momentum with modulator cathode voltage.

Figure 45

Slower drifts in photoinjector power and amplitude (normalized) and temperature, and the associated variation in the beam momentum. The data were smoothed using a moving Gaussian window with 2 s width.