Longitudinal transport measurements in an energy recovery accelerator with triple bend achromat arcs

Longitudinal properties of electron bunches (energy spread and bunch length) and their manipulation are of importance in free electron lasers (FELs), where magnetic bunch length compression is a common feature of beam transport. Recirculating accelerators and energy recovery linac accelerators (ERLs) have been used as FEL drivers for several decades and control of longitudinal beam transport is particularly important in their magnet lattices. We report on measurements of longitudinal transport properties in an ERL-FEL, the ALICE (Accelerators and Lasers in Combined Experiments) accelerator at Daresbury Laboratory. ALICE is an energy recovery research accelerator that drives an infrared free electron laser. By measuring the time of arrival of electron bunches, the canonical longitudinal transport quantities were measured in the beam transport and bunch compression sections of the lattice. ALICE includes a four-dipole bunch compression chicane providing fixed longitudinal transport, and triple bend achromat arcs including sextupole magnets where the first and second order longitudinal transport can be adjusted. The longitudinal transport properties in these lattice sections were measured and compared with the theoretical model of the lattice. A reasonable level of agreement has been found. The effect of sextupoles in second order, as well as first order, longitudinal correction is considered, with the measurements indicating the level of alignment of the beam to the center of the sextupole.

Figure 1

Layout of the ALICE energy recovery linac accelerator (ERL) showing locations A (main linac entrance), B (main linac exit), C (first arc exit) and D (compression chicane exit).

Figure 2

Relative path length vs beam momentum from linac exit (location B in Fig. 1) to arc exit (location C in Fig. 1) and chicane exit (location D in Fig. 1). Points are measurements, lines are parabolic fits. The colors represent different quadrupole strengths in the first arc, the values of which are indicated in Fig. 3. The nominal ALICE beam momentum is $26.5\mathrm{MeV}/c$.

Figure 3

$R_{56}$ from linac exit to arc exit (red) and from linac exit to chicane exit (black) vs arc quadrupole strength. The dashed lines show the theoretical predictions and the points show the measurements. The reference momentum is $26.5\mathrm{MeV}/c$. The error bars are the errors from the parabolic fits used to extract the $R_{56}$. The faint dashed vertical and horizontal lines indicate the theoretical closed arc condition where the arc is both isochronous and achromatic.

Figure 4

$T_{566}$ from linac exit to arc exit (red) and from linac exit to chicane exit (black) vs arc quadrupole strength. The dashed lines show the theoretical predictions.

Figure 5

Relative path length from linac exit to arc exit vs beam momentum, with the first sextupole at zero strength (black line) or at maximum field strength (colored lines). The different colors indicate data taken with different strengths of the first arc dipole, i.e. different beam steering through the first sextupole.

Figure 6

Change in $R_{56}$ from linac exit to arc exit (at the reference momentum of $26.5\mathrm{MeV}/c$) resulting from increasing the strength of the first arc sextupole from zero to full strength, as a function of the first arc dipole field strength. The points are the data at the three dipole values used in the measurement, the black dashed line is the line of best fit. The dipole strength for the trajectory that passes through the center of the sextupole (in which case there is no change in $R_{56}$ in response to changes in sextupole strength) is indicated by the blue dashed line.