Partial design of a 980-MeV Energy Recovery Linac (ERL)

Abstract

We describe the partial design of a 980-MeV energy recovery linac (ERL) with radiofrequency (RF) of 1.5 GHz. We model the linac, recirculation arcs, beam spreader/combiner, and beam compression. The electron gun, gunline, and beam dump are not modeled. We consider a one-up/one-down design in which 20-MeV bunches are accelerated and decelerated by a 960-MeV superconducting linac. We also consider a two-up/two-down design in which 20-MeV bunches are accelerated by two passages through a 480-MeV superconducting linac and decelerated by two subsequent passages. For both designs, which incorporate graded-gradient linac focusing with cavity gradients of 15 MeV/m, the beam breakup (BBU) thresholds exceed 100 mA. In the two-up/two-down design, we achieved a large degree of longitudinal compression in both recirculation arcs by accelerating and decelerating off-crest in the linac. For low current operation with normalized emittances in both transverse directions of 0.1 mm-mrad, this high-compression design was studied by tracking without consideration of synchrotron radiation using two codes. In tracking with the MAD-with-acceleration code, bunches with initial rms bunchlength σ = t 1.85 ps are compressed to 22 fs in the 500-MeV arc, to 15 fs in the 980-MeV arc, and to 32 fs in the second passage through the 500-MeV arc. The compressed bunch transverse dimensions are slightly larger than those given by conservation of the normalized emittance. In tracking with the elegant code, bunches with initial rms bunchlength σ = t 1.85 ps are compressed to 8.5 fs in the 500-MeV arc, to 9.2 fs in the 980-MeV arc, and to 48 fs in the second passage through the 500-MeV arc. Incoherent synchrotron radiation (ISR) was then included in tracking with elegant, indicating a slight increase in the compressed bunch lengths and horizontal emittance. The effect of coherent synchrotron radiation (CSR) was also studied by tracking with elegant. For bunch charge of 1 pC (corresponding to average ERL current of 1.5 mA), the CSR has a small effect, while for bunch charges of 2 and 4 pC, the longitudinal bunch compression and horizontal focusing are both significantly worsened by CSR. For isochronous transport at 500 MeV and high compression (factor of ~180) at 980 MeV, bunch charges ≤ 2 pC suffered little degradation from CSR. With isochronous transport in both recirculation arcs, bunch charges ≤ 10 pC suffered little degradation from CSR. The tracking results suggest that average ring currents in the tens of milliamperes will be strongly affected by CSR, degrading high-current operation. For ring currents of several mA, high performance operation with a large degree of longitudinal compression is expected from our design.