A. Definition and Overview of Solution

B. Transport to the Wiggler

C. Energy Recovery Transport

D. Component Summary

1. The dipole end field rolloff integral value has been set to K₁=0.27; this is based on magnetic field measurements of a prototype.
2. A short slot (~20 cm) for a cavity monitor has been added after the cryomodule, just upstream of energy recovery dump dipole.
3. A reinjection match has been added to support a potential 75 MeV upgrade.
4. Injection line mismatch present in earlier versions has been fixed by Yunn by rematching with equally spaced quads.
5. The transport system injection point to reinjection point path length is set to 40*13-18.5=501.5 rf wavelengths, so as to support multipass monitoring.
6. The main dipole powering and shunting scheme has been modified to support first light operation and to reflect the most recently developed requirements on power supply stability. The two optical cavity chicanes are on a single string, with the second chicane shunted by up to 1% (software limited) below the first. The recirculation are is is on a single string, with the pi-bends shunted by up to 3% (and nominally operating at 1.5%) below the reverse bends.

Match to Wiggler Following the cryomodule, a six quadrupole telescope (comprising two quadrupole triplets) is used to betatron match into the wiggler. A four dipole "optical cavity chicane" embedded between the triplets provides mechanical clearance between the FEL optical cavity and the electron beam, and generates momentum compaction for the compression of the electron bunch, which, as noted previously, has been accelerated off crest. The chicane geometry is limited by the allowable range of momentum compactions. Larger footprints (and compactions) give more room for the optical cavity and allow more bunch compression, but lead in turn to very stringent RF stability requirements so as to limit variations in bunch time of arrival at the wiggler and maintain optical/electron beam pulse synchronism within the FEL.

As noted on the " Designing a High Power FEL Driver" page, space charge has little effect at the machine final energy. It does, however, affect the injector through the cryomodule transport. The injection matching telescope is therefore adjusted to provide appropriate matching into the module for each operating current; the cryomodule to wiggler match is similarly adjusted to compensate for space-charge induced variations in the beam phase space at the end of the module. At the present time, a single optical solution for the gun to wiggler transport appropriate for all current levels and bunch charge scenarios has not been found, nor has a current (beam-power) ramp scenario been developed that would allow for real-time adjustment of quadrupoles. Instead, beam operations will utilize a fixed single bunch charge (60 pC for first light, 135 pC for full power operations), and vary the average beam current by altering the bunch repetition rate. Insofar as wakefield effects are negligible, the impact of space charge will then remain the same for all currents, and a separate betatron solution can be utilized for each of the bunch charge scenarios. Work on this issue continues.

Recirculation Arc Design The transport following the match is a large momentum acceptance, nominally isochronous recirculation arc. The design is based on the successful solution used at Bates, in which a dipole chicane is symmetrically split by a 180 degree bend; dipole parameters (bend and entry/exit angles) and drift lengths are then set to provide isochronous transport (from wiggler to reinjection point) achromatic, betatron stable motion in x (with a tune of 5/4) and imaging transport (M=-I) in y, across each end loop. The end loops are connected with a FODO transport channel comprising six 90 degree cells for an over-all matrix of M=-I in both transverse planes. This provides M=I in x and M=-I in y over the full arc proper, and gives significant aberration suppression. Each end loop has a system of 4 trim quads for dispersion control and momentum compaction variation to optimize the energy recovery transport and RF system performance and to provide energy compression during energy recovery. Each end loop has a system of four sextupoles for suppression of chromatic aberrations (T₁₆₆, T₂₆₆, and T₅₆₆ are driven to zero, others are controlled by the choice of system parameters). The system is nominally 501.5 rf wavelengths long (chosen to be 40n+/-18.5 to optimise mulitpass BPM monitoring); this can be increased and decreased by somewhat over 1/2 wavelength by steering in the 180 degree dipoles using a set of adjacent trim magnets.

Reinjection Match Upon return to the crymodule axis, a 4 quad telescope is used to match into the module for energy recovery. This is not strictly necessary for the existing driver, as RF focusing during energy recovery will provide adequate beam envelope control. It is introduced to simplify the installation of projected upgrades using multiple cryomodules, which, due to reduced RF focusing in the back end of the acceleration cycle and the front end of the energy recovery cycle will necessitate additional betatron matching.

Diagnostic and Correction Systems A system of optical transition radiation based beam viewers and electromagnetic beam position monitors provide beam position and profile information at various locations around the machine. An orbit diagnostic is typically placed approximately every quarter wave length in betatron phase advance. Small horizontal and vertical air-core dipoles are placed adjacent to the diagnostics to allow beam steering for orbit correction and lattice diagnostics. The FODO backleg transport is heavily instrumented to support beam dynamics studies (emittance measurements) intended to investigate CSR effects. Bunch arrival time/beam phase monitors are placed before and after the crymodule to allow for measurement and correction of transport system momentum compactions.

Last modified: 25 March 1997
http://www.jlab.org/~douglas/ is maintained by: douglas@jlab.org