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VISA Beamline Schematic.

Initial Long. Phase Space (ELEGANT).

Final Long. Phase Space (ELEGANT).

CAD model of Chicane Compressor.

Optical Transport Line along undulator.

Sextupole

Sextupole (2)

VISA II Experiment (continued)

VISA II Experiment

Linearization of Transport Line

In order to maintain a chirped electron bunch from the linac to the undulator, the transport to beamline 3 must be linearized. Linearization requires a stronger control of the dispersion along the dog leg. The addition of sextupoles along the dispersive line will allow VISA II to run in two distinct modes. By diminishing the T₅₆₆, the linear phase space chirp is maintained.
The sextupoles are 5 cm long and have a calculated gradient of 22.0 T/(m²A).

Compression

The chicane compressor consists of four dipole magnets with a nominal field of 0.2 T. The dipoles have a bend radius of 1.2 m and a length of 41 cm. The effective magnetic length is 44.6 cm, with a path length of 41.89 cm. Once the chicane is installed, the electron bunch is expected to compress from 300 µm to less than 30 µm. The current of the bunch is projected to increase from 60 A (VISA I uncompressed running conditions) to 1 kA.

CAD Model of Chicane	Chicane Installed at ATF

Simulations also show that coherent synchrotron radiation (CSR) may amplify during compression and in the process microbunch the electron beam. Emittance degradation of 4-10 mm mrad is anticipated. The initial experiment to be performed at VISA II is running the FEL with a short compressed beam (current of 1 kA, charge of 200 pC). According to GENESIS 1.3 simulations, the short compressed bunch should produce extremely high gain. Saturation is expected by the 3rd meter of the 4 meter long undulator. With additional compression, the slippage can be so severe that the gain may not be purely exponential.

Simulations

Transport Calculations

Fig. 1: Initial Longitudinal Phase Space for VISA II operating conditions. The initial phase space after acceleration through the linacs has a clear energy chirp. This graph was produced using ELEGANT.

Fig. 2: Final Longitudinal Phase Space for VISA II operating conditions. This is the final phase space before injection into the undulator. There are two sextupoles installed along the dispersive line (or F-line) near the points of highest dispersion. The initial chirp of the beam is preserved through the lattice due to the mitigating of the second order dispersion by the addition of sextupoles.This graph was produced using ELEGANT.

Grenouille