Experiments: Completed / Terminated

Superconducting Micro-undulator FEL Experiment

The visible FEL oscillator will employ a superconducting electromagnet 68 period micro-undulator (with a period of 0.88 cm and 0.5 Tesla peak field at a gap of 0.44 cm).

Field lines of one 8.8 mm period (POISSON calculation)

Prototypes of this undulator (up to 23 periods long) were built and performed extremely well in superconducting tests. The quench magnetic field is as predicted by the load curve comparing field on the Nb:Ti wire (obtained from POISSON simulations) and the measured short-sample quench current:

Peak magnetic field on the wire and short-sample quench field vs. current.

The load curve predicts a wiggler quench current of about 230 amperes. Measurements have shown the quench to be about 240+/-10 amperes (the spread is for various individual sections measured separately).

The field errors of the undulator were very low as constructed, making this perhaps the first undulator that will require no trimming after manufacture.

Magnetic field scan measurement of a superferric undulator section.

The 2nd integral of the magnetic field scan measurement.

As can be seen from the field scan, the magnetic field looks very much like a sine function. Indeed, a power spectrum of the measured field vs. the reciprocal period shows harmonics are at the noise level:

Power spectrum of the magnetic field scan.

The FEL experiment makes use of the short period of the undulator and the low emittance beam generated by the RF photocathode gun to operate at 500 nm with a 50 MeV electron beam. Emittance scales with energy so that at 250 MeV a wavelength of 100 nm would be practical.

At the present, the visible FEL oscillator beam line has another undulator in position. This undulator, developed by MIT, is a pulsed device. Its parameters are similar to those of the superferric undulator. Spontanious emission has already been observed from this device and preparations are under way to get lasing in the near future.

Last Modified: December 3, 2007
Please forward all questions about this site to: Christina Swinson

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