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The initial goal of PEGASUS will be to test a novel photoinjector, the PWT photocathode gun. The Pegasus plane wave transformer injector has been conditioned to 20 MW of RF power. Initial operations show a 15 MeV dark current beam that will be used for beam radiation studies. The design of a new LaB6 cathode will allow for both thermionic emission and photoinjection operation. Experiments currently planned include novel beam instrumentation, surface effects in optical transition radiation, and waveguide SASA-FEL.


Pegasus Design Parameters

12-18 MeV
Energy Spread
0.15 %
<4 mm-mrad
Bunch Charge
1 nC
Bunch Length
900 microns
Beam Size
150 microns

Laboratory Description

RF Injector

The PWT gun is a novel standing-wave S-band electron source designed to provide 1nC, 17 MeV electron beams. The PWT gun consist of a 60 cm long, 12 cm diameter tank loaded with 11 discs (picture), creating 10 full and two half accelerating cells. The peak gradient is expected to be 60 MV/m. The PWT has a shunt impedance of approximately 50 MOhm/m and a QL of roughly 6000, which makes an efficient structure with a moderate fill time of 2-3 micro seconds. The compact design allows for a simple emittance compensation solenoid. Cathode source flexibility is enhanced by an insertable cathode. At present, a thermionic LaB6 cathode serves as the electron source. Other cathodes to be tested at PEGASUS will include OHFC copper, single crystal copper, Cs2Te, and diamond coated Ti.

Thermionic Emitter

The PEGASUS thermionic cathode is a compact and cost efficient emitter designed to provide beam charges of up to 1 nC.  By virtue of its simple design, the thermionic cathode can also operate as a photocathode.  The thermionic cathode assembly consists of a cylindrical LaB6 cathode conductively heated by a UHV cartridge heater, all incased in a molybdenum body.  The LaB6 cathode is 3.5 mm in diameter, with an active area of approximately 0.28 cm2, and 1 mm thick.  The heater is a commercially available HeatWave UHV Standard Series cartridge heater rated at 1200 C at 7.5 Watts. A schematic of the major components of the later version is shown in Figure 1.

RF System




Control System


Beam energy and energy spread is measured in the dispesive section after the dipole spectrometer. Beam charge is determined using Faraday cup beam dumps. Beam position and charge is monitored using YAG crystals imaged with CCD cameras.


Optical Transition Radiation (OTR) [1]

SASE FEL Physics

SASE FEL studies will revolve around the 2m Kurchatov/UCLA undulator. Undulator alignment is facilitated by the multipole field measurements conducted using a pulsed-wire apparatus developed at UCLA. While the beam provided by the PWT would not drive the FEL to saturation, GENESIS 1.3 simulations show that the additional of a 1 mm square waveguide will significantly enhance gain and reduce the saturation length by 30 % [2]. Insertion of a waveguide in the undulator has been investigated. The waveguide will enhance FEL performance by compensating for the diffractive effects, eventually yielding saturation. Of particular interest is the purity of the waveguide mode as well as the power losses of the proposed hollow glass waveguide [3].


[1] S. Reiche, et. al., Proceedings of 2001 Particle Accelerator Conference, Chicago (2001) 1282.

[2] S. Reiche, et. al., Presented at the 2000 International FEL Conference.

[3] Y. Matsuura, et. al., Applied Optics Vol. 35, No. 27 (1996) 5395.