The UCLA PEGASUS Plane-Wave Transformer Photoinjector

G. Travish, G. Andonian, P. Frigola, S. Reiche, J. Rosenzweig, and S. TelferUCLA Department of Physics & Astronomy, Los Angeles CA. USA

Features:Standing-wave S-band structurePlane-Wave Transformer design Replaceable cathode1/2 + 10 + 1/2 cell configurationPeak field-gradient is 60 MV/mFinal beam-energy is 17 MeV. Fill time of 2-3 µsShunt impedance of 50 M/mQL of 6000

Ti:S basedMostly commercially

availableDiode-pumped everythingRegen only amplificationStretcher w/ maskNo pulse shaping for nowRep rate of 500 Hz – 1Khz(RF only at 1 - 10 Hz)

Drive Laser Photoinjector T3 Laser

http://pbpl.physics.ucla.edu/

Applications:For photon-electron interactions.Femtosecond science & diagnosticsThomson scattering source

Features:Seeded by a second regenBoth regens pumped by same laserMultipass “bow-tie” amplifier

Work supported by DOE grant DE-FG03-98ER45693

A new drive laser has been designed for the PEGASUS Photoinjector. Procurement awaits final design details and bidding.

The PEGASUS drive laser, as with all photoinjector drive-lasers, must provide a sufficient number of photons with an energy above the cathode workfunction, and within a pulse-length short relative to the RF period. In practice, this implies a UV (~266 nm) laser, with ≈200 µJ of energy deliverable to the cathode, and a pulse length adjustable from about 1 to 10 ps. The pointing stability, energy stability and reliability have been only qualitatively considered, but should be near state-of-the-art as the design calls for an all diode-pumped system. In addition to these general requirements, the drive laser needs to be operable by non-specialists (i.e. no dedicated laser operator), and be flexible enough to allow for reconfiguration to meet new research directions (i.e. addition of a pulse shaper, diagnostics, etc.).

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Laser Parameter Value

Wavelength 266 nm

Energy > 200 µJ

Pulse length 1 - 10 ps

Repetition Rate 500 - 1000 Hz

Beam Parameter Value

Energy 12 - 18 MeV

Energy Spread (rms)

0.15%

Emittance (norm. rms)

4 µm

Bunch Length 1 mm

The PEGASUS photoinjector is based on the novel, but proven Plane Wave Transformer linac. The injector has been conditioned to high power, but awaits a laser. In the interim, thermionic operation is being prepared.

Due to the compact and simple design of the gun, a simple solenoid can used for emittance compensation. Simulations indicate that the design specifications in the table should be readily achievable.The interchangeable cathode design allows for a variety of cathode materials to be tested including the planned use of copper, magnesium, LaB6, and conventional thermionic emitters.

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Long term plans call for PEGASUS to install a “table top terawatt” (T3) laser for photon-electron interactions and femtosecond time-scale science. Specific plans call for a Thomson x-ray source.Amplifier Parameter

Value

Wavelength 800 nm

Energy 100 - 200 mJ

Pulse length 50 - 100 fs

Repetition Rate 10 Hz

The head-on interaction of the electron beam focused to a 50 µm spot with a transversely matched laser of 1 TW (100 mJ) gives an x-ray flux of about 2 x 108 photons at about 2 Å. Increasing the laser power to 2 TW and focusing the beams to a difficult to achieve 25 µm spot size, yields more than an order of magnitude more x-ray photons and two orders of magnitude improvement in the brightness. However, the head-on scattering produces long x-ray pulses. In order to achieve shorter pulses, 90 degree scattering will be required, with the penalty being a substantial reduction in the photon flux (down to about 2 x 106 even in the aggressive case).

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RF

Vacuum

Solenoid

CathodeBeam