The Pegasus laboratory is located in the NW corner of the sub-basement of Knudsen Hall in the UCLA Department of Physics and Astronomy. It is composed by a laser room, a control room and a radiation shielded bunker with an automated door and personnel protection system.
Pegasus radiation area monitor locations
The PEGASUS photocathode gun currently consists of a version of the UCLA/SLAC/BNL 1.6 cell gun which has recently been retuned, extracting the tuners from the cavity in order to avoid arcing, allowing operation at a high power level (10 MW). Such a level gives a peak accelerating field of 120 MV/m and a beam output energy >5.5 MeV. Because the beam charge for the UED application is intentionally maintained low (1-10 pC), it becomes very important to limit the amount of the dark current out of the gun to maximize the signal to noise ratio in the diffraction images as well as for the other beamline diagnostics. The amount of dark current is a function of the peak accelerating field and the rf pulse length (in our case 1.5 ms) whose only requirement is to be larger than the gun filling time (0.8 ms). In order to minimize the dark current level we choose to operate with as short as possible rf pulse and a peak accelerating field ~ 95 MV/m. The cathode material is another aspect of the photoinjector that deserves some attention. It determines the quantum efficiency, i.e. the number of electrons for a given laser energy and their initial thermal spread (related to the difference between the material work function and the laser photon energy). The Pegasus laboratory has been commissioned using Mg and Cu cathodes. Innovative cathode solutions with protective MgF2 and diamond layer are under test.
The photocathode drive laser is a state-of-the-art all diode pumped Titanium:Sapphire based system from Coherent. A 79.33 MHz optical train from the master oscillator synchronized with the RF with an active phase locking loop at a level better than 250 fs seeds a 1 KHz regenerative amplifier. After the final compressor infrared (800 nm) pulses with energy greater than 3 mJ and length less than 35 fs are obtained. The pulse-to-pulse energy fluctuations for the system are less than 1% rms, and the mode quality is particularly good (M2 < 1.35). After a two stage nonlinear harmonic conversion, 266 nm pulses of energy up to 300 mJ can be obtained. The large bandwidth of the Ti:Sa laser medium allows superior flexibility in adjusting the laser pulse length (easily changeable between 50 fs and 5 ps) and time profile as required by different kind of experiments. Measurements of the laser temporal profile are obtained by a cross correlation technique varying the time delay on the 800 nm arm after the first harmonic generation crystal before recombination with the 400 nm light into the tripling crystal. More than 1.5 mJ of unconverted 800 nm laser pulse are available for other uses (pumping a laser-induced process/ phase transformation in a target, ultrafast diagnostics). A relay imaging system with a variable diameter iris located in one of the image planes delivers on the cathode a beam with controllable transverse dimensions (150-1000 mm).
An operating system independent architecture based on PostgreSQL serves as a three level control system enabling remote operation. Four Windows PC, and three Linux boxes with ISA and PCI cards for Digital IO, ADC, DAC and video are linked together and to a Linux lab central server by Gigabit Ethernet. Scope, video multiplexer and trigger box are also attached to these computers by serial connection. The user side interface is Labview based. This system has been developed by PBPL to more closely mimic modern accelerator control systems. The ease of use of this system is critical in order to allow the students the capability of quick adjustments and additions to the system. Since all the data acquisition and control runs via the central server via Ethernet, the system can be controlled from the web easily enabling the remote operation mode for outreach and education purposes. Moreover all the results and machine parameters are stored on a web-accessible electronic logbook (E-Log) that allows the information to be quickly shared and available to everybody.
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.