The ALICE Energy Recovery Linac (ERL) photoinjector drive laser

The drive laser is a HighQ IC10000 Picotrain model, delivering > 10 W at 1064 nm wavelength (infrared). The lasing medium is diode pumped neodymium yttrium vanadate (Nd:YVO4). It is mode locked using a SESAM and generates horizontally polarised 7 ps FWHM pulses at a repetition rate of 81.25 MHz, this being the 16th sub harmonic of the 1.3 GHz RF frequency chosen for the ALICE ERL. The drive laser optical system defines and delivers macropulses to the cathode at a repetition rate up to 20 Hz. The length of a macropulse can vary from a single laser pulse, up to a 100 μs pulse train comprising some 8,125 laser pulses. The layout of components on the optical table is shown in figure 1.

Macropulse generation

The 81.25MHz pulse train generated by the laser is first chopped into 130 μs macropulses at a 100 Hz repetition rate. A shutter following the chopper reduces the macropulse repetition rate to 1, 2, 5, 10 or 20 Hz. The beam polarisation is then rotated from horizontal to vertical using a half-wave plate. Finally, a KD*P (potassium di-hydrogen phosphate) Pockels cell defines the beginning and end of the macropulse by rotating the polarisation back to the horizontal plane, allowing the beam to pass through a polarisation analyser aligned to the horizontal plane. The slopes on the leading and trailing edges of the macropulse are removed (these arise as the chopper wheel sweeps across the pulse train), leaving a macropulse with well defined edges as the Pockels cell is fast enough to operate between consecutive laser pulses. The process is illustrated in figure 2.

Laser beam processing

Second harmonic Generation (SHG) is carried out after macropulse definition to generate visible green light. The non linear nature of the SHG process serves to enhance the contrast ratio of the Pockels cell. With the SHG bolted to the front of the laser oscillator, the IC10000 generates 67 nJ per pulse at 532 nm. However, in our configuration, following power loses in the macropulse generation optical elements which precede the SHG, there is typically 40 nJ per pulse at 532 nm, or about 3.3 W. A pinhole with approximately 70% transmission is located immediately after the SHG unit to define a circular transverse beam profile. Diagnostics which can be viewed remotely are present for both the 1064 and 532 nm beams, as shown in figure 2.

The turning mirrors before and after the SHG unit transmit 1% and 2% of the infrared and green beams respectively. The diagnostic beams then pass through beamsplitters giving two separate beams which are used to generate images via screen and camera combinations, and to feed photodiodes to monitor pulse energy and macropulse structure. A motorised half-wave plate and polariser serve as a beam attenuator, the amount of laser light transmitted simply depending on the angle between the wave plate and polariser.

A pulse stretcher comprised of two separate YVO4 crystals can be included in the optical setup to increase the natural 7 ps FWHM pulse length to either 13 ps if one crystal is used, or 28 ps with both. 2% of the 1064 nm beam emerging from the laser oscillator is preserved and passed into the laser beam transport system (L-BTS) for delivery to the photoinjector in tandem with the high power 532 nm beam. This is for the purpose of synchronising the laser oscillator with the RF accelerating systems, with the ultimate aim of forming a closed feedback loop to stabilise timing and reduce jitter to the lowest levels possible.