Gemini Laser Area
28 Jan 2010







​The Gemini laser area is on the upper floor of the building housing the Gemini facility, and contains the two amplifiers, their pump lasers, the two pulse compressor chambers, optics to split and direct the input beams and a suite of diagnostic instruments.

The experimental area lies directly below the pulse compressors, so the output beams can be sent down vacuum pipes through holes in the floor to the target chamber below. The walls of the experimental area are made of 1-metre thick concrete to provide radiation protection, and the main optics tables in the laser area are positioned directly above these thick walls to ensure the best possible stability.

The beam from the Gemini front end reaches the area by a complicated route through a trench in the floor below and a pipe fixed to the concrete wall of the target area bunker. The beam travels nearly 40 metres between​ Amp 3 and Gemini, and in that path it is expanded from 31 mm to 50 mm in an image-relaying beam telescope which also preserves the beam profile.

Shortly after entering the Gemini laser area the beam is split into two halves, one of which is directed to each amplifier table. The relative timing of the beams can be set at this point, to allow for changes in the optical layout in the experimental area while still ensuring the beams arrive on target at the same time. Some experiments require one beam to have a variable delay with respect to the other, and this fine adjustment can also be made here.

One beam goes to each amplifier table, where it makes four passes through the amplifier crystal. The pump laser for each amplifier is a two-beam commercial Nd:glass laser (Quantel SA, Paris), delivering a total of 60 joules of second-harmonic light at 527 nm. These green beams are homogenized using custom-made diffractive optical elements, which convert the non-uniform beam from the laser into a smooth top-hat profile of the correct size with little loss of energy. Between successive passes there are image-relaying vacuum spatial filters to preserve the beam profile and minimise the growth of small-scale modulations. After the final pass the pulse energy can be as much as 25 joules. This beam is expanded to 150 mm diameter in an achromatic expanding telescope, then sent to the pulse compressor.

The vacuum chamber housing the compressor is 3.5 metres long and 1.25 by 1 metre in section. It holds the two gratings and the retro mirror that make up the compressor, plus three steering mirrors that send the beam to the experimental area. All these optics and their mounts are supported on two solid aluminium breadboards, which in turn are supported on pillars fixed to the floor of the area. The pillars are mechanically isolated from the stainless steel vacuum chamber so the optics do not move when the chamber is pumped down. Each chamber has its own turbomolecular pump, with a backing pump in the laser services area on the ground floor. The two compressor chambers and the interaction chamber share a common roughing pump, so only one can be pumped from atmospheric pressure at any time. As the compressors normally remain under vacuum for long periods, this does not cause any operational problems.

The final element in the laser area is the beam diagn​​​ostics. Samples of the compressed beam leave the compressor chamber through windows and travel to the diagnostics table. There is a full-aperture beam transmitted through the final turning mirror, and also a 15 mm beam that has passed through a hole in the final mirror, and is therefore a true sample of the compressed pulse. The full-size beam is stretched by its passage through the mirror and large window, so is used for measurements that do not depend on a short pulse. It is telescoped down to a few mm diameter, and used to measure the energy, focal spot and beam profile. The short-pulse beam leaves the chamber through a thin window, and is used for pulse length and contrast measurements, which can only be made with a short pulse.

Diagnostic information is captured on each shot and made available to the users immediately, as well as being archived.

Contact: Pattathil, Rajeev (STFC,RAL,CLF)