Amplifiers

The Asterix IV/PALS laser chain includes besides preamplifier five power amplifiers. They boost the pulse supplied by the oscillator section up to an energy of one kilojoule. The size of the individual amplifiers gradually increases along the laser chain. The final amplifier is nearly 13 m long (see the picture) and provides the laser beam of 29 cm in diameter. A fraction of second before the laser shot, the amplifiers are energized by discharging the large capacitor banks placed in the underneath floor into the flashlamps that surround the amplifier’s tube containing the gas active medium. The intense flash of incoherent ultraviolet light produced by the flashlamps creates in the amplifier’s tube abundance of excited iodine atoms that are “ready” to add their extra energy to the laser pulse supplied by the oscillator section.

The advantage of using a gaseous active medium in the iodine power amplifiers is that this cannot be optically damaged or cracked, in contrast to solid-state lasers. The use of gaseous active medium allows furthermore at Asterix IV/PALS to produce the quality of the laser beam superior to the beam quality achieved by most of solid-state lasers.

The amplifiers are built as arrays of modules each consisting of a quartz tube surrounded by a flashlamp block. For example, the final amplifier is composed of eight modules about 1 m long, each carrying 12 flashlamps mounted on two half-shell units easing access and maintenance. The table shows the active length, the beam diameter, and the typical partial pressures of the working and the buffer gases for each amplifier.

The flashlamps energizing the amplifiers consist of quartz tubes and are filled by xenon. They are driven at 30 to 40 kV – depending on the desired laser output energy – through a discharge circuit resulting in white light pulses with maximum spectral intensity at 250-300 nm and duration of 2-10 μs. Yellow-green light near 500 nm also dissociates the iodine molecules I₂ that spontaneously form in the active medium.

Using short UV light pulses to pump the active medium has two principal advantages.

First, the efficiency of generation of the inverted iodine atoms from the parent alkyliodide molecules is fairly large – nearly 50 % – since fewer “adverse” secondary products are generated during the photolysis. This makes it possible to easily regenerate the lasing medium after each shot, allowing a low-cost operation of the system.

Second, the short pumping creates population inversion only in a few microseconds. Amplification may thus take place in a homogeneous active medium prior the acoustic waves, which develop over a tens-of-microsecond time scale as a result of local heating of the wall of the amplifier tube by the flashlamp light, destroy the uniformity of the medium. This allows producing an extremely uniform profile of the output beam.

The flashlamps design ensures the electric discharge to always occur near the flashlamp axis, minimising the load of the quartz tube and hence the risk of its damage or disruption. The electric current flowing through the discharge is led back along the flashlamp by four current rods surrounding the tube and connected to the ground potential. The metallic reflector serving to couple the pump UV light from the flashlamp to the active medium is floating. Thus the field inside the flashlamp is fully determined by the electrodes and the surrounding current rods, giving it the axial symmetry.

In all amplifiers the Xe filling of the flashlamps is regularly exchanged to remove the molecular impurities which reduce performance and lifetime. All flashlamps are cooled by a closed nitrogen loop which re-establishes their thermal balance over several-minutes (this system additionally cools the amplifier’s quartz tube). Typically 600 – 1 000 full energy shots are possible before the flashlamps need to be inspected and replaced.

The laser medium is replenished from cryogenic units located in the underneath floor, which store the C₃F₇I liquid at a temperature providing the saturated vapour pressure at which the amplifier operates. The system removes the post-shot products such as I₂ that would otherwise strongly quench the population inversion. The regeneration of the medium of the A3, A4, and A5 amplifiers is enhanced by local heating which generates pressure gradients and allows a full energy shot every 25 minutes.