Clusters

By synchronising the firing of a vapourisation laser with the opening of a supersonic valve then a metal plasma can be produced at the peak of the carrier gas density over the target rod. As the vapour cools in the high pressure helium the metal atoms cluster and an intense beam of neutral clusters is formed. This intense pulsed beam source provides the necessary cooling to convert initially hot metal atoms to large concentrations of ultracold clusters.

This method of pulsed-laser driven metal vapourisation is particularly attractive since it is only the metal target itself which needs to be heated to the vapourisation temperature. This is technically superior to the use of a furnace since it is not necessary to heat any other part of the apparatus to extreme temperatures, therefore reducing the need for complex cooling systems. In addition, the pulsed nature of these sources lessens pumping requirements therefore enabling higher carrier gas pressures to be used. This is important because the gas pressure has been found to be one of the key parameters which controls the cluster production.

The most important feature of this design is expansion chamber in which vapourisation, thermalisation and cluster formation can occur. The dimensions of this chamber are chosen to optimise the cooling of the plasma (through conduction of heat to the cavity walls) and minimise the loss of material to depositing on the walls. Furthermore, by using an adjustable plug the dimensions of this chamber can be adjusted to yield optimal cluster production for different metals.

Correct alignment and focusing of the laser onto the target has been found to be essential in producing cluster beams of high intensities and stabilities. To improve the laser alignment a channel, which is coaxial with the laser channel, is situated under the target. By using this channel when setting up the equipment it is possible to ensure that the laser is correctly aligned.