Intense hydrogen-cluster beams having velocities of about 500 m/sec can be produced by expansion of precooled hydrogen gas out of a nozzle into high vacuum. They are of interest for the compensation of particle losses in thermonuclear devices. The dependence of the penetration depth into the plasma on the velocity and the size of the clusters is still unknown. In order to study the influence of the latter the available beams should cover a range of cluster size as wide as possible. Using a cryostat which can be operated at temperatures above the critical one, the range has been extended by about three orders of magnitude up to a mean size of about 10 9 H atoms per cluster. The cluster sizes have been determined by light-scattering technique. This method is particularly useful during the injection of cluster beams into a plasma.

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The reflection of a beam of nitrogen clusters from a polished stainless steel surface is investigated. The scattered flux shows a strong maximum at an angle of reflection almost 90°, independent of angle of incidence. The mean velocity of the reflected beam is about equal to the tangential velocity component of the incident beam. Measurements with increased background pressure demonstrate that the reflected beam still consists essentially of clusters.

The feasibility of D-3He reactor plasma conditions in a tokamak of the NET/INTOR class is investigated. It is found that, depending on the energy confinement scaling law, energy breakeven can be achieved in NET without significant modification of its design. Significant improvement in Q (ratio of fusion power to injected power) can be achieved by removing the tritium producing blanket and replacing the inboard neutron shield by a thinner shield optimized for the neutron spectrum in D-3He; this allows the plasma major radius and aspect ratio to be reduced and higher beta and Q-values (up to about 3) to be achieved. The implications of D-3He operation for neutron shielding, the heat loads on the first wall and the divertor as well as plasma refuelling are considered.