Experimental studies of the concept of an efficient radiation trap based on the adiabatic photon confinement between curved mirrors is presented in the paper. Such a trap is free from restrictions typical to the Fabry-Perot cells, relating to the confined radiation quality, accuracy and stabilization of optical elements. Experiments have demonstrated the high efficiency of confinement, which mainly depends on the reflectivity of mirrors and their surface defects.

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Experimental results on neutralization of negative ion beams in a photon stripping target are presented. For the energy of beams used in these studies (6–10 keV), the maximum neutralization efficiency is 95%. In contrast with gas or plasma neutralizers, the generation of positive ions from the negative ion beam was negligible. A non-resonance photon trap with highly reflecting mirrors of special shape was used as a stripping target. Due to a special mirror shape, in the process of photons reflections and propagation inside the trap there are some adiabatically conserved invariants that limits the volume accessible for photon motion even if the mirror surface is not closed. This approach makes it possible to overcome limitations typical for the Fabry–Perot cells, which require high radiation quality, high quality of the optical elements, and very high mechanical stability of the cell. The trap was pumped by commercially available fiber laser (λ  =  1070 nm, Δλ  =  7 nm, P  =  2.1 kW) through a small hole in the mirror. The experiments were carried out with H- and D-beams. The observed neutralization efficiency depended mainly on the reflectivity of the mirrors, on defects on their surface, and on the laser pumping power.

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A two-color infrared interferometer has been developed for the investigation of high-density weakly ionized tantalum plasma in x-ray complexes based on linear induction accelerators (1.6 kA electron beam current, 4.6 MeV energy, and 100 ns pulse duration). Simultaneous probing at two different wavelengths makes it possible to independently measure the density of neutral and electron components. The interferometer uses wavelength values of 1.064 µm (Nd:YAG laser) and 10.6 µm (CO2 laser). To attenuate the effect of sample beam refraction in inhomogeneous plasma, the interferometer used a refraction suppression scheme composed of spherical mirrors focusing the object beam into the region occupied by the plasma. In addition, the power of the sample beam transmitted through the plasma was controlled in order to analyze whether there was a distortion of the interference pattern because of strong sample beam refraction and absorption in the plasma cloud. To calibrate the initial phase shift of the probe radiation, a movable mirror mounted on a piezoelectric element and oscillating according to a harmonic law with amplitude greater than the laser wavelengths was used. In initial experiments, the parameters of target plasma registered by this interferometer are as follows: the linear density of neutrals reached 1.5 · 1017 cm−2, and the degree of ionization was of the order of 10−2. The target plasma expansion velocity is determined as ∼6 km/s.

A diagnostic has been developed to study the focusing dynamics of a high-power electron beam on a target. The diagnostic uses a pinhole camera to observe bremsstrahlung from the converter target. A data acquisition system yields images of the beam focal spot with a frame duration of 20 ns and an almost unlimited recording duration. The focal spot dynamics of a linear induction accelerator with an energy of 1.5 MeV, a current of 1.2 kA and a pulse duration of 350 ns were studied. The focal spot was found to be disrupted within the first 100 ns of the pulse. Defocusing has a complex 3-D character. The feasibility of additional cleaning of the target using an accelerator pre-pulse was shown. Stable beam focusing for up to 200 ns was demonstrated.

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