Design and fabrication of gas cell targets for laboratory astrophysics experiments on the Orion high-power laser facility
1 Introduction
As part of its commitment to the UK academic plasma physics community the Central Laser Facility (CLF) supports academic access to the Orion laser facility at the Atomic Weapons Establishment (AWE) Aldermaston[1], which constitutes up to 15% of the Orion experimental programme through the supply of target components and subsistence provision. This target fabrication support is in addition to the experimental support provided at the Rutherford Appleton Laboratory (RAL) for the high-power laser experiments carried out for the CLF programme. Over the past 40 years the CLF Target Fabrication Group has developed extensive skills in a range of capabilities that are directly applicable to target manufacture and has dedicated laboratories for thin film coating, micro-assembly, metrology, wafer based target manufacture, laser micro-machining and precision micro-machining. In addition, the group runs a number of development programmes to increase the level of capability for UK user groups and to drive forward target technology. One such development that has been delivered is the implementation of a gas-filled microtarget capability. This was developed in response to an AWE academic access experiment[2, 3] carried out by a consortium led by Imperial College London and drawing on knowledge from AWE, Imperial College London, Observatoire de Paris and RAL, a capability was introduced and then transferred to RAL for the user community access for CLF experiments.
2 Orion academic access experiment
The requirement was to develop an experimental platform (target assembly) to produce and study counter-streaming radiative shocks relevant to astrophysics. This is a further development of previous designs to enable the study of the more complex phenomena and instabilities that can arise in such a system[4]. Since 2000 there have been a number of designs for gas targets used in this field that have either had the features of tubes down which the plasma flow propagates or cells with windows[5–7]. The target design was optimized to allow for the shocks to propagate without interacting with the cell walls and for optical and X-ray diagnostics to be able to image the shocks as close as possible to their point of origin. The target cell was required to be filled with either argon, neon, krypton or xenon to pressures ranging from 0.1 to 1 bar with control of the internal gas pressure from shot to shot. The ends of the gas cell were sealed with solid ablators made from plastic disks; these are driven by the lasers and act as pistons in the gas cell to drive shocks. Other key target features were brominated plastic layers to prevent preheat of the gas, copper cones to shield the interaction point of the lasers from the diagnostics and a number of alignment features.
3 Target design
3.1 Overall design
The proposed experimental layout is shown in Figure
Through several iterations, a more complex cell was designed with an octagonal shape and two large (5 mm, subsequently changed to 3 mm) open apertures at each end for the plastic ablators. The gas cell was machined from a single piece of aluminium using a computer numerical control (CNC) micro-mill to provide a joint-free component; this design is shown in Figure
Two different sets of windows were used for both X-ray imaging and optical interferometry. The chosen material for the X-ray windows was a polyimide plastic, laser-cut and adhered to the aperture on opposite sides. The thickness of the plastic was defined as
The pusher design consisted of a
Fig. 2. (a) Fully assembled target for Orion gas cell campaign and (b) the base gas cell design.
3.2 Brominated plastic disks
The brominated plastic disks were produced using hot-pressing techniques as a foil of the required bromine content, and thickness was not commercially available. Previous academic access experiments[8] have used CHBr disks developed using hot-pressing a brominated plastic bromo-4-polystyrene, which although pressed under temperature and pressure can form a foil with a granular structure, which would be unsuitable for the experiment as it would seed instabilities in the plasma flow.
The use of a different form of CHBr (poly-4-bromostyrene) allowed the production of a clear disk with a reduction in the granularity in the bulk and at the surface. Characterization of the bromine content confirmed a 31.10 wt% Br composition by mass[9]. An image of the improved disk is shown in Figure
Fig. 3. A brominated plastic disk before adhering to the polypropylene support on the gas cell.
Fig. 4. (a) Picture and (b) schematic of the gas system used for filling targets to specified pressures.
3.3 Adhesive selection
Three main types of adhesive were used to glue the windows onto the cells: Superglue (a cyanoacrylate), Loctite ‘Double Bubble’ (a 2-part epoxy adhesive) and Araldite 2010. Initial tests with the different adhesives showed that each one had varying performance with respect to sealing the gas cell windows and foils. Using a gas-fill rig (described in Section
4 Target filling and testing
A system for filling targets with gas (Figure
The deflection tests on the end windows (pusher) of the target were important as the Orion laser beams would be focused onto a specific point on the target and this point would be referenced from alignment marks and features. A gas target in the Orion target chamber that had a foil deflection and therefore offset from its intended position would mean a defocused beam on the surface and a lower intensity than expected. The results from the first deflection test in which a target was pressurized in an evacuated testing chamber (see Figure
A second set of tests was carried out using a target that was filled in the range of 0.1–1 bar in an external environment of atmospheric pressure. This test was carried out to test the cell integrity when transporting from the testing chamber to the Orion target chamber. In this case the target would be at a lower pressure than the external environment. The target bowed inwards and the relative deflections are shown in Figure
Errors in both deflection tests are determined from the observed variation in the confocal positioning sensor that was used to measure the position of the foil. This is observed to vary when measuring a static foil by
Fig. 7. Deflection of target pusher window in an atmospheric pressure environment at a range of fill pressures.
5 Pressure testing and experimental performance
Initial pressure testing showed an average leak rate of
Three 1-week experimental campaigns were carried out in March 2015, July 2015 and October 2015. In addition to the gas cell targets backlighter targets used were a standard design as in previous campaigns[8].
The leak rates for the targets when in the chamber were improved from the first to the second weeks of shots by the aforementioned steps and the target shot leak rates can be seen in Figure
Fig. 8. Target leak rates for a number of targets in the Orion chamber (sample time every 10 min).
The leak rates, improved from the first tests, were characterized to be approximately 1–2 mbar per minute in the first week targets with the improved fabrication and adhesive performance leading to the production of targets that could hold pressure at a relatively stable level for up to 2 h for the subsequent experiment. This was essential as the target was installed in the Orion chamber and held at pressure and the shots were prepared. It can be seen in Figure
Fig. 9. Example X-ray backlighting data from the Orion Imperial College academic access experiment.
6 X-ray target imaging
A sample backlighter image is shown in Figure
7 Conclusions
A total of 164 targets (75 gas cells and 89 backlighters) were manufactured for three separate 1-week academic access experiments on Orion. The development of several new target fabrication techniques in collaboration with Imperial College London and AWE has enhanced the capabilities that are available and has contributed to a highly successful academic access campaign[2, 3]. Further work is to be carried out to enhance the quality of the gas cell targets to enable further experiments with other geometries.
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Article Outline
C. Spindloe, D. Wyatt, S. Astbury, G. F. Swadling, T. Clayson, C. Stehlé, J. M. Foster, E. Gumbrell, R. Charles, C. N. Danson, P. Brummitt, F. Suzuki-Vidal. Design and fabrication of gas cell targets for laboratory astrophysics experiments on the Orion high-power laser facility[J]. High Power Laser Science and Engineering, 2017, 5(3): 03000e22.