Target fabrication for the POLAR experiment on the Orion laser facility Download: 801次
1. Introduction
The target-fabrication group within the Central Laser Facility (CLF) has extensive expertise in the areas of micro-assembly, thin-film coating, target characterization and micromachining. The group was established to support the academic user community having access to the lasers within the CLF. The Atomic Weapons Establishment (AWE) Orion laser–plasma interaction facility[1] became operational in 2013 and as part of its remit makes up to 15% of its operational time available for experiments by the UK academic community and their international collaborators. The CLF supports this access by providing laser targets for these experiments.
The POLAR project[2, 3] aims to replicate in the laboratory some of the hydrodynamic processes occurring in X-ray binary star systems known as magnetic cataclysmic variables[4, 5]. In these binary stars, matter from a cold star is gravitationally accelerated towards a dense, white-dwarf companion star. Collimation of this flow by the white-dwarf’s intense magnetic field results in a so-called accretion column of material at the poles of the star, instead of the usual accretion disc. The flow of material towards the star is impeded by the white-dwarf atmosphere and a reverse shock wave that is established in this column, near the surface of the white dwarf. The exceptionally high temperatures generated by this shock wave result in X-ray emission, and so give rise to the classification X-ray binary star. The purpose of the Orion experiment was to build a laboratory analogue of this process, in which the hydrodynamic processes could be studied in a controlled manner. The experiment uses high-power laser pulses to generate high-velocity plasma flow (collimated not by a magnetic field as in the star, but by a miniature surrounding tube) which impacts on the surface of an obstacle that mimics the white dwarf, resulting in the reverse shock wave that the experiment aims to study. The experimental layout is shown in Figure
The main interaction target was the POLAR target, designed to produce the high-temperature plasma conditions that were of interest to the experimental group. This is shown in Figure
Secondary to this, a backlighter target was required to provide an X-ray image of the target during the interaction[6, 7]. This target is shown at the top of Figure
The targets utilized the full scope of the target-fabrication group’s capabilities and also required research and development into new, cost-effective materials.
2. POLAR interaction target
2.1. Specification
These numbers correspond to the features in Figure
Fig. 2. A CAD representation of the target sliced through the centreline (annotations are described in the text above).
Fig. 3. Shows a 3D CAD cut away of the target and indicates the positions of the pusher foil position and also shows the foam insert at the bottom of the tube.
The detailed design of this target underwent a number of modifications during the campaign. A number of alignment features and coatings were added to protect the target and to allow for the positioning of the target in the Orion chamber. These included small-diameter copper wires placed transversely across the tube as fiducial reference points to measure the propagation of the shock, and fibres on the target stalk to provide reference points for very precise alignment in the Orion target chamber. A 3D representation is shown in Figure
Fig. 4. A fully processed Brominated plastic disk showing internal structure (dark areas are the un-melted material).
2.2. Brominated foil
Brominated plastic for the pusher material was not commercially available in the thickness required and purchasing a bespoke made foil was not possible within the budget. A research programme was initiated to press plastic films of brominated plastic from a commercially available powder. A heated press was used that had previously been used to manufacture deuterated plastic films. A number of tests were carried out to heat and mould bromopolystryene (C8H7Br) that had a bromine content of 43% by weight.
Initial results of pressing CHBr had mixed success, and temperature, pressure and mass were all important variables in producing a good quality film. At low temperatures () the powder did not show significant melt and was just fused together to form a brittle pressed pellet that was too thick. When the temperature was increased to the sample showed signs of melt and the use of the correct amount of material produced a film thick. However it can be seen in Figure
To produce more uniform films a temperature of would be required. Initial scanning electron microscope (SEM) analysis using energy-dispersive X-ray spectroscopy (EDX) confirms that there is bromine content still present after processing although there is some structure that is present in the pressed disk that is due to the target foil not going through a full melt when forming as shown in Figure
Fig. 5. A low-density sample placed at the end of the PI tube next to the quartz obstacle.
Further characterization work using oxygen flask combustion, followed by ion chromatography has shown that the bromine levels of the samples analysed by mass percent was [9] in correlation with the initial brominated powder that was 43% by weight. There is some loss of Bromine that is seen that requires further investigation.
2.3. Foam insert
Low-density foam of density was to be placed at the end of the PI tube. The foam was required to be positioned next to the obstacle and was required to be in length. The foam was manufactured using a critical point drying (CPD) technique[10] and a number of techniques were trialled to form the foam inside the tube. Firstly to make the foam
Fig. 6. Images of the POLAR target from the obstacle side (left) the Au/CH pusher (right).
2.4. Final target assembly
The remaining target components were produced using precision micromachining (washer and cone shield) and thin-film coating techniques (gold–plastic multi-layered pusher), and were integrated into the final target assembly as shown in Figure
3. Backlighter target
The backlighter target assembly design is shown in Figure
Fig. 10. Sample backlighter images showing: (a) the PI tube with the quartz/steel obstacle at the top, the diagnostic slits cut in to the tube and the gold resolution grid attached to the target and (b) the PI tube with the quartz/steel obstacle at the top and a low-density foam insert below the stop.
Table 1. Details of the Orion laser and POLAR target parameters for the images in Figure 10 .
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4. X-ray target imaging
Sample backlighter images are shown in Figure
5. Conclusions
A total of 80 targets (34 POLAR and 46 backlighter) were delivered for two periods of two weeks of academic access on Orion during November 2013 and February 2014. The development of a number of new target-fabrication techniques at the CLF, Rutherford Appleton Laboratory has enhanced the capabilities that are available and has contributed to a successful academic-access campaign. Further work is to be carried out to enhance the quality of the brominated plastic pushers to remove defect points in the target that may lead to instabilities in the plasma as it propagates down the tube and also to produce more uniform foam next to the obstacle.
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Article Outline
C. Spindloe, D. Wyatt, D. Haddock, I. East, J. E. Cross, C. N. Danson, E. Falize, J. M. Foster, M. Koenig, G. Gregori. Target fabrication for the POLAR experiment on the Orion laser facility[J]. High Power Laser Science and Engineering, 2015, 3(1): 010000e8.