In recent years, petawatt class lasers operating at ultra-high intensities to target have become relatively common in national laboratories and even university environments[
High Power Laser Science and Engineering, Volume. 6, Issue 3, 03000e47(2018)
400 TW operation of Orion at ultra-high contrast
The Orion facility at the Atomic Weapons Establishment in the United Kingdom has the capability to operate one of its two 500 J, 500 fs short-pulse petawatt beams at the second harmonic, the principal reason being to increase the temporal contrast of the pulse on target. This is achieved post-compression, using 3 mm thick type-1 potassium dihydrogen phosphate crystals. Since the beam diameter of the compressed pulse is mm, it is impractical to achieve this over the full aperture due to the unavailability of the large aperture crystals. Frequency doubling was originally achieved on Orion using a circular sub-aperture of 300 mm diameter. The reduction in aperture limited the output energy to 100 J. The second-harmonic capability has been upgraded by taking two square 300 mm 300 mm sub-apertures from the beam and combining them at focus using a single paraboloidal mirror, thus creating a 200 J, 500 fs, i.e., 400 TW facility at the second harmonic.
1 Introduction
In recent years, petawatt class lasers operating at ultra-high intensities to target have become relatively common in national laboratories and even university environments[
The Orion laser facility at the Atomic Weapons Establishment (AWE) has been operating since 2013[
The short-pulse architecture is based on chirped pulse amplification[
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A key capability of the Orion facility is the ability to frequency-double one of the short-pulse beamlines. The frequency doubling achieves two advantageous conditions. First, the wavelength enables a different regime of laser–plasma interactions to be studied. Second, and more importantly, it provides an increase in the temporal contrast of the laser pulse. Previous investigations of type-I frequency doubling of femtosecond and picosecond laser pulses from Nd:glass[
Frequency doubling on Orion is achieved by passing the amplified compressed pulse through a 3 mm thick potassium dihydrogen phosphate crystal cut for type-I frequency doubling. Unfortunately, it is not currently possible to obtain a 3 mm crystal at the full 600 mm beam diameter with the required transmitted wavefront quality; the largest possible during facility construction was a 325 mm circular aperture. It was necessary therefore to reduce the beam to 300 mm using a segmented absorbing glass apodizer, prior to the conversion crystal. This reduced the maximum pulse energy of the short pulse from 500 J (1 PW) at 1054 nm to 100 J (200 TW) at 527 nm. The experiments carried out in the second harmonic () proved very successful[
2 Enhanced second-harmonic capability design
The modelled spatial profile of the first-harmonic () light is shown in Figure
Evaluating the options showed that the 500 mm large aperture crystal and the twin 320 mm square crystal schemes gave the greatest improvement to the energy. For the large aperture option the 500 mm crystal posed a significant manufacturing risk. In addition to this there were concerns that the larger dichroic mirrors required for this design would not fit within the existing chamber and would be prohibitively expensive to implement. For the twin crystal option the greatest challenge is ensuring that the two emerging beamlets can be spatially and temporally overlapped on target. This would require the use of two separate motorized mirrors, vertically stacked (‘double deck’), each capable of tilt and piston control. Although the mirror mounts would need to be redesigned, it was possible to reuse the existing dichroic mirrors used in the frequency conversion process. Considering the relative risk, complexity and cost it was decided to proceed with the twin square crystal option.
The final engineering design for the twin beamlet frequency conversion chamber is shown in Figure
The second mirror mount is motorized to spatially and temporally overlap the two beamlets on target. The required angular resolution of the mounts is approximately . In addition, the path length of the two beamlets must be matched to within a fraction of a wavelength if the beamlets are to combine coherently and act as a single beam when focused. Therefore the mirrors must be translated, without introducing additional tilt, with a resolution of around 100 nm. To assess the performance of the new mirror mount an autocollimator was used to measure the pointing as a function of actuator position, as shown in Figure
The final focusing optic for the short-pulse beamline is an off-axis parabolic mirror mounted on a motorized hexapod. For operation this is an 720 mm diameter parabolic mirror coated for use at 1054 nm. The original design utilized an 360 mm diameter dichroic mirror coated for use at 527 nm; this was mounted onto the same hexapod using an aluminium adaptor ring. For the new twin beamlet design, an 720 mm ( for each beamlet) dichroic parabolic mirror coated for 527 nm operation is used.
3 Enhanced second-harmonic performance
The original frequency conversion components were removed from the vacuum chamber and replaced by the new twin beamlet design. A microscope camera was used to image the focused beams at the TCC. Pulses from the OPA were propagated along the beamline and through the frequency conversion chamber. There is sufficient intensity for the frequency-doubled pulses to be visible when focused by the parabola at the TCC. This camera was then used to angle-tune the frequency conversion crystals for maximum conversion efficiency. The top dichroic mirror on the second ‘double-deck’ mount was then tilted to bring the two beamlets together near focus. A piston scan was then performed using this mirror to scan its path length relative to the lower deck. The pulse coherence length of ps gave an interaction length of approximately over which interference between the two beamlets was visible. For this measurement an apodizer was placed in the near-field to sub-aperture each beamlet to mitigate the imperfect focal spot profile and therefore improve the interference fringe clarity, as shown in Figure
The microscope camera was then used to optimize the focal spot of the two beamlets using the off-axis parabola as described previously. Unfortunately, the four reused dichroic turning mirrors had developed a significant defocus aberration since their initial installation (total of ). Although this could be adequately compensated for in the case of the single beam, it posed an additional challenge for the dual beamlet design. A solution was to share the affected mirrors between the two beamlets. The individual weakly defocused beamlets could then be optimized through the application of tilt using the final turning mirrors to bring the centres of the apparent defocus wavefront errors together such that they could be largely compensated for by the positioning of the off-axis parabola. The residual aberrations from these mirrors, together with those of the second diffraction grating, are the dominant sources of wavefront error for the focal spots. The remaining optical components, including the new turning mirrors, off-axis parabola and the two frequency conversion crystals, impart a negligible effect. Through optimization of the off-axis parabola position it was possible to achieve a focal spot of , correlating to less than twice the diffraction limit. A scheme to further optimize the focal spots using a wavefront sensor at the TCC is currently under development.
A series of test shots were then carried out to measure the performance. A direct measurement of the pulse energy of the two beamlets was not feasible as the conversion chamber and target chamber are both required to be at vacuum for shot firing operations. A method to calculate the total energy in the two beamlets is used based on a cross calibration using the diagnostic station. This diagnostic station measures a sub-aperture of the two beamlets and uses a common calorimeter to measure both beamlets simultaneously. Comparison of this energetic data with the original data (where the diagnostic beam was the full aperture of the main beam) allows for the extrapolation of the energy in the new configuration. The maximum energy measured on a shot with this technique is 210 J. As the pulse duration of the beam (prior to frequency conversion) is typically measured to be of the order of 500 fs, this relates to a power of approximately 400 TW.
This enhanced capability is now being utilized on full energy plasma physics experiments. Figure
4 Conclusion
The second-harmonic performance of one of the Orion short-pulse beamlines has successfully been enhanced. The change in design from a single 300 mm circular beam to twin 300 mm square beamlets has resulted in a 2.1 times increase in achievable energy. The capability of coherently recombining these two beamlets on target has been demonstrated.
The upgraded performance has been utilized for plasma physics experiments. The increased energy is able to heat through more material, enabling experiments at higher density than previously. Also, the ability to heat larger volumes of material reduces the temperature gradients within a prepared sample of high energy density material, leading to more accurate results. The maximum pulse energy of 210 J demonstrated in these experiments, with a typical pulse duration of 500 fs, corresponds to a total power deliverable to target of approximately 400 TW.
[5] M. T. Girling, S. J. F. Parker, D. Hussey, N. W. Hopps. Proc. SPIE, 7721(2010).
[10] D. I. Hillier, M. T. Girling, M. F. Kopec, N. W. Hopps, J. R. Nolan, D. N. Winter.
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Stefan Parker, Colin Danson, David Egan, Stephen Elsmere, Mark Girling, Ewan Harvey, David Hillier, Dianne Hussey, Stephen Masoero, James McLoughlin, Rory Penman, Paul Treadwell, David Winter, Nicholas Hopps. 400 TW operation of Orion at ultra-high contrast[J]. High Power Laser Science and Engineering, 2018, 6(3): 03000e47
Special Issue: HIGH ENERGY DENSITY PHYSICS AND HIGH POWER LASERS
Received: May. 1, 2018
Accepted: Jul. 20, 2018
Published Online: Aug. 22, 2018
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