LhARA pre-CDR Meeting 20th Dec 2019

Meeting dedicated to discussion of interface between source and capture Gabor lens.

Material:

• Required aperture size versus distance from source -- J. Pasternak
• Initial thoughts on Gabor lens design -- C. Whyte

Present: KL, CW, AK, JPas, OE

Notes

• AK:
  • Current proposal is to allow 10cm for diagnostics. Based on Jaroslaw's slide, that would need an 8mm diameter hole in the differential pumping plate.
• CW:
  • Laser diagnostics are required and while the space that they occupy should be reduced, it is unclear that the distance source to aperture can be reduced to make the the hole significantly smaller.
  • Also, debris that comes through the hole will cause contamination within the volume of the lens.
  • Consequence is that capture lens needs to be designed to accept poor vacuum and significant particle contamination. The target vacuum vessel operates around 10^-4 to 10^-5 whereas the Gabor lens operates at a couple of orders of magnitude less.
  • Various mitigations to large hole (8mm hole at 10cm). This could be small-diameter mylar (or other material) window. Have made 50 micron thick 50mm diameter mylar windows that can hold off 1 bar. Even may need to accept a solenoid rather than a Gabor lens.
  • Can calculate what the pressures would be based on a thick plate with a tapered hole. It's not a problem to set up the simulation to look at a number of different lengths and aperture diameters.
• OE:
  • Raised issue of particle diagnostics in addition to the laser diagnostics/laser set-up. There is no solution for online non-destructive diagnostics. Current methods include radiochromic films and magnetic spectrometers.
  • Running the laser at 10Hz means everything gets coated (with debris from the target) fairly rapidly.
• KL:
  • Can use a re-entrant shape for the plate.
• JPas:
  • Beam optics solutions are not a problem (i.e. are within the specs of the Gabor lens) for distances up to 30cm.

Summary

• 10cm has been chosen as the distance from the target to the front face of the first Gabor lens. This is a compromise between allowing enough space for laser diagnostics and having a small enough aperture for differential pumping without significantly collimating the beam.
• CW will calculate the differential pumping performance of a re-entrant, tapered aperture.
• JPas will recalculate the optics for a target to lens distance of 10cm.

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