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Safety Studies for the ATF CO2 Laser System

Safety Studies for the ATF CO2 Laser System. Mark Palmer and the ATF Team. Background. We expect the ATF CO 2 Laser System to continue to increase in peak power over the next 3-5 years Main amplifier energy ~20J (limited by exit window damage threshold) Peak power increases by

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Safety Studies for the ATF CO2 Laser System

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  1. Safety Studies for the ATF CO2 Laser System Mark Palmer and the ATF Team

  2. Background • We expect the ATF CO2 Laser System to continue to increase in peak power over the next 3-5 years • Main amplifier energy ~20J (limited by exit window damage threshold) • Peak power increases by • Concentrating all energy in a single pulse (achieved 2018) • Compressing the pulses from ~5ps (2017) to ~2ps (2018) to <500 fs (2020) • Improving the transport efficiency of the system (better optics, vacuum transport, etc) • Critical issue: • Protecting main laser system from back reflections returning from user experiments • NOTE: Laser system will be severely damaged within a handful of shots if a major back-reflection occurs! • Solution: Plasma Shutter • CO2 long wavelength IR (LWIR) pulse passes through a pinhole • “Low” power near-IR laser strikes edge of pinhole to create plasma • Plasma fills pinhole creating a mirror for LWIR back-reflected pulses • RSC Safety Issue: A laser system misalignment fault can cause a high peak power LWIR pulse to strike the pinhole material creating x-rays (and also a back- reflection that will quickly cause laser damage and failure).

  3. Present-day laser configuration and planned upgrades • Upgrade R&D thrusts: • Reduce pulse expansion via stronger pulse stretching, reducing amplifier window thickness, beam profile control (apodizing, adaptive mirror) • Energy increase by improving compressor efficiency, stronger pulse stretching, implementing a bigger amplifier and window (long term) • Femtosecond regime with NLPC • Variable polarization • Material studies • Ti:Al2O3 Amplifier 1 • 20 µJ • 350 fs • OPA • 5 µJ • 60 ps YAG 1 Stretcher • 20J • 30 ps • 30 mJ • 40 ps 5J Operating Point Amplifier 2 In-vacuum transport to user experiments (prevents filamentation at high power) • 4 ps • Work on improving: • Rep. rate (faster HV switches, optical pumping) • Beam quality • Energy/power stability • System reliability • Pulse • 2.3 ps • 10J • Energy YAG 2 Compressor • Efficiency 50% Plasma Shutter ps fs

  4. Test Configuration • Note: Prior reviews have utilized estimates and external observations • Test utilized pre-existing experimental chamber • Goals: • Directly measure the x-ray source field within the vacuum chamber • Apply a thin Al filter (thickness of 3.35 mm) to distinguish low energy (< few 10s of keV) x-ray dose from high energy dose • Obtain measurements near likely ultimate parameters for B820 operation to aid future analysis of upgrades [extrapolate to lower fluences for radiation safety analysis] TLD IR camera TLD Interaction Chamber TLD TLD Plasma Shutter sphere OAP TLDs laser lens target Test sphere 6” diam.; windows are for placing TLDs around a target Optical transport

  5. Experimental Study Parameters Designed to significantly exceed the present operational configuration of the plasma shutter!! Relevant laser parameters for test dose rate estimate: • Laser pulse length 2.3 ps • Laser pulse energy 5 J • Laser pulse rep rate 0.03 Hz • Energy fluence @ 5 J 6.25E5 J/cm2 • Laser intensity @5J and 2.3 ps 2.7E17 W/cm2 • Plasma kTH(Ref 1) 976 keV • Hard X-ray energy / laser energy (Ref 1) 0.011 (range is 0.00737 to 0.0165) Ref 1: Physical Review A Volume 32, Number 6 December 1985, Superhot-X-Ray and -Electron Transport in High- Intensity CO2-Laser-Plasma Interactions, G. D. Enright and N. H. Burnett.

  6. Results Filtered Unfiltered

  7. Estimated Dose Rates in Experimental Configuration • 16 rad/hr at 3 inches with 3.35 mm Al shielding a 1 rad/hr at 1 foot (with no additional shielding) • Add 1 inch steel shielding: 803 mR/hrassuming 400 keV photons THIS DOES NOT REPRESENT AN OPERATIONAL CONFIGURATION!!!

  8. Operational Parameters Pinhole is a conical pinhole with a 10:1 surface aspect ratio Relevant laser parameters for test dose rate estimate: • Laser pulse length 2.3 ps • Laser pulse energy 5 J • Laser pulse rep rate 0.03 Hz • Hard X-ray energy / laser energy 0.002 • Normal incidence energy fluence at focal point @ 5 J 1.24E5 J/cm2 • Maximum energy fluence on pinhole surface @ 5 J 1.24E4 J/cm2 • Maximum Laser intensity on pinhole surface @5J and 2.3 ps 5.4E15 W/cm2 • Plasma kTH(Ref 1) 190 keV • Hard X-ray energy / laser energy (Ref 1) 0.002 Ref 1: Physical Review A Volume 32, Number 6 December 1985, Superhot-X-Ray and -Electron Transport in High- Intensity CO2-Laser-Plasma Interactions, G. D. Enright and N. H. Burnett.

  9. Shielding Requirements Starting point estimate… Estimate of unshielded dose rate in operational configuration (excluding x-rays < 30 keV, which are assumed to be filtered by the vacuum chamber components everywhere) • 5 J laser pulse x 0.002 x 0.03 Hz = 0.0003 J s-1 • 0.0003 J s-1 = 1.9E12 keV s-1 • Assuming all x-rays are 400 keV yields 4.7E9 photons s-1 Micro Shield yields 312 mR in 1 hour at 1 foot in air assuming an unshielded 400 keV point source. Again, references 1 and 2 do not account for x-rays below 30 keV. Ref 2: Physical Review Letters Volume 47, Number 23 December 1981, Hard-X-Ray Measurements of 10.6 mm Laser Irradiated Targets, W. Priedhorsky, D. Lier, H. Day, and D. Gerke.

  10. Shielding Approach – Version 1 Apply 1” Steel shielding wherever possible Explicitly deal with chamber penetrations • Ref 2: 5.5-fold decrease in hard x-ray production at operational intensity relative to experimental configuration:803 mR/hr (slide 7) a 165 mR/hr • This remains a significant overestimate of exposure rate since the spectrum in this calculation assumes all photons are 400 keV, and photon energies below 35 to 40 keV are not included. • Calculations do not account for the Maxwellian thermal distribution of x-rays! Newly published TVL data just became available, which properly accounts for the above thermal distribution! a Ref 3 Ref 3: Radiation Protection Around High-intensity Laser Interactions with Solid Targets, Liang, Taiee Ted; Bauer, Johannes M.; Liu, James C.; Rokni, Sayed H., Health Physics: December 2018 - Volume 115 - Issue 6 - p 687–697

  11. TVL Data Curves for Fe TVLe TVL1

  12. Shielding Approach – Updated for NEW TVL Data • TVLs monotonically decreasing at lower laser fluences due to lower temperature for x-ray production • Utilize 1017 W/cm2 values for all calculations • Start with 2767 rad/hr as measured in experimental configuration • Application of TVLs a shielded dose rate 1.68 mrad/hr @ 1 foot • Operational Config: Reduction of 5.5x as discussed on slide 10 • 307 mrad/hr @ 1 foot • Back reflection to cause laser failure in <10 shots • Realistic fault duration (~6 min) a<25 mrad @ 1 foot

  13. Shielding Approach – Special Features of Chamber • Chamber has penetrations • Upstream and downstream for CO2 Laser entrance and exit • YAG laser entrance port • View ports • Pinhole positioning stages on top of chamber

  14. Upstream and Downstream Shielding • Upstream and downstream apertures are limited to maximize solid angle coverage of Steel chamber • Upstream and downstream paths are “blocked” by thick Cu mirrors • Additional Steel shielding to be added in limited regions where coverage is not complete

  15. Plasma Shutter Chamber Shielding • 1 inch steel wall thickness • Vertical assembly protected by: • 1 inch steel layer where practicable • Design of pinhole support provides minimum 1 inch path through steel to any ”thin” region in top-hat (see pictures on following slides for clarification) • View ports • Block with 1 inch steel caps • Administratively control (see proposed OPM) • YAG Port…

  16. 2 spanner holes to rotate insert • 2 set screws @ 120° • Aluminum insert • SS body • Provides a shielding “umbrella” to cut out solid angle for vertically produced x-rays

  17. Shielding Approach - YAG laser Entrance Hole • Rely on fact that high energy x-rays will penetrate SiO2 optics • Require any exiting x-ray to need at least 2 x-ray reflections off of Steel surface (max. reflectivity ~0.05) • Utilize mesh guard to prevent physical access within 1 foot of YAG entrance hole Reflectivity of x-rays from SiO2 B. L. Henke, E. M. Gullikson, and J. C. Davis, “X-Ray Interactions: Photoabsorption, Scattering, Transmission, and Reflection at E = 50–30,000 eV, Z = 1–92,” At. Data Nucl. Data Tables 54, 181 (1993). YAG entrance hole

  18. Shielding Approach – YAG Laser Entrance Hole Radiation Calculation: As before – • Experimental Configuration Source 2767 rad/hr @ 3 inches • Assume 1 TVL1 (~6 mm) for SiO2 for exit window from Ref 3 (see slide 11) • 277 rad/hr @ 1 foot • Low energy x-rays have been filtered • Operational Config: Reduction of 5.5x as discussed on slide 10 • 50.4 rad/hr @ 1 foot • Remaining x-rays will penetrate the YAG SiO2 mirror (take no attenuation credit) • 2 reflections from steel required for x-ray to escape secondary chamber (0.05 reflectivity) • 126 mrad/hr • Realistic fault condition of duration ~6 min • < 10 mrad on contact with YAG entrance hole • Install barrier at minimum 12-inch radius from hole • Assuming 0.6-inch dia. hole a<3 mrad @ 1 foot • Note: may want to increase design hole diameter somewhat for ease of alignment a shouldn’t be a factor

  19. Conclusion • Plasma shutter configuration will be maintained under C-AD configuration control • X-rays created in alignment fault conditions can be shielded successfully to maintain a safe environment in the ATF FEL room • We propose to also add TLDs around the plasma shutter to provide long-term radiation monitoring • Exact location TBD a Questions and comments

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