S Astbury 1 , A Alejo 3 , S Bedacht 4 , P Brummitt 1 , D Carroll 1 , R Clarke 1 , S Crisp 1 , C Hernandez-Gomez 1 , A Higginson 2 , P Holligan 1 , S Hook 1 , J S Merchan 1 , D Neely 1,2 , A Ortner 4 , D Rathbone 1 , P Rice 1 , G Schaumann 4 ,
G Scott 1 , C Spindloe 1 , S Spurdle 1 , A Tebartz 4 , S Tomlinson 1 , F Wagner 4 , M Borghesi 3 , M Roth 4 and M K Tolley 1
1 Central Laser Facility, STFC, Rutherford Appleton Laboratory, Harwell Oxford, Didcot, Oxon, OX110QX, UK 3 Queen's University Belfast 2015, University Road Belfast, BT7 1NN, Northern Ireland, UK
2 Department of Physics, SUPA, University of Strathclyde, Glasgow G4 0NG, UK 4 Institut für Kernphysik, Technische Universität Darmstadt, 64289 Darmstadt, Germany
A progress review of the cryogenic targetry system being developed at the Central Laser Facility for high-power laser experiments.
Figure 1. Left: Vacuum test chamber and temperature/gas flow control systems. Centre: Inner chamber showing coldhead and radiation shielding. Right: Solid deuterium target inside growth chamber o Sumitomo RP-082B pulse tube cryocooler allows 5K minimum temperature at coldhead. o Automated mass flow and Lakeshore 336 temperature control systems (LabVIEW compatible) o Motor driven growth chamber sealing/exposure mechanism o 4-sensor temperature readout with surface and volumetric mountings. o 3-stage radiation shielding (and superinsulation blanketing) o 150W heating power. Coldhead from 5k – 293K in 30mins. o 2x27000rpm roughing-backed turbo pumps (2000l/min capacity, P min
= 5x10 -7 mbar)
Figure 2.
Under-view of coldhead
Copper target mount/foil
Stainless steel growth chamber with sapphire glass optical windows
Motor drive mechanism
Gas injection capillary
Temperature sensors
Figure 3.
Front view of target and growth chamber
Around 10 shots incident on cryogenic deuterium were delivered over a period of two weeks.
Many issues from the previous experiment had been addressed to allow for better survivability, thinner targets and better characterisation.
Target growth substrate reduced by a factor of 30 from
3mm to 100um, reducing the overall ice thickness from
Figure 9.
Installation of system and diagnostics in TAP target chamber.
A HeNe side probe diagnostic was installed which allows a comparative image of the target thickness before growth and after ice exposure to the outer vacuum.
Figure 10.
Left: 3mm thick foil from TAP 2014, Right: 100um etched foil from TAP 2015.
Through vastly improving the thermal radiation shielding, deuterium targets have been produced which can survive on the order of days rather than seconds.
100um
~400um
Figure 11.
Side probe images of target foil before
(left) and after (right) ice growth/exposure.
~4mm (estimated) to under 400um.
Figure 12.
Time-lapse of deuterium ice survivability over a period of 14 hours exposed to a high vacuum.
Following the phase diagram, there are five relevant stages within the growth procedure of solid hydrogen:
1. Regulate temperature above H
2
(<20K)
boiling point
2. Hydrogen gas injection: gas bled into chamber until pressure reaches
~500
20mbar.
3. Temperature dropped and gas bled in for pressure regulation.
4. Liquefaction occurs and liquid flows over and saturates target foil aperture.
5. Further cooling below 13.8K freezes the hydrogen into a thin solid membrane.
Figure 4. Low-pressure phase diagram of hydrogen, showing the solid, liquid and gas regimes and their dependence on pressure and temperature.
Figure 5.
Time-lapse of the growth process, from left to right: Initial state of system (~20K), the system in liquidphase and the solidified target.
One of the major issues of the project is the period of time the ice targets survive for upon exposure to high vacuum.
Minimum achievable coldhead temperature with varying exposure to ambient thermal radiation o Target sublimation upon exposure to low pressure environment decreases with lower base temperature (thus increasing target survivability). o Further modelling work shows that thermal radiation is main contributing factor. o An in depth study into the effects of thermal radiation on target survivability was undertaken.
Figure 8 . Effect of thermal radiation on minimum coldhead temperature.
• System modified to allow for spray deposition of tens of nanometres of deuterium gas onto micron thick gold foils.
• Adjustable trigger system for gas jet installed (nominally ~250ms prior to laser pulse)
• Gas jet and subsequent deposition manipulated by varying backing pressure of deuterium and trigger time.
• Dual target foils installed for higher shot rate.
Figure 13.
Dual target mount setup
275
265
235
225
255
245
Deposition layer thickness
215
Time
Figure 14. Deposition graph of deuterium on 6K cooled
QCM
• Using the principle of thin-film deposition monitoring systems the CLF have developed a cryogenically compatible quartz crystal microbalance (QCM).
• Able to detect deposition of deuterium after triggering as a method of calibrating gas jet parameters.
Figure 15.
CLF’s cryogenically compatible thin-film deposition system
Repetition rate improvements:
• Damage mitigation of target foil– extended target stalk, retractable mount.
• Decreasing warmup time
• In vacuo replacement of target foil
Characterisation:
• Further research into integrating confocal chromatic sensors