Laser rays on TPC mylar strips

1
Laser rays on TPC mylar strips

• Technical Board, 18 September 2002
• Børge Svane Nielsen and Jørn Westergaard
• Niels Bohr Institute

• Action point from TB in May
• Study effect of laser rays on TPC mylar foils
• Study laser ray reflection off mylar foils

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Laser rays will hitTPC mylar strips
Thin laser rays originate from rods at outside
of field cage Traverse 2 m of TPC volume Hit
mylar strips near inner field cage (or inner
field cage itself)
Mylar strips
What is the effect of the laser rays reflected
from mylar strips?
What is the effect on the mylar strips from the
laser rays?
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Effect of rays on mylar
Test at NBI

• We use aluminized mylar strip received from CERN
• Shoot on strip with laser beam reflected from a
  micromirror (? 1 mm)
• at 2 m distance
• Results
• 260-300 ?J/pulse (10 Hz) 1 mm hole through
  one Al layer after 15 min !
• 230-290 ?J/pulse (10 Hz) small damage after
  30 min
• 170-220 ?J/pulse (10 Hz) no visible damage
  after 4 hours
• 150-200 ?J/pulse (10 Hz) no visible damage
  after 20 hours (4 days)

Design energy is 20 ?J/pulse at ? 10 Hz, well
below the tested values. We believe that the
damage is due to melting of the Al layer.
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Image on ray onto mylar
Image of 1 mm laser beam after 200 cm
z200cm
FWHM0.95 mm
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Reflection from mylar
Image of 1 mm laser beam reflected off mylar foil
Distance mylar foil ? screen ? 70 cm

• ionisation down
• by factor 500
• compared to
• primary laser ray

FWHM23-26 mm
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Conclusion

• We have tested the effect of a thin laser ray
  hitting
• an aluminized mylar strip received from CERN
• Results
• We see damage of the strip above 230 ?J/pulse
  _at_ 10 Hz
• The effect looks like melting of Al layer
• No damage is seen below 200 ?J/pulse _at_ 10 Hz
  after 20 hours

Design power is 20 ?J/pulse, well below the
tested values. Laser frequency during calibration
and physics runs could be lower than 10
Hz. We have reasonable safety margin.