1. Technical Board, 18 September 2002Børge Svane Nielsen, NBI1 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

2. Technical Board, 18 September 2002Børge Svane Nielsen, NBI2 Laser rays will hit TPC 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 on the mylar strips from the laser rays? What is the effect of the laser rays reflected from mylar strips?

3. Technical Board, 18 September 2002Børge Svane Nielsen, NBI3 Effect of rays on mylar We use aluminized mylar strip received from CERN Shoot on strip with laser beam reflected from a micromirror (  1 mm) at 2 m distanceResults:  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. Test at NBI:

4. Technical Board, 18 September 2002Børge Svane Nielsen, NBI4 Image on ray onto mylar FWHM=0.95 mm Image of 1 mm laser beam after 200 cm z=200cm

5. Technical Board, 18 September 2002Børge Svane Nielsen, NBI5 Reflection from mylar FWHM=23-26 mm Distance mylar foil  screen  70 cm Image of 1 mm laser beam reflected off mylar foil  ionisation down by factor ~ 500 compared to primary laser ray

6. Technical Board, 18 September 2002Børge Svane Nielsen, NBI6 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 @ 10 Hz  The effect looks like melting of Al layer  No damage is seen below 200  J/pulse @ 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.