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Maxwell Calculations

Maxwell Calculations. SB Tent Crew. Apr 22, 2008. Maxwell Model. Single GEM. dV GEM = 500V drift gap = 1.5mm dV drift = 120V (FB) induction gap = 1.5mm dV induction = 500V P1 = F1 = 2*P2/cos(60 °) = 140um Cu hole diameter: 86um Kapton hole diameter: 80um

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Maxwell Calculations

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  1. Maxwell Calculations SB Tent Crew Apr 22, 2008

  2. Maxwell Model • Single GEM. • dVGEM = 500V • drift gap = 1.5mm • dVdrift = 120V (FB) • induction gap = 1.5mm • dVinduction = 500V • P1 = F1 = 2*P2/cos(60°) = 140um • Cu hole diameter: 86um • Kapton hole diameter: 80um • 2um x 2um x 2um resolution. P2 “solid” mesh drift gap GEM induction gap solid pad plane

  3. E-Field at GEM Surface Forward Bias 120V • Slice of Ez along red line: • ~flat Ez away from hole. • Near hole, large pe extraction field. • in hole, Ez drives e’s through hole. • <Ez> on GEM surface (>17u from hole edge) ~4.8 kV/cm • Ezmin ~3.9 kV/cm

  4. E-Field at GEM Surface Reverse Bias -30V • Slice of Ez along red line: • ~flat Ez away from hole. • Near hole, large pe extraction field. • in hole, Ez drives e’s through hole. • <Ez> on GEM surface (>17u from hole edge) ~5.9 kV/cm • Ezmin ~5.0 kV/cm

  5. PE Extraction Efficiency great news! Extraction Efficiency Extraction Efficiency Reverse Bias -30V Between 80-90% extraction efficiency • Bob Azmoun measured PE extraction efficiency vs GEM surface Ez. • 5 kV/cm is well on the plateau. • Even better news… • Extraction efficiency increases with RB.

  6. E-Field Line Directions Line density does NOT represent field strength! • In this simulation… • electrons were created at the surface of the GEM. • electron path follows E-Field lines • recorded PE’s final destination. • Reverse Bias (-30V): • most (~91%) e’s from the GEM surface end up on pads. • ~3% are swept up to mesh. • ~6% lost to GEM bottom. Reverse Bias Forward Bias note: field line destinations and actual charge destinations are differentdue to diffusion and avalanche. Lines go to mesh Lines go to pad Lines go to GEM bottom

  7. Photoelectron Destinations • In this simulation… • Reverse Bias (-30V) • electrons were created at the surface of the GEM. • electron path follows E-Field lines • recorded PE’s final destination. • most (~91%) e’s from the GEM surface end up on pads. • ~3% are swept up to mesh. • ~6% lost to GEM bottom.

  8. E-Field Line Directions Reverse Bias Line density does NOT represent field strength! Lines go to mesh Lines go to pad Lines go to GEM bot • For an intuition of what different negative drift fields look like… • notice that “reverse bias” doesn’t turn on immediately

  9. E-Field Line Directions Forward Bias Line density does NOT represent field strength! Lines go to mesh Lines go to pad Lines go to GEM bot • For an intuition of what positive (and zero) drift fields look like… • more positive drift field → more “squished” lines.

  10. Reproducing the “150μm” Number Line density does NOT represent field strength! Lines go to mesh Lines go to pad Lines go to GEM bot • Define Ez “turnover”: –0.005<Ez<0.005 (kV/cm) • represents green line • for every (x,y), look along z for place where–0.005<Ez<0.005 (kV/cm) • Ez is completely positive (ie. e’s pulled to mesh) ~130μm above GEM surface.

  11. Summary • Some common beliefs have been upheld about RB: • Reverse Bias -30V gives minimum extraction field of ~5 kV/cm • PE extraction efficiency btwn 80-90%. • Reverse Bias -30V, ~3% of GEM surface area has field lines terminating on mesh. • Careful about conclusions here… diffusion plays a major role. • Reverse Bias doesn’t immediately turn on with negative drift field. • at RB -10V, no field lines go from top GEM surface to mesh. • 130-150 μm is reasonable for ionization electron cut-off.

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