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Photogun Talking Points

Photogun Talking Points. Poelker, EIC, Jan 10, 2010. 1mA at High Polarization*. * Note: did not actually measure polarization. High Initial QE. Vacuum signals Laser Power Beam Current. How Long Can We Run at 1mA?. 1mA operation, 3.6 C/hr, 86 C/day.

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Photogun Talking Points

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  1. Photogun Talking Points Poelker, EIC, Jan 10, 2010

  2. 1mA at High Polarization* * Note: did not actually measure polarization High Initial QE Vacuum signals Laser Power Beam Current

  3. How Long Can We Run at 1mA? • 1mA operation, 3.6 C/hr, 86 C/day. • Photocathode with 1% initial QE, 2W at 780nm and gun with 250C charge lifetime (i.e., what we have at CEBAF) • Initial laser power = 160mW to produce 1mA • Should be able to operate at 1mA for 7 days before running out of laser power. Time to move to fresh photocathode spot (10 minutes), swap photocathode (1 hour), heat/reactivate photocathode (8 hours) • Imagine a 10W laser and 1000C charge lifetime, we should be able to operate at 1mA for 48 days before “doing something”!

  4. How about 25mA? • 25mA operation, 90 C/hr, 2160 C/day. • Photocathode with 1% initial QE, Need initial laser power = 4W to produce 25mA • If gun provides 1000C charge lifetime and you have 25W laser power, you can operate at 25mA for 20 hours before you run out of laser power. Time to move to fresh photocathode spot (10 minutes), swap photocathode (1 hour), heat and reactivate photocathode (8 hours) • For 100W laser and 10,000C charge lifetime, you can operate at 25mA for 15 days before “doing something”. • To date: ~ 2 Watt laser and 250C lifetime, so we have some work to do….

  5. How about 250mA? • 250mA operation, 900 C/hr, 21600 C/day. • Photocathode with 1% initial QE, Need initial laser power = 40W to produce 250mA • If gun provides 1000C charge lifetime and you have 100W laser, you can operate at 250mA for ~1 hour before you run out of laser power. Time to move to fresh photocathode spot (10 minutes), swap photocathode (1 hour), heat and reactivate photocathode (8 hours) • For 100W laser and 10,000C charge lifetime, you can operate at 250mA for 10 hours before “doing something”

  6. MeRHIC Polarized Source R&D Issues • High average current, 25mA operation • Low repetition rate = high bunch charge = high peak current • Good emittance? small beam?....higher peak current…space charge induced emittance growth • Child’s Law Space Charge Limit? • Surface charge limit • Need >> 100kV. Field emission • Laser power and photocathode cooling • Cathode/anode design for 100% transport • Photocathode R&D: try to get higher QE (~10%), something more rugged than superlattice GaAs

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