Pressure measurements for the gun Total pressure Extractor
- Slides: 8
Pressure measurements for the gun Total pressure: Extractor Gauge May 21, 2018: https: //logbooks. jlab. org/entry/3577517 • N 2 equivalent in gun with gauge on 2 hours: 5. 4 e-12 Torr • Subtract x-ray limit of 1. 5 e-12: 3. 9 e-12 Torr • Assume all hydrogen, correct 0. 46: 8. 47 e-12 Torr Gas composition: Prep chamber RGA traces: use as worst case scenario of gun gas composition (see last 2 slides for details) Gas (amu) N 2 equivalent reading (Torr) Ionization correction factor Partial Fraction of pressure (Torr) total Hydrogen (2) 2. 2 e-10 0. 46 4. 8 e-10 93% Methane (16) 1. 88 e-11 1. 4 1. 3 e-11 2. 6% CO (28) 2. 33 e-11 1. 05 2. 2 e-11 4. 3%
Pressure simulation along beam axis: Molflow+ software from CERN • This uses a cylinder to approximate electrode • New Y/laser chamber geometry • Refine to more closely resemble CEBAF, particularly in cathode anode gap area
Assumptions in the data • Heated steel outgassing 1 e-12 Torr. L/scm^2 • Y chamber and laser chamber at 3 e-12 Torr. L/scm^2 • May be underestimated • NEG coating sticking function 0. 005 (from literature) • Pumping in DP can all at bottom, pumping in Gun on walls (low detail for these approximations) • Gun NEG pumps 60% activated as in 250 C bake
Hydrogen vs. z: Molflow simulation 8 E-12 Pressure (mbar) 7 E-12 6 E-12 • Wiggles in distribution due to short simulation time • Pressure lower than what was measured Hydrogen pressure distributions 3 anode tube holes, hydrogen 5 E-12 4 E-12 3 E-12 2 E-12 1 E-12 0 E+00 -160 -110 -60 -10 Distance along gun axis (cm) 40 • Pressure can be scaled up by factor of ~3 to match • Extractor gauge raises gun pressure • Extractor gauge reading will decrease for ~2 weeks after energization • Simulation mbar, measurements Torr
Hydrogen vs. z for different configurations Hydrogen pressure distributions 8 E-12 7 E-12 without anode tube holes 3 anode tube holes, hydrogen 6 holes anode tube Pressure (mbar) 6 E-12 5 E-12 4 E-12 3 E-12 2 E-12 1 E-12 0 E+00 -160 -140 -120 -100 -80 -60 Distance along gun axis (cm) -40 -20 0
Pressure (mbar) Gas composition Distance along gun axis (cm)
CEBAF Prep chamber RGA traces: HV valve closed (left) and open (right)
AMU Partial Pressure (Torr)