1. Charlie Sinclair Cornell University (retired)
Vacuum Considerations for High Average Current Photoemission Electron Guns
Charlie Sinclair
Cornell University (retired)
12/2/2018
PESP2008

2. For the case of NEA GaAs photocathodes in HV DC guns with very good vacuum (ca mbar), ion back bombardment is the only significant cause of QE decay There is very strong evidence for this from a number of polarized electron sources operating at high average current
12/2/2018
PESP2008

3. several locations over a period of many weeks. The QE in the
QE scans of a GaAs
cathode in one of the JLab 100 kV polarized guns. The cathode was illuminated by a
small laser spot at
several locations over a period of many weeks. The QE in the
unilluminated areas
is essentially unchanged over this time.
12/2/2018
PESP2008

4. We characterize QE degradation by ion back bombardment in terms of the charge delivered per unit illuminated area – Qo Coulombs/cm2
If the maximum available laser power is Pmax, a cathode of initial absolute QE h, uniformly illuminated at wavelength l (mm) over an area A, can deliver a constant current Io for a time T
12/2/2018
PESP2008

5. Performance approaching this has NEVER been demonstrated.
The current best reported values for Qo for GaAs are about 2 x 106 C/cm2. Thus, with an initial QE of 10%, and a 1.8 mm diameter spot illuminated at 520 nm, it should be possible to deliver 100 mA average current for over 200 hours.
Performance approaching this has NEVER been demonstrated.
200 hours is only 8 days – it is clearly desirable to increase Qo significantly. Improving the base pressure is the only path to accomplish this.
12/2/2018
PESP2008

6. Three topics Where does the residual gas come from?
Hydrogen and methane outgassing
How do we know the pressure?
Problems with existing gauges and RGAs,and potential solutions
How do we reduce the residual gases more effectively?
High temperature (400 – 450 C) bakeout
Elimination of methane source
NEG pumping
Ion pump improvements
12/2/2018
PESP2008

7. Cornell H2 Outgassing Measurement
Sample chambers were 1.5 m long, 15 cm i.d., with
1.65 mm walls. Both 304L and 316L stainless were
tested. End caps were 1.65 or 20 mm thick. Temperature was 25 +/- 0.1 C during measurement.
12/2/2018
PESP2008

8. Bakout and measurement protocol
Initial bake to 400 C for 96 hours, either under vacuum or in air
Outgassing measurement
Exposure to air for 8 hours
Bakeout to various temperatures – 150 – 250 C
Measurements done by rate-or-rise with a SRG, checked against throughput method
12/2/2018
PESP2008

9. Outgassing results – thin walls
12/2/2018
PESP2008

10. Outgassing results – thick end caps
Also note recent JLab FEL gun 400 C, 100 hour
bake, giving 1.5 x torr-l/sec-cm2 outgassing
12/2/2018
PESP2008

11. Conclusions
Air or vacuum bake to 400 C for 96 hours reliably gives very low hydrogen outgassing from thin walled stainless chambers
Moderate bake temperatures – 150 C – are sufficient for subsequent bakes following extended air exposure
316L modestly better than 304L, air bake modestly better than vacuum bake
Thick walls limit the obtainable outgassing
Many more details in Park et al., JVST A 26, 1166 (2008)
12/2/2018
PESP2008

12. Conclusions, cont’d.
There is good news – we can bake systems in air to ~ 400 C, not under vacuum, without risk of leaks and such, and then follow up with vacuum bake to modest temperatures – ~ 150 C – to finish
It is important to absolutely minimize the use of materials thicker than about 3 mm – mm thick materials give ~ 100 times more outgassing than much thinner materials
Titanium???
12/2/2018
PESP2008

13. Methane outgassing
Widely believed to originate from the titanium in the plates of getter ion pumps
Known to be produced on hot filaments
Very large methane peaks, which pump away only slowly, observed during HV processing of guns – where does this gas come from?
With ion pump unpowered, methane level rises at a constant rate for many hours
12/2/2018
PESP2008

14. Possible methane solutions
Construct ion pump with highly pure titanium plates, to limit methane production
Eliminate the ion pump altogether, in favor of either a (maglev) turbopump backed by an ion pump, or a cryopump. These are expensive solutions, as a metal valve at the pump throat is necessary as well. Maglev pumps are large and expensive. Use the external turbo/cryo pump only during HV processing, and have no ion pump on gun?
12/2/2018
PESP2008

15. Improved XHV ion pump
Use very high purity titanium to reduce methane formation
Increase cell diameter
Increase magnetic field
Incorporate radioactive source(s) to sustain discharge
Would it be worth it?
12/2/2018
PESP2008

16. Problems with gauges and RGAs
Hot filament heats surrounding areas, causing outgassing
Hot filament may chemically alter residual gases
Electrons striking anode generate X-rays, leading to X-ray limit
Electrons striking anode cause ESD, distorting the residual gas spectrum and altering pressure readings
12/2/2018
PESP2008

17. Redhead’s original
extractor gauge
12/2/2018
PESP2008

18. Possible gauging solutions
Make anode cages of beryllium wires or pyrolytic graphite, to reduce X-rays and ESD – NO!
Collector wire of 7 mm carbon fiber – NO?
Modulate gauge collector to separate signal plus X-ray and X-ray only, use lock-in amplifier (gauge and setup dependent)
Employ a cold cathode – FE array or photocathode. FE arrays are gassy?
12/2/2018
PESP2008

19. An “ultimate” gauge Form an electron beam from a cold cathode
Pass the beam through a strong focusing channel
Collect ions produced along the beam channel
Collect the spent electron beam in a depressed collector at ~ cathode potential
This solution would do no chemistry, generate no temperature rise, and produce no X-rays or ESD
12/2/2018
PESP2008

20. Conclusions
A path to very low outgassing, based on 400 C air bakeout and very limited use of thick materials, seems clear. Only 150 C bakes of system are required (BUT – NEG activations???)
Eliminate ion pump to eliminate methane, and use external turbo/cryo pump only during HV processing?
Gauge/RGA changes necessary to KNOW the pressure you reach
12/2/2018
PESP2008