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

For the case of NEA GaAs photocathodes in HV DC guns with very good vacuum (ca. 10-11 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

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

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

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

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

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

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

Outgassing results – thin walls 12/2/2018 PESP2008

Outgassing results – thick end caps Also note recent JLab FEL gun 400 C, 100 hour bake, giving 1.5 x 10-13 torr-l/sec-cm2 outgassing 12/2/2018 PESP2008

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

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 – 10-15 mm thick materials give ~ 100 times more outgassing than much thinner materials Titanium??? 12/2/2018 PESP2008

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

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

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

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

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

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

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

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