Schneider, W J

CERN Document Server

A LOAD-LOCKED GUN FOR THE JEFFERSON LAB POLARIZED INJECTOR*W.J. Schneider#, P. Adderley, J. Clark, A. Day, B. Dunham+, J. Hansknecht, P. Hartmann, J. Hogan,R. Kazimi, D. Machie, M. Poelker, J.S. Price+, P.M. Rutt, K. Ryan, C.K. Sinclair and M. SteigerwaldThomas Jefferson National Accelerator Laboratory, Newport News, VAAbstractConstruction is underway at Jefferson Lab on a load-locked polarized electron source.  The design incorporatesall of the essential features of the existing non load-locked gun and improves on the designs of existing load-locked guns operating at other labs.  When complete, weexpect the new load-locked gun to enhance the versatilityof the JLAB polarized injector.1  INTRODUCTIONHistorically, load-locked guns have been constructed as ameans of circumventing seemingly insurmountableobstacles that have prevented labs from deliveringreliable polarized beam to physics end-stations.  Forexample, at SLAC, prior to the construction of their load-locked gun, full cathode activation in the main gunchamber caused high voltage breakdowns.  It is believedthat the high voltage breakdowns were associated withcesium deposition on the cathode electrode during theinitial activation [1].   Once this process (i.e., initial fullactivation of the photo cathode) was performed in thepreparation chamber of their load-locked gun, the highvoltage incidents ceased. At MAMI, short cathodelifetimes (~ hours) necessitated frequent cathodereplacement, a situation that prevented reliable beamdelivery to nuclear physics users during a typical months-long experiment [2].  The load-locked gun at MAMI nowallows the accelerator staff to change photo cathodes withminimal delay (few hours) to the nuclear physicsprogram. At Jefferson Lab, we have demonstrated that a load-locked gun is not essential to meet the demandingrequirements of the Jefferson Lab nuclear physicsprogram. For example, unlike SLAC, we do not have anyhigh voltage problems associated with doing cathodeactivation in the gun proper and, unlike MAMI, ourcathode life at high current is excellent. Over the past twoyears, we have identified a number of mechanisms thatcontribute to the decay of photo cathode quantumefficiency. Understanding these decay mechanisms hasallowed us to implement design changes to our non-load-locked gun that have resulted in exceptional lifetime (1/elifetime > than 100 H at 100 µA, > 1000 H at 10 µA) [3]._______________________*Work supported by U.S. Department of Energy DE-AC05-84-ER40150 # Email: schneide@jlab.org +Now at GE Medical Systems Milwaukee, WI With our present non-load-locked gun, 10’s ofCoulombs can be delivered to Users before intervention(i.e., re-cesiation or heat treatment followed by fullactivation) is required.  Such polarized sourceperformance means that intervention can occur during ascheduled-maintenance day, (every other week at JLAB)with no impact on the physics program. Still, the obvious disadvantage of the present non-load-locked gun is that a bake-out is required when photocathode replacement is necessary. We have reduced theentire replacement process (photo cathode replacement,bake-out and gun re-commissioning) down to fifty-twohours beam to beam - albeit small, but still a delay.Although we do not expect to improve the inherentperformance of the non-load-locked gun, we believe aload-locked gun will greatly enhance the versatility of thepolarized injector.  With a load-locked gun, we couldrapidly change photo cathodes to meet changing demandsof the nuclear physics program, and during biannualfacility development periods; we could change photocathodes quickly to conduct photo cathode research usingthe superb diagnostics in the injector. Before making specific design plans, we outlined thebasic features of the Jefferson Lab load-locked gun.These features are based on our experiences at JLAB andthe experiences of our colleagues at other Labs. Theyinclude:• Installation of the photo cathode from air to the gunchamber must take less than six hours.• The load-lock vacuum chamber must be at groundpotential and there must be no moving parts at highvoltage.•   The gun and bend magnet must produce beam in thehorizontal plane; the bend magnet must not deflectthe beam more than 15 degrees.• There will be four chambers; a) main gun chamber,b) cathode preparation chamber, c) heat cleaningchamber, and d) atomic hydrogen cleaning chamberwhere samples are inserted into the load-lockmechanism and cleaned with atomic hydrogen• A fifth chamber may be added for storage of photocathodes or cleaned wafers ready for activation.•   Gun features that have proven to be essential on thenon-load-locked gun (superb vacuum, electrodesdesigned specifically for Jefferson Lab beam currentrequirements, electron optics that minimize strayelectrons hitting vacuum chamber walls, etc.) mustbe incorporated into the load-locked gun design.0-7803-5573-3/99/$10.00@1999 IEEE. 1991Proceedings of the 1999 Particle Accelerator Conference, New York, 1999A brief description follows of the load-locked gun beingassembled with specific detail on the major points ofinterest.2  MAIN GUN CHAMBERThe gun design (figure 1) is a novel one as it makes useof the better electron optics of a horizontal configuration,has no moving parts at HV and has all adjacent chambersat ground potential. Low base pressure (with and withoutbeam extraction) and wise choice of materials is thoughtto be the most important ingredient for long photocathode lifetimes. The main gun chamber is manufacturedfrom a standard six-way stainless steel cross. It has one220 l/s ion pump and three GP 500 MK 2, SORB-ACSAES cartridge pumps symmetrically located around thephoto cathode.  The non evaporable getter (NEG) pumpsare well suited for pumping CO, CO2 and greatly enhancethe pumping speed (~ 4000 l/s) for hydrogen, thedominant gas species in the vicinity of the photo cathode.Pressure in the gun chamber is further reduced, becauseall cathode preparation is done in a separate chamber. Wehave achieved pressures below 1x10-12 Torr with a similargun [4] pumped by a massive NEG array.  That wasmeasured by an extractor gauge with a measurement limitclaimed to be 1x10-12 by the manufacturer.A stainless steel tee supports the cathode electrodewhich has a shape designed for high current operation andis made of titanium alloy (Ti-6Al-4V).  The titaniumexhibits better high voltage performance (ie conditioning,low field emission current at full gun fields, etc.)[5]. Theceramic, to isolate the 100 kV (120 kV peak) highpotential, is located vertically so that photo cathodepreparation is performed at ground potential. Amolybdenum puck, which carries the cathode wafer, issimilar in concept to the SLAC design. The puck is heldin place inside the tee supporting the cathode with a stemspring holder and sapphire rollers. The electrode holder,triple point protectors and internal surfaces of the six-waycross are electropolished while the electrodes aremetallographically polished with diamond paste. Theanode, also manufactured of titanium alloy, is mountedon a stainless steel spider with a large open area toincrease vacuum conductance from the gun properthrough a 2.5-inch beam line. A channel cesiator isprovided behind the anode for in situ “touch-up”.Alignment of the electrodes relative to the beam axis wasaccomplished to better than a ½ mm.A Surface Interface (SI) manipulator [6], on the beamaxis which can both translate and rotate, is used to movethe puck between the preparation chamber and the gun(figure 2). A silver plated stainless steel adapter, mountedon the manipulator engages a set of transfer ears insidethe puck to allow attachment and release of the puck.Movement of the puck into and out of the gun has workedsmoothly although we have not yet baked out the entireapparatus. Isolation between the gun and the cathodepreparation chamber is accomplished through a 1.5-inchultra high vacuum metal sealed VAT valve [7].3  CATHODE PREPARATION CHAMBERThe cathode preparation chamber contains all of thecomponents to produce negative electron affinity (NEA)photocathodes: a stainless steel chamber with eight portsplaced around the circumference. Two ports are used forthe SI manipulators, the others ports are for a 40 l/ssputter ion pump, a GP 100 SAES NEG, a channelcesiator, a NF3 oxidizer, an optical window with a mirrorfor light and a ring anode.  In addition, a SRA [8] residualgas analyzer (RGA) and an extractor gauge have beenadded for vacuum diagnostics. On beam axis of thechamber is the SI manipulator, previously mentioned, thattransfers the puck into the gun. The puck is transferredfrom the on axis to the transverse manipulator via analuminum clamp that attaches to the molybdenum puck.This transfer from gun manipulator to load lockmanipulator has also worked smoothly; again we have notbaked. Pressure in the cathode preparation chamber ismaintained at better than 1E-9 Torr. At some point, wemay add a storage area to the preparation chamber thatwill allow us to activate a number of wafers during anaccelerator maintenance day and store them for futureuse.  Separating the cathode preparation chamber fromthe next chamber - heat-cleaning chamber is a 2.5-inchVAT ultra high vacuum metal sealed valve.4  HEAT CLEANING CHAMBERHeating the photocathode samples is accomplished in aseparate chamber in a manner that differs from techniquesused at other labs. The heat-cleaning chamber isfabricated from two six-way stainless steel crosses andone water-jacketed spool piece where the heating takesplace. A SI manipulator allows transfer of the puckbetween the load-locked or the cathode preparationchambers into the heat-cleaning chamber. A ResearchInc. model 4085 infrared spot heater, powered by aChromolox Port –12221 control system is used to heat thewafer to ~ 600 C at a ramp rate up and down of 1 degreeC per second. The chamber is also equipped with activecooling. The heater is capable of 750 W although we havefound that 375 W appears to be sufficient and havelimited the power supply.  A thermocouple in the IRbeam is presently used for control of the heater. We are inthe process of developing the parameters (rates ofheating, thermocouple location, pressure rise, etc.) for theheat-cleaning chamber.  Separating the cathodepreparation chamber from the heat-cleaning chamber is a2.5-inch VAT ultra high vacuum metal sealed valve.  Wehope to maintain ultra high vacuum using a combinationof a 40 l/s ion pump and a GP100 SAES NEG tominimize the pressure rise during the heating cycle and toremove the hydrogen which is desorbed from the waferdue to the hydrogen cleaning process. A similar 2.5-inch1992Proceedings of the 1999 Particle Accelerator Conference, New York, 1999VAT ultra high vacuum metal sealed valve separates theheat cleaning chamber from the atomic hydrogen cleaningchamber.5  ATOMIC HYDROGEN CLEANING ANDLOAD LOCK CHAMBERThe primary function of this chamber is to introduce anumber of different wafers on pucks and store them foreventual processing. Processing that takes place in theload lock is atomic hydrogen cleaning which is now theonly cleaning method used to prepare photo cathodes atJLAB [9]. To maximize quantum efficiency we haveincorporated an in situ atomic hydrogen cleaner.  Thehighest quantum efficiencies achieved at JLAB have beenwafers that were cleaned in situ in a low voltage testchamber. Similar efficiencies have been achieved whensamples obtained in a portable cleaning chamber weretransferred through air and installed in a gun. A Balzermodel TMU-071 turbo-pump is used during atomichydrogen cleaning where typical values for temperatureand pressure near the sample are 350 C and 1x10-5 Torrrespectively. The load lock makes use of the identicalinfrared heater presently used in the heat-cleaningchamber to raise sample temperature for atomic hydrogencleaning. It is anticipated that in situ atomic hydrogencleaning will provide a means to clean exotic photocathode materials for which wet chemistry techniques areincapable; for example, chalcopyrites that contain silicon.6  CONCLUSIONSIn summary, we have discussed the design of thepolarized electron gun and its associated processingchambers. We have developed a 100 kV photoemissionelectron source, which currently supports the delivery ofhighly, polarized, high average current CW electronbeams with long cathode operational lifetime. The newload-locked source incorporates the capability ofexchanging, cleaning (both atomic hydrogen and heat)and activating (co-deposition of cesium and NF3) thephoto cathodes without breaking the necessary ultra highvacuum. The gun chamber proper is separated from thepreparation, heating and cleaning load lock chambers sothat it is capable of reaching the extreme high vacuumregime to obtain long cathode lifetimes.7 REFERENCES1. J.E. Clendenin, “Polarized Electron Sources”, IEEE 0-7803-3053,(1996).2. E. Reichert, Nucl. Inst. Meth. A, Vol. 391, Pg 498-506, (1997).3. M. Poelker et al, LE 98 Workshop, St. Petersburg, Russia, (1998).4. C.K. Sinclair, invited paper this conference.5. R. Latham, High Voltage Vacuum Insulation, Academic Press, (`1995)Fig. 1Fig. 26. Surface Interface Inc., 110 Pioneer Way, Suite D, Mountain View,CA  94041, (415) 965-8205.7. VAT Inc., 500 W. Cummings Pk, Woburn, MA  01801, (617) 935-1446.8. Stanford Research Systems, SRS, 1290 D Reamwood Ave.,Sunnyvale, CA  94089, (408) 744-90409. C.K. Sinclair, et al, Atomic Hydrogen Cleaning of SemiconductorCathodes PAC ’97 Proceedings, Vancouver, B.C., (1997).1993Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

A Load-locked Gun For The Jefferson Lab Polarized Injector

Abstract

Similar works

Full text

Available Versions

CERN Document Server