Proton Source Repairs Dan Bollinger 2/1/10

Cockcroft-Walton accelerators FNAL has 2 Cockcroft-Walton Pre-accelerators (Pre-acc) that provide beam to the entire complex (sans HINS) I- H-

Each Pre- acc consists of 2 main parts H- ion source Accelerating column Ref: C. Schmidt; Proton and H- sources at Fermilab Jan

Negative ion source (magnetron) Ref: Handbook of ion Sources; Bernard Wolf Cesium on cathode lowers work function of molybdenum (Mo) cathode to enhance H- production: Work function for Mo is 4.6eV Work function for Mo + Cs is 1.8eV Lowest work function when there is a 0.6 monolayer of Cs Source operation relies on surface plasma effect Plasma produces energetic particles that strike the cathode surface H- ions are produced by desorption or reflection from a cesium coated cathode surface Ref: C. Schmidt; Proton and H- sources at Fermilab Jan

Negative ion source (magnetron) con’t Source from column perspective Ion source points down. The H- is extracted and bent 90deg to enter accelerating column The ion source resides in the dome at the upstream end of the column Ref: C. Schmidt; Proton and H- sources at Fermilab Jan

Negative ion source (magnetron) con’t Typical source aging related failures Aperture restricted by cathode flakes that have fallen off Cathode erosion Hydrogen inlet/valve blocked by cesium hydride Sources last on the order of 3 months and usually fail from one of these type of problems

H- source since start-up Source has been taken out and rebuilt twice Source output not all that stable over long run New source has a good arc/beam current flattop Old sources have flattops that fall off on the trailing edge which is usually why they are taken out and rebuilt Typical signs of aging source

H- source since start-up con’t Water resistor needs flushed 1/month Conductive material accumulates around electrodes and causes voltage drops/imbalances on the accelerating column

H- source since start-up Ion pump out gassing due to end of lifeOut gassing affects on source operation Increases in gas pressure perturbs feedback loops Instabilities in source take fair amount of time to recover from Increasing frequency and magnitude of ion pump out gassing near end of pump life

H- source since start-up Ion pump was replace during shutdown last week 2400 L/s ion pump We reached a point where we were not able to increase the hydrogen gas pressure to keep the source operating (cesium hydride blocking gas inlet)

I- column failure Prior to shut down the I- column would not hold off above 500kV with out arcing over Hipot of column showed 1 accelerating gap was the problem

I- column repair 1 st time new column has been built in over 25yrs !! Ceramic severely damaged New column built on site Column pulled off of wall New column has been installed and conditioned to 760kV

I- source problems We have installed 2 sources for a total of 7 times into the I- column I- source arc current since we have tried to start sources after rebuilding the column The longest lived source has been about 18hrs The sources die off has been the same every time Possibilities: Cathode poisoning by some contamination Gas leak that is displacing the hydrogen Something with new accelerating column ?

I- source problems con’t Problems so far with sources: Badly eroded cathode Ion gauge Gas valves (both sources) Extractor cable failed Cs power supply failed HV regulation Ion pump occasional out gassing (not as bad as H-) Arc modulator powersupply Work done to try and fix overall problem: Leak checks RGA scan (nothing out to 100amu) Further RGA scans to 200 amu Tried source on test stand (longevity test) Cleaned cesium boiler tubes Lots of tuning Changed hydrogen valves On top of mysterious die off of the sources there have been equipment problems

Shameless Plug….. Based on Brookhaven design: round aperture dimpled cathode vacuum chamber: cube with lots of pumping Operationally: lower arc current higher extractor voltage better vacuum pumping at the source longer lifetime (hopefully) H- current40-60mA Extraction Voltage35kV Arc Voltage V Arc Current~10A Rep Rate15Hz Pulse width100ms Duty Factor0.12% Accelerating Voltage35kV Proposed upgrade to existing 750kV system