Proton Source Repairs

Dan Bollinger 2/1/10

Cockcroft-Walton acceleratorsFNAL 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 sourceAccelerating column

Ref: C. Schmidt; Proton and H- sources at Fermilab Jan 31 2002

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 31 2002Ref: C. Schmidt; Proton and H- sources at Fermilab Jan 31 2002

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 31 2002

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 life Out 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 repair1st time new column has been built in over 25yrs !!

Ceramic severely damaged

New column built on siteColumn pulled off of wall

New column has been installed and conditioned to 760kV

I- source problemsWe 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- current 40-60mA

Extraction Voltage 35kV

Arc Voltage 140-160V

Arc Current ~10A

Rep Rate 15Hz

Pulse width 100ms

Duty Factor 0.12%

Accelerating Voltage 35kV

Proposed upgrade to existing 750kV system