Pelletron Battle – Field Report

All Experimenters’ Meeting

August 15, 2011

L. Prost (for A. Shemyakin)

Terminology for the Pelletron HV regulation system (simplified)

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Column current

ICorona ≈ IChain – IColumn - INeedle

Corona current

Corona current ≡ Lost current

Chronology: onset of problems and 1st access July 29, 2011

Following a lightning strike, the entire beam line needed to be re-tuned for an unknown reason in order to pass the beam through. The Pelletron was left off (no pbars to cool).

Aug 1, 2011 Turning Ecool on in attempt to understand what went wrong, we found that

we can’t operate because of a high corona current from the terminal. Aug 2, 2011

Staying with HV on and conditioning did not help. Started tank opening. Aug 3-4 access

Nothing special is found. Regular maintenance after 6-month operation (without opening)

• Included replacement of a water pump drive pulley Because ~1 liter of water had to be added to the collector water system, the

desiccant column (which removes water vapor from SF6) was re-generated

At startup: Very bad lost current

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Chronology: 2nd access Aug 5

Second regeneration of desiccant (tank filled with SF6)

Noticeable improvement, but still below operational voltage

Needles stopped moving, triggering the second access

Aug 6-8 access Needle motion problem disappeared in

air• Reset relevant connectors

Water pulley was found to be heavily damaged; plastic and aluminum chips everywhere

• A temporary spare was installed Needles and the triode (part of a HV

regulation system in the terminal) were replaced by new ones

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At startup: Very difficult conditioning The new triode did not work at high

current Electronics at one of Pelletron levels

stopped working, forcing an access (3rd)

Intermittent readings of the column resistive divider

Chronology: 3rd access

Aug 9-10 access Electronics and the triode were replaced; divider contacts tightened;

installed a permanent pulley Made a HV test in air: very high lost current Removed non-conductive residue from the tank walls Second HV test in air: noticeable improvement

Startup (Aug 11) Conditioning is better than any of previous times A BPM in the tank stopped working

• Significantly complicated commissioning Established cooling

• HV is less stable than usual; higher corona current; a full discharge

Addressing the issues Activated Titanium Sublimation pumps near the Pelletron Conditioned two top sections Took SF6 sample for analysis

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SF6 gas quality

All indicates that quality of SF6 has significantly deteriorated Partly compensated by regeneration of the desiccant and by cleaning the

tank walls Were not able to find any precedents

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3.5

4

4.5

5

5.5

6

20 30 40 50 60 70 80

Pressure, psig

Pel

letr

on

vo

ltag

e, M

V

Max HV Limits Full discharge Fit

Vacuum limit

Operating Voltage

HV limitations at various gas pressure in the Pelletron tank. Blue line indicates maximum observed voltage at the given pressure. Brown dashed line is a rough extrapolation from the point with a full discharge based on the data for pure SF6.

18-Dec-2008

Now, difficulties to maintain operational voltage with nominal tank pressure

Summary/Status

The long downtime was caused by a combination of several unrelated failures Thanks to everybody involved

Present status Ecool is operational to some extend

• Trips ~ once every 10 min ⇒ Getting better with time• Requires cycling of HV (after a full discharge)~ once a shift

Will work with MCR to optimize performance in this state Added SF6 (~5%) to increase HV strength (proportional to pressure)

• More conditioning is taking place… right now!

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