Pelletron Battle – Field Report All Experimenters’ Meeting August 15, 2011 L. Prost (for A. Shemyakin)

Terminology for the Pelletron HV regulation system (simplified) 2 Column current I Corona ≈ I Chain – I Column - I Needle Corona current Corona current ≡ Lost current

Chronology: onset of problems and 1 st 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 SF 6 ) was re-generated  At startup:  Very bad lost current 3

Chronology: 2 nd access  Aug 5  Second regeneration of desiccant (tank filled with SF 6 )  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 4  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 (3 rd )  Intermittent readings of the column resistive divider

Chronology: 3 rd 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 SF 6 sample for analysis 5

SF 6 gas quality  All indicates that quality of SF 6 has significantly deteriorated  Partly compensated by regeneration of the desiccant and by cleaning the tank walls  Were not able to find any precedents 6 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 SF 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 SF 6 (~5%) to increase HV strength (proportional to pressure) More conditioning is taking place… right now! 7