# Presentation: 582824

## Report on SNEAP’08and notes about the ECool’s Pelletron 

- Recycler Departmental Meeting
- October 22, 2008
- A. Shemyakin

## SNEAP

- Symposium of Northeastern Accelerator Personnel
- “a community of personnel involved with electrostatic particle accelerators and their use”
- Founded in 1968 by three Northeastern universities
- The name is kept for historical reasons
- Annual meetings
- Exchange of mostly very practical information
- everything relevant to the operation of  electrostatic accelerators
- electrostatic accelerators, ion sources, telemetry and control systems, maintenance, safety issues etc.
- “use RAG IT-II wipes to clean the tank, it improves reliability”
- Accelerator physicists, engineers, technicians
- SNEAP’2008
- 32 participants
- At maximum, in 80’s, SNEAP had more than 100 participants
- People from USA, Australia, Italy, France, Canada, New Zealand, Finland
- Reports and a lot of discussions

## Trends (G. Norton, NEC) 

- Growth of voltage/energy culminated with installation of ORNL’s 25 MV Pelletron tandem in 1976
- 25.5 MV with beam (1988)
- The only machines that reliably runs with beam above 20 MV
- Nuclear physics
- Breaking the Coulomb barrier for heavier nuclei
- No new machines above 10 MV for ~15 years
- Dozens are used for nuclear physics for many years
- Knowledge how to manufacture is getting lost
- NEC keeps changing the Pelletron design/fabrication
- Now all acceleration tubes are high-pressure rinsed with  distilled water
- After proposal and tests at JAERI (Japan)
- Installing the better conductivity chain sheaves to eliminate conductive bands
- Better software
- The last Pelletron in Pakistan was commissioned remotely
- Improved terminal electronics

## Trends (cont.)

- Most of new Pelletrons are < 3 MV
- Horizontal tandems
- Atomic Mass Spectrometry
- 14C/12C, 10Be/9Be, 26Al/27Al,...
- Background 10-16, 1 mg samples, 2000 samples in 5 days (2% precision), ~70 labs
- Fast growing field is Biomedical applications
- Material analysis
- Rutherford backscattering, Proton-induced X-ray emission,...
- Material modifications and tests
- The main stress is on the design of equipment outside the accelerator
- Accelerators are highly reliable and are key-ready
- True unattended operation
- Beam handling and analysis are more and more sophisticated
- Some of older accelerators run for many years without opening
- UEC Inc.’s 1.7 MeV:  twice in 12 years
- Alabama A&M: once per 5 years
- Decreases the need for simple replicas of older accelerators
- Machines work for decades, are transferred between labs, are modified

## Specifics

- Many of advances come from high voltage (> 10 MV) machines
- Larger groups, more frequent openings, more efforts are required to provide a reliable operation
- ANU (14 MV): 109 openings in 35 years; lately once in 10 months
- Almost all machines are with positive terminal voltage
- Needles are at the ground as well as all HV regulation circuitry
- Slow travel time for electrons to the terminal => limits the frequency response
- Often only stripping foil or gas are in the terminal
- Negative ion source is at the ground potential
- Usually determines reliability
- Electronics protection depends on the amount of stored energy
- Minimum at < 2 MV
- Steel boxes at > 3 MV
- Double shielding at >10 MV
- ECool’s Pelletron ran between 2007 and 2008 shutdowns ~92% of time
- ~8000 hrs/year
- More than other machines (1000 – 5000)

## Practical experience (relevant for us)

- GVM
- Motor or GVM failures are a common cause for tank openings
- Nobody has two GVM
- Bearing replacement is considered a routine procedure
- Some have 4” GVMs, but with 4 blades (from HVE)
- GVM problems may be related to bad bearings, dust, grease from bearings...
- Energy stability is typically by at least several  times worse than ours
- To catch coming bearing failures, one can carefully listen to the noise during openings
- Typically the chain and the shaft can be rotated with people in the tank
- In large machines, there are switches to turn them on/off from inside the tank
- Chain quality can controlled by a dedicated capacitive pickup sensitive to the charge of individual pellets
- Make one of the pellets not-chargeable, use it as a marker, and look at evenness of charges
- Can replace individual pellets
- Nobody reported about replacements of the chain or shaft motors

## Gas quality

- A large absolute amount of oxygen may result in a fire
- One the main reasons that prevents of using just a compressed air
- Reliability depends critically on the gas quality
- Life time of bearings and chains
- Difficult to analyze SF6 byproducts
- If a sample is taken with SS cylinder (as we are doing), the byproducts quickly react with walls
- ANU (Australia’s 14 MV Pelletron) developed a dedicated device
- Notre Dame:
- Replacement of the old N2/CO2/SF6 gas by a new N2 + 10%SF6 increased dramatically the chain life time
- Typical pressure is 100 -120 psia
- One of checks  is the weight of a bottle with liquefied SF6
- Air stays gaseous

## ANU: chain life time

_**CHAIN LIFETIME ENVIROMENTAL PERIODS**_
- 1, 1973-1983. CORONA PTS, STANDARD PURIFIER, HIGH BREAKDOWN PRODUCT CONTENT.			 _**TYP HOURS 4000**_
- 2, 1983-1988. CORONA PTS, ENLARGED PURIFIER, LOW BREAKDOWN PRODUCT CONTENT. 			_**TYP HOURS 25000**_
- 3, 1988-TO PRESENT. RESISTORS INSTALLED, STANDARD PURIFIER, LOW BREAKDOWN PRODUCT CONTENT. 	 _**AT +77000 HOURS**_
- PELLET GAP CHAIN 3 BETWEEN 6.5 AND 7.5 mm; GAP ON A NEW CHAIN IS 5.4 mm	**	****	****	****	**

- THIS PIN IS 0.010” SMALLER ON RADIUS.

- ECooler’s chains: ~15 000 hrs

## History of HV – related failures in Fermilab’s cooler

- ~ 5 years with no failures
- 5 failures in the last year
- There may a reason for the worsened reliability

| Date | Indication of the problem | Repair | Downtime to repair, hours |
| --- | --- | --- | --- |
| 27-Sep-01 | Intermittent HV regulation; lost control over needle motion | Needle assembly was re-assembled and connections in the needle current circuitry improved. | 4 days |
| 30-Apr-02 | GVM motor failed | New motor installed | 4 days |
| 9-Feb-07 | Intermittent HV regulation | Replaced failed PS in TPS | 7 |
| 18-Dec-07 | Tungsten needles evaporated | Replaced by sewing needles | 27 |
| 19-Jan-08 | Large HV noise | Cleaning and soldering connections | 30 |
| 10-Apr-08 | Poor HV regulation | Replacement of GVM bearings | 47 |
| 22-Jul-08 | No HV regulation | Needle current triode was replaced | 25 |
| Oct-08  | Increased HV noise and energy drift | Replacement of GVM bearings | 1 week |

## SF6 analysis in the ECool’s Pelletron (%)

- No water is found
- In agreement with the low dew point (< -80 C) reported by a sensor on SF6 skid
- No SF6 byproducts
- Could be related to the test procedure
- A large amount of oxygen and nitrogen is reported
- A 4th sample has been taken in Oct 08 and is being analyzed

- Two year history of SF6 pressure and temperature in the tank

- Events
- SF6 was purified while moving into MI-31 in 2004
- May 2006 – new SF6 blower
- Leak rate ~2 psi/year since then
- 17-Feb-08 – beginning of high leak rate (~2 psi/month)

- 25-Oct-06

- 22-Oct-08

- Steady leak ~2 psi/year

## Discussion

- In 2006 -2007, SF6 loss did not correlate with adding air into the system
- The leak may be in the new blower assembly which always stays at a positive SF6 pressure
- Significant degradation of SF6 quality occurred during the year when a large leak through tank flanges occurred
- The air might go in during the SF6 vacuum pumping stage
- The leak has been repaired
- The equilibrium pressure is 5 mTorr
- At the time of SF6 leak repair, the vacuum gauge was replaced
- Before the repair, the (previous) gauge read 300 mTorr
- If the report is correct, the problems with reliability might be caused by oxygen
- We should think hard about possible sources of the contamination
- It might have been resolved by the leak repair
- We may think about changing the procedure to send some of SF6 to atmosphere when the tank pressure is deeply negative
- A gas purification needs to be scheduled for the April 2009 shutdown
- Lengthy process (a week)
- May need to add 1/3 of the total SF5 mass
- Shall we consider N2/SF6 mixture?