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

Recycler Departmental Meeting

October 22, 2008

A. Shemyakin

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SNEAPSNEAP

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

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Trends (G. Norton, NEC) 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

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Trends (cont.)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

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SpecificsSpecifics

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)

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Practical experience (relevant for us)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

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Gas qualityGas 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

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ANU: chain life timeANU: 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

o 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

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History of HV – related failures in Fermilab’s coolerHistory 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 days9-Feb-07 Intermittent HV

regulationReplaced 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

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10-Apr-08 Poor HV regulation Replacement of GVM bearings

47

22-Jul-08 No HV regulation Needle current triode was replaced

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Oct-08 Increased HV noise and energy drift

Replacement of GVM bearings

1 week

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SF6 analysis in the ECool’s Pelletron (%)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

Date 12-Sep-07 22-Jul-08 2-Sep-08SF6 98.7 62.2 61.9O2 0.3 7.2 7.5N2 1.1 30.6 30.6

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

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DiscussionDiscussion

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?