# Presentation: 013091

## Test of the high voltage strength of Pelletron’s gas insulation (18-Dec-2008)

- Recycler Meeting
- January 21, 2009
- A. Shemyakin

## Introduction

- By now, we are almost certain that SF6 gas in the Pelletron tank is heavily contaminated by air (up to 40%).
- Cost of replacement by a pure SF6 is ~100 k$
- The Pelletron performance can be affected by the contamination in several ways
- Increased frequency of equipment failures due to a high oxygen content
- A serious concern; however, there were no failures in the last 2 months
- Replacing today’s gas by a SF6 + N2 mixture would be a solution
- Possible changes in  cooling efficiency of elements inside the tank
- Data logger data do not show any measurable changes
- Changes in HV electric insulation properties of the gas
- Subject of the test
- Question for the test to answer:
- How close is the operational voltage to the HV limit in the gas?

## Two types of HV limitation in the Pelletron

- Emission and finally a discharge in vacuum
- The main limit in operation
- Much worse with the beam in the tube
- Back in 2004, it was the reason  to increase the total length of acceleration tubes
- Always accompanied by a burst in the vacuum pressure
- Should not significantly depend on properties of the outside gas
- Discharge on the gas side
- Before the test, there was only one instance of gas –side discharge in this Pelletron
- The first day of HV operation in WB, in air
- Strongly depends on the gas type and pressure
- In operation, the HV strength in the gas should always exceed the one in vacuum.

- Section of acceleration tube.

## SF6 as an insulation gas

- Two types of SF6 applications
- Arc quenching in breakers
-  needs pure SF6
- HV insulation
- While pressurized, most of effect is reached by adding a small portion of SF6 to N2

- Conversion:
- 1 atm = 14.7 psi
- 73.5 psig = 6 atm abs

- DC breakdown voltage.
- From IEE Trans.on Dielectric and Electric Insulation, V2 N5 (1995), p.977

- 60 Hz AC breakdown voltage for SF6/N2 mixture.

- 230kV/cm

- 6%

- Range where the test was done

- Just straight line

## Motivation for the HV test

- Because dependence on N2 contamination is weak, one may hope that operation with 60% of SF6 in the mixture is OK
- Still, it’s better to make sure by a direct measurement
- Usually, such measurements are done by conditioning  and observing full discharges  until the breakdown level is reproducible
- We prefer to have the gas strength is ~twice above the HV strength on the vacuum side
- To protect the tubes during discharges in vacuum
- Also, we are afraid of breaking the equipment in the Pelletron in the time of discharges
- The idea of the HV test
- Look at the lost current as a precursor of a full discharge
- Make measurements at several values of the gas pressure to interpolate toward the nominal one

- R2

- R1

- R2/R1= e; R1 = 1m; Eterm= Uterm/R1;
- For Uterm= 5 MV
- Eterm= 50 kV/cm

## Lost current

- The current lost from the terminal can be calculated from the balance of currents in the Pelletron column
- R:LOSTI = Chain current -(Current through all resistive dividers + Needle current)
- Works only for a steady state
- If the terminal voltage changes, the R:LOSTI reading needs to be corrected
- If changes are not fast (several sec),
- Rterm*Cterm ~15 sec
- R1*C1~ 1 sec

- Terminal-to-tank capacitance

- Chain current

- Lost current from terminal

- Needle current

- Resistive divider current (3)

- C1

- R1

- Corona current through the last gaps

## Typical measurement

- HV test at 50.2 psig.
- Large peaks of R:LOSTI correspond to fast increasing of HV. Also, it has ~ -2 microA offset.

- HV (R:GVMVLT), 0.5MV/div

- Chain current (R:CHN1I), 20microA/div

- Lost current (R:LOSTI), 4microA/div

- Pressure (R:IGA06), 1.E-09/div

- 30 min

- 0

- Here the lost current never comes to its minimum steady state level.

## Corrected Lost current

- Example of data with Lost current corrected with the HV time derivative
- The first voltage increase at 60.2 psig
- Later HV was increased up to 4.9 MV, so the dust redistribution is probably significant
- For this measurement, the point of the Lost current jump is taken as the “limitation”

## Results

- “Limitation” points were determined typically by “noticeable” changes in the lost current
- Maximum voltage
- At 70 and 60 psig, HV was eventually limited by vacuum activity
- At 28 psig, HV was increased up to a full discharge
- For other pressure values, the rise of the lost current was too scary

- 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 from Slide 4. It promises a safety factor of 1.9 at nominal pressure.

- Vacuum limit

## Summary

- There is a good safety factor in the HV strength of gas insulation in the Pelletron
- Most likely, the strength is close to its design value, and the gas contamination does not have a dramatic impact on HV performance of the Pelletron
- From the point of view of HV strength, replacing the gas by a pure SF6 is not needed
- Judging by this measurement and by the publish data, adding small amount SF6 or N2 into the tank in a case of a leak should have a similar effect
- The procedure used  in the test for determining the limiting voltage with the lost is not reliable enough to be used in a quantitative scaling
- After each gas manipulation, the lost current first appeared at a significantly lower voltage than it eventually did after several voltage increases. Therefore, a necessity of conditioning the gas side after accessing the tank could be related primarily to the gas transfers and to the access itself.