logo

area

Paschen test results on MBH CR002 coil

Arnaud Foussat, Jan Petrik, Carlos Lopez TE-MSC

Thanks to P. Cruikshank & TE-MSC VSC section

on behalf of WP11 team

11T task force meeting, 15/05/2019

logo

area

Outline Paschen test on QH-coils insulation

Observations, objectives, test protocol

Paschen law

Layout of coils Paschen test vessel

CR-002 coil in vessel assembly

Test conditions, results

CR-002 Paschen hi-pot test results

Comparison with previous paschen tests

Air voltage breakdown-Yuxuan Chen’s results,

deviation from air ideal Paschen law.

Summary

2Arnaud Foussat, 15/05/2019

logo

area

Paschen test on QH-coils insulationInitial idea, observations:

Sound insulation is guaranteed when a complete encapsulation of all high voltage

active parts by solid insulation ensures a reliable high voltage operation

independently of the surrounding vacuum conditions.

Objectives:

To reveal a possible influence on breakdown voltage onto Quench-coil electrical

insulation by the level of partial air pressure ( or helium gas).

Start campaign with CR002 which was not cooled down.

Note: The calculation of a single high voltage test at a pressure from the theoretical

Paschen- curves minimum is usually not reliable, because of the undefined path of

the electric arc.

Test protocol:

Excursion of vessel pressure from 0.01 Pa ( 10-4 mbar) to 50000 Pa ( 500 mbar)

using a stabilisation holding time of 2 mins by steps. The DC breakdown voltage (

leakage current) across heater to coil insulation is measured at the different

pressure. Arnaud Foussat, 15/05/2019 3

logo

area

Paschen law

The breakdown voltage Vb between electrodes

is function of P.d, the product of pressure inside

chamber and distance between electrodes.

The Paschen law is derived as follows with A,B,

SE as fit parameters A and B, the Second

Townsend coefficient (𝛾SE), which describes the

mean number of generated secondary

electrons per ion,

Arnaud Foussat, 15/05/2019 4

logo

area

Layout of coils Paschen test vessel

Vacuum section built a test vessel (SPS) with turbo pump group,

micrometric leak valve, isolation valve and HV feedthroughs connected

to coil and QH leads

Arnaud Foussat, 15/05/2019 5

logo

area

CR-002 coil in vessel assembly

Arnaud Foussat, 15/05/2019 6

Fig.1 PE HD insulating supports spacers to keep coil away from vessel

walls. Insulated leads connections to avoid paschen discharge on leads

side.

logo

area

Test conditions

The test voltage was applied over 30 sec with a ramp of circ. 1 kV/min using MEGGER1025.

(due to large current charging time the voltage dwelltime of 30 sec was chosen as a comparisonreference)

The test voltage steps were refined every 10 V in the vicinity of breakdown value.

Both quench heaters were independently Hi-pot tested over pressure range [5.10-3 mb – 200 mb].

Arnaud Foussat, 15/05/2019 7

logo

area

Test results

Both breakdown voltages minima found at 575 V

@ 0.4 mbar and 675 V @ 0.5 mbar.

If we consider P.d minima as invariant in air at 8

mbar.mm then the charateristic minnimum defect

involved with the breakdown in CR02 would be

around 14 mm. (note that distance QH strip to

conductor in surface of about 4 mm, through QH

insulation kapton thickness of 50 microns).

Arnaud Foussat, 15/05/2019 8

logo

area

DOI: 10.1109/TASC.2011.2179393

Arnaud Foussat, 15/05/2019

CR-002 Paschen hi-pot test results

p.d=3000 mbar.mm

(see next curve p 12) 9

logo

area

Comparison with previous paschen tests

Previous work of comparison between the experimental curve and theoretical curve in air [Yuxuan Chen, [2]] , showing same offset in the minima beakdown voltage tendency, but with a constant shift in voltage values. That is because the theoretical curve are based on the uniform electrical field, the γ and value of A and B are constant under the prediction of uniform electrical field.

“Any irregular electrodes shape have non uniform electrical field. It seems reasonable to have a constant difference value. The detail reason for this phenomenon need future research in the future…”

Arnaud Foussat, 15/05/2019 10

logo

area

Air voltage breakdown-Yuxuan Chen’s results [2]

Arnaud Foussat, 15/05/2019 11

logo

area

Deviation from ideal Paschen law

Arnaud Foussat, 15/05/2019 12

p.d=3000 mbar.mm

logo

area

Summary

Arnaud Foussat, 15/05/2019 13

The hi-pot test under partial air pressure of CR02 coil to QH insulation

shows a minima voltage of 585 V at 0.4 mbar which corresponds to defect

size of about 14 mm. This tends to indicates discharge on surface between

QH and coil ( Gnd) and good conditions of QH-coil insulator.

This result is coherent with the fact that the good CR002 coil-QH insulation

had hold at 1 atm at least 4kV DC. However, the Paschen test is destructive, extrapolated Brkd on paschen curve at 1 atm

might be lower after test campaign.

Paschen test of coils (200K, 1 b) in He gaz should consider limitation in leakage current as

risk of damage when presence of submillemetric defects .

The shift of minima voltages values from ideal curves (180-300V) is

explained through non uniform electrical field, different electrode material. The P.d Paschen trend on coil does not seem to follow ideal Brkd.Paschen law.

Note that any expected minima Paschen breakdown voltage through coil

QH insulation would produce a minimum breakdown around 80 mbar air

pressure level.

logo

area

References

[1] R. Massarczyk, “Paschen’s law studies in cold gases”, Los Alamos National

Laboratory, Los Alamos, NM, US

[2] Yuxuan Chen, ELECTRICAL BREAKDOWN OF GASES IN

SUBATOMOSPHERIC PRESSURE, Thesis, Alabama

Arnaud Foussat, 15/05/2019 14

logo

area

Thank you

logo

area

Paschen breakdown conditions at 1 bar, for He

and air atmosphere

Arnaud Foussat, 15/05/2019 16

logo

area

Density properties of Helium vs temperature

Arnaud Foussat, 15/05/2019 17

fit curve: y = 60.344x-1.048

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

0 50 100 150 200 250 300 350

Density

[g/c

m3]

He density vs. temperature @ P 1 bar

logo

area

DOI: 10.1109/TASC.2011.2179393

Paschen curve for air

Arnaud Foussat, 15/05/2019 18

logo

area

Typical Paschen pressure increase

DOI: 10.1088/1009-0630/15/2/15}Arnaud Foussat, 15/05/2019 19