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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
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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
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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
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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,
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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
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CR-002 coil in vessel assembly
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Fig.1 PE HD insulating supports spacers to keep coil away from vessel
walls. Insulated leads connections to avoid paschen discharge on leads
side.
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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].
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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).
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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
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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…”
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Air voltage breakdown-Yuxuan Chen’s results [2]
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Deviation from ideal Paschen law
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p.d=3000 mbar.mm
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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.
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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
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Thank you
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Paschen breakdown conditions at 1 bar, for He
and air atmosphere
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Density properties of Helium vs temperature
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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
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DOI: 10.1109/TASC.2011.2179393
Paschen curve for air
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Typical Paschen pressure increase
DOI: 10.1088/1009-0630/15/2/15}Arnaud Foussat, 15/05/2019 19