hv feedthrough tests for the lbne meeting status of r&d 8/18/2015hv feed for bnl meeting1 hans...

HV Feedthrough Tests for theLBNE Meeting

Status of R&D

04/19/23 HV Feed for BNL Meeting 1

Hans Jostlein

This is a summary of the Fedthrough R&D status at Fermilab.

It will concentrate on very recent results,and on problems.

The hope is to identify promising tests and solutions.

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This weeks improvement:

Use a 17” ID cryostat.

Now we can see where the breakdown occurs,and, maybe, why.

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The old, small-diameter, Dewar:

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Picture of the New Dewar:

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Mirrors in the bottom;Copper tube “fence” as the anode

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Two types of Feedthrough are being tested:

--The ICARUS Style, and

-- the Cable-type

Micr

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The ICARUS Style Feedthrough

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ICARUS style FT installed:

The existing Prototype Feedthrough

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ICARUS style FT installed, now wet:

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High Voltage Tests in Lar:

Reached 120 kV first day, 90 kV on subsequent days.

Breakdowns originate at the bottom of the PE sleeve, and go up into the vapor

phase to the SS outer tube of the FT.

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The reason for the breakdown:

Bubbles emerging from the bottom of the PE sleeve.Bubble size is approx 13 mm diameter.

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Bubble Frequency.

Note;Short times observed in undisturbed LarLong times after stirring or discharge.

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Bubbles

An interesting Paper (thanks, Igor):

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 9, No. 1, February 2002 17

Thermally and Electrically Induced Bubbles in Liquid Argon and Nitrogen

A. Denat, F. Jomni, F. Aitken and N. Bonifaci

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Condition for Electric Breakdown Initiated by Bubbles:

In an electrically stressed liquid, an insulating bubble elongates and takes the shape of a prolate ellipsoid.

This elongation could be large if the electrostaticpressure overcomes the capillary one created bythe surface tension S of the liquid [71]: epsilon0 *(epsilonr-1) * E^2/2  >> 2 S / R

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Condition for Electric Breakdown Initiated by Bubbles:

Here, epsilonr is the relative permittivity of the liquid. In cryogenic liquids where E, is low, this condition is only satisfiedfor large enough bubble radii and high electric field,

e.g. in LN, for R = 10 and 100 micron, Equation (2) gives E320 kV and > 100 kV/cm, respectively. For a 10 mm bubble (our case), E min = 10 kV/cm in LN2.

Moreover, as the vapor pressure remains practically constant during theelongation step and considering that Paschen law applies,elongation favors electric discharges in the vapor and,then, the bubble becomes conducting.

In this case, a large increase of the electric field occurs at the poles of theellipsoid by a factor larger than a2/b2, if the major axis 2ais parallel to the applied field, 2b being the minor axis.

This could induce a transition to a streamer and, then, theliquid breakdown (i.e. a bubble initiated breakdown).

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Bubble Model

• A fixed amount of heating power is conducted down the center conductor tube

• The tube is cooled via heat conduction through the PE sleeve wall and the stagnant gas surrounding the center tube.

• If the tube is above the LAr boiling temperature where it emerges from the PE sleeve, we get boiling and bubbles

• Stirring the liquid eliminates the temperature layering and stops bubbles temporarily, until layering re-establishes itself

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Brief aside on PE Cooling

• Note that the PE Sleeve is cooled directly by the LAr;

• We see boiling only right at the LAr surface;

• The submerged PE must be at LAr temperature.

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Possible Bubble Fixes

• Try using a thin wall inner conductor (thinking of SS corrugated hose)

• Calculate heat transfer and bubbling to be sure it will work

• Greatly improve cooling of center tubeby providing direct contact with the PE sleeve

We will also add a grooved annulus around the PE to reduce surface flash-over (thanks, Huangho!)

See next Drawing:

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Possible Unintended Consequences

The annular space above the new “Rod” is now sealed.

This may be OK,or it may pose a threat as a virtual leak, contaminating the Lar.

This space can be vented through the center tube if the tube has many small holes along its length.

Spontaneous Bubbling

The Bubbling Model

We use three ingredients:

-- a nucleation site--the “pumping action” of bubbles, i.e. they cause a local LAr upflow--the presence of super-heated LAr

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Nucleation sites are places with a sharp point, which can be tiny.They are almost unavoidable.

If a string of bubbles persist for some time it will cause local LAr upflow, which brings LAr from lower layers to the nucleation site.

This Lar can be warmer than the upper layers because the hydrostatic pressure raises the boiling point:

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Note that 18 “of LAr correspond to 0.1 at,raising the boiling temperature by 0.8K.

This is huge !

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Slope is 1.56 at/ 20K

We have now a direct measurement of LAr temp versus depth in LAPD. We have yet to compare the slope with expectations.

This result seems to say there is not good mixing in the (shallow-- 2ft) LAr mass in LAPD. Mixing is expected to be much better in Microboone.

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The stratification by temperature requires two ingredients:

-- heat influx to the vessel-- no significant stirring.

My test dewar is 17” in diameter and is vacuum insulated with superinsulation.While the heat influx is small, there is also essentially no thermal convection stirring.

And we see spontaneous bubbling random site, e.g. the tops of the anode copper tubes.These tubes have zero heat input, except from heat conduction form the warmer deep layers. This heat input plus the superheat from the up-flowing LAr provide the energy to make bubbles. Strings of bubbles have been seen to go on for several minutes.

If one stirs the LAr, either by sparking, pouring in Lar, or stirring with a stick, bubble formation stops for a couple of minutes and resumes, presumably, after thermal layering has been re-established.

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Bubbles from the Center Conductor

In my test dewar I see large bubbles being formed at the bottom of the PE tube.

I think now that they are due to a combination of heat input:

--conduction down the center conductor--superheated LAr.

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The Cable-Type Feedthrough

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Pre-test of the cable idea

• Use the existing Tevatron cable # 2134• It has an outer braid, no conducting layer• Install smooth end fittings on the cold end to allow high

voltage levels• See next picture

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Success

• Reached 178 kV in LN2 before discharge• Afterwards ran 160 kV for one hour• On disassembly noticed that the end fixture had failed,• See next picture:

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End fixture Failed

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This is how it was meant to look:

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Feedthrough went to 160 kV

Problem with “small discharges” ?

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Advice from and Expert

Jerry Goldlust from Dielectric Science, Inc, was kind enough to review the prototype design and suggest corrections.

His main observation:

•PE can be slightly conductive•Charge accumulates on the surfaces of the insulating tube•We have a gas gap between the center rod and the PE sleeve, and also between the PE sleeve and the outer grounded SS tube•When the charges get large enough they arc over

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Why the Gaps are there

We wanted to avoid trapped gas volumes.Also there is no easy way to close those gaps.For instance, when cold, the PE sleeve shrinks away from the outer SS ground tube.

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This Cable:

This cable has several important properties:•Rated to 200 kV DC•It has a solid center conductor—makes it vacuum tight•It has a conductive PE layer around the center conductor-no electric field, no small discharges there•It has a conductive outer PE layer, coextruded—

--no electric field, no discharges--No need for a braid--can seal directly against it with O-rings

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Micr

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This one is older and shorter

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Micr

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Semi-conducting layer

Ground Nut

HV Nut

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Cable type Test in LAr

No Bubbles.Went to 100 kV.

At 110 kV breakdown between ground nut and HV nut.

Subsequently broke at 75 kV and lower.

Found the Cable cracked.This was used 3 times before it failed.

Not understood.

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SummaryThe Saga goes on.The ICARUS Style should work once we fix the bubble problem.

The cable type FT is attractive because of its utterly simple design.But we must understand and fix the cracking problem first.

Ideas and Suggestions are Welcome.