1. HV Feedthrough Tests for the LBNE Meeting Status of R&D 6/26/2016HV Feed for BNL Meeting1 Hans Jostlein

2. 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. 6/26/2016HV Feed for BNL Meeting2

3. This weeks improvement: Use a 17” ID cryostat. Now we can see where the breakdown occurs, and, maybe, why. 6/26/2016HV Feed for BNL Meeting3

4. 6/26/2016HV Feed for BNL Meeting4 The old, small-diameter, Dewar:

5. 6/26/2016HV Feed for BNL Meeting5 Picture of the New Dewar:

6. 6/26/2016HV Feed for BNL Meeting6 Mirrors in the bottom; Copper tube “fence” as the anode

7. 6/26/2016HV Feed for BNL Meeting7 Two types of Feedthrough are being tested: --The ICARUS Style, and -- the Cable-type

8. Micr 6/26/2016HV Feed for BNL Meeting8 The ICARUS Style Feedthrough

9. 6/26/2016HV Feed for BNL Meeting9 ICARUS style FT installed:

10. The existing Prototype Feedthrough 6/26/2016HV Feed for BNL Meeting10

11. 6/26/2016HV Feed for BNL Meeting11 ICARUS style FT installed, now wet:

12. 6/26/2016HV Feed for BNL Meeting12 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.

13. 6/26/2016HV Feed for BNL Meeting13 The reason for the breakdown: Bubbles emerging from the bottom of the PE sleeve. Bubble size is approx 13 mm diameter.

14. 6/26/2016HV Feed for BNL Meeting14 Bubble Frequency. Note; Short times observed in undisturbed Lar Long times after stirring or discharge.

15. 6/26/2016HV Feed for BNL Meeting15 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 brings warmer LAr up from the lower depth; Should increase bubbling; Why does it and stop bubbles temporarily?

16. 6/26/2016HV Feed for BNL Meeting16 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.

17. 6/26/2016HV Feed for BNL Meeting17 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 Possibly install a small PE “vent tube” (length 1m or so) to let bubbles exit safely.

18. 6/26/2016HV Feed for BNL Meeting18 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

19. 6/26/2016HV Feed for BNL Meeting19 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 electrostatic pressure overcomes the capillary one created by the surface tension S of the liquid [71]: epsilon0 *(epsilonr-1) * E^2/2 >> 2 S / R

20. 6/26/2016HV Feed for BNL Meeting20 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 satisfied for large enough bubble radii and high electric field, e.g. in LN, for R = 10 and 100 micron, Equation (2) gives E  320 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 the elongation 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 the ellipsoid by a factor larger than a2/b2, if the major axis 2a is parallel to the applied field, 2b being the minor axis. This could induce a transition to a streamer and, then, the liquid breakdown (i.e. a bubble initiated breakdown).

21. The Cable-Type Feedthrough 6/26/2016HV Feed for BNL Meeting21

22. 6/26/2016HV Feed for BNL Meeting22

23. 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 6/26/2016HV Feed for BNL Meeting23

24. 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: 6/26/2016HV Feed for BNL Meeting24

25. 6/26/2016HV Feed for BNL Meeting25

26. End fixture Failed 6/26/2016HV Feed for BNL Meeting26

27. This is how it was meant to look: 6/26/2016HV Feed for BNL Meeting27

28. Feedthrough went to 160 kV Problem with “small discharges” ? 6/26/2016HV Feed for BNL Meeting28

29. 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 6/26/2016HV Feed for BNL Meeting29

30. 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. 6/26/2016HV Feed for BNL Meeting30

31. 6/26/2016HV Feed for BNL Meeting31

32. 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 6/26/2016HV Feed for BNL Meeting32

33. Micr 6/26/2016HV Feed for BNL Meeting33

34. This one is older and shorter 6/26/2016HV Feed for BNL Meeting34

35. Micr 6/26/2016HV Feed for BNL Meeting35

36. 6/26/2016HV Feed for BNL Meeting36 Semi-conducting layer Ground Nut HV Nut

37. 6/26/2016HV Feed for BNL Meeting37 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.

38. Found the Cable cracked. This was used 3 times before it failed. Not understood. 6/26/2016HV Feed for BNL Meeting38

39. 6/26/2016HV Feed for BNL Meeting39 Summary The 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.