CNGS horn / reflector in 2005: The story of the glass ring insulator “A brief summary” by K. Elsener, A. Pardons, S. Rangod

“history” April 2004: horn-1 delivered from LAL to CERN April 2004: horn-1 electrical tests o.k. May 2004: extensive list of compulsory modifications (S. Rangod) June 2004: CNGS review – horn modifications indorsed July 2004: during dis-assembly of horn, glass ring splinters discovered -> mistake in LAL design -> additional modifications necessary March 2005: horn modifications completed; ready for electrical tests April 2005: horn electrical tests o.k. Mai 2005: horn installed on new support frame; transport from BA7 to 887 June 2005: two cracks in horn glass ring discovered July 2005: (7th-11th) NBI workshop at Fermilab - discussions July 2005: (18th) CERN “mini-review” on glass-ring enigma Aug 2005: L. Bartoszek visit to CERN – discussions; decision on new glass rings (with grafoil seals) Sept 2005: new glass rings / grafoil seals ordered Oct 2005: reflector connection plate / glass ring assembly complete Nov 2005: reflector electrical tests o.k. Dec 2005: (5th) reflector installed in target chamber

Glass Disk Enigma Any suggestions? Received from LAL Problem Solution slide presented by A. Pardons at NBI2005, 9 July 2005 Glass Disk Enigma Received from LAL Received from LAL After modification Solution Problem Number of screws & distance Glass plate broken Problem = conceptual After modification Crack discovered ~weeks after electrical tests More cracks present or to be expected? Ceramic ring? Pieces instead of ring? Any suggestions?

(Preliminary proposal 1) slide presented by S. Rangod at CERN “mini-review”, 18 July 2005 (Preliminary proposal 1) For centering pins 12 identical sectors Al2O3 (thickness 30 mm) 12 coaxial springs used during assembly phases 6 necessary gaps to guarantee the contact on lateral faces Critical angular geometry

Conclusions of “mini-review” The CERN “mini-review” did NOT encourage solutions with pieces of ceramic. Separating the functions (i.e. pressure for electrical contact and sealing against water / vapor leaks) was strongly encouraged. A solution with an insulating ring other than a glass ring would have been preferred. Conclusion (after more discussions and tests): New glass ring (smaller width, no holes) Softer seals (GRAFOIL) Since work on horn, reflector and spare horn, installation in the target chamber, stripline installation etc. had to proceed almost in parallel, it was decided that the first object to be equipped with the new assembly was to be the reflector.

Explanation of final assembly (see the drawing next page) (from a document written by A. Pardons) The idea is to only have a limited force on a fragile glass disk (insulator disk between two connection plates of different polarities). Using 6 Belleville washers in a 2-2-2 configuration gives the correct compression force on disk & seal to ensure leak tightness without risking breaking the disk. The washers allow us to choose our target force quite precisely, while giving the assembly a good elasticity and absorption of shocks. Specification: Belleville Washers M16 P# M16-L-2.4-301 thickness 2.4mm

20 Sept. 2005: glass-rings cut out of plate (water/sand jet cutting)

GRAFOIL-rings cut out of plate (water/sand jet cutting) GRAFOIL details: - 98% graphite - needs only little contact pressure (we have applied 5 MPa) max. temp. 500o C available in 1mx1m pieces our rings: 3 mm thick horn R=705-775mm reflector R=1105-1175m (3 pieces of 120o)

Reflector – “Sandwich” Assembly (1)

Reflector – “Sandwich” Assembly (2)

Reflector – “Sandwich” Assembly (3)

Reflector – “Sandwich” Assembly (4): quality control

Reflector – “Sandwich” Assembly (5) before placing the second graoil ring and the second connection plate after

Reflector – “Sandwich” Assembly (6) NOTE: final configuration has 6 (not 5) Belleville washers

Reflector: mounting the “sandwich” of connection plates with glass insulating ring

Reflector: “short-circuit” side before installation of connection plate inside the inner conductor

Reflector on test-bench (18 - 23 Nov. 2005, 20000 double pulses) 180 kA double pulse (Dt=50 ms) Reflector on test-bench (18 - 23 Nov. 2005, 20000 double pulses)

Reflector transport in access gallery (5 November 2005)

Reflector in target chamber top: passage near target shielding right: installation on base frame

CERN horn team (with reflector, in assembly lab)