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

2. “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

3. 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?

4. (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

5. 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.

6. 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 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
thickness 2.4mm

8. 20 Sept. 2005:
glass-rings cut out of plate
(water/sand jet cutting)

9. 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= mm
reflector R= m
(3 pieces of 120o)

10. Reflector – “Sandwich” Assembly (1)

11. Reflector – “Sandwich” Assembly (2)

12. Reflector – “Sandwich” Assembly (3)

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

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

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

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

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

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

21. Reflector transport in
access gallery
(5 November 2005)

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

23. CERN horn team (with reflector, in assembly lab)