To: Distribution 2 Apr 97

From: Martin Nordby

Subject: IR Engineering and Physics Meeting Minutes: 7 March 97

Hard-Copy Distribution:

Bob Bell	41		Nadine Kurita	18
Gordon Bowden	26		Harvey Lynch	41
Pat Burchat	95		Tom Mattison	17
Scott Debarger	17		James Osborn	LBL B71J
Hobey DeStaebler	17		Andy Ringwall	17
Jonathan Dorfan	17		John Seeman	17
Stan Ecklund	17		Mike Sullivan	17
John Hodgson	12		Uli Wienands	17
David Humphries	LBL 46-161		Mike Zisman	LBL B71J
Roy Kerth	LBL 50-340
David Kirkby	95
Jim Krebs	41

Electronic Distribution:

Curt Belser	Tom Elioff	Lew Keller	Natalie Roe	Dieter Walz
Lou Bertolini	Kay Fox	J. Langton	Ross Schlueter	Rick Wilkins
Adam Boyarski	David Fryberger	Georges London	Ben Smith	Fran Younger
Catherine Carr	Fred Goozen	Rainer Pitthan	Steve St Lorant	Ron Yourd
Al Constable	Alex Grillo	Joseph Rasonn	Joe Stieber
David Coupal	Keith Jobe	Jeff Richman	Jack Tanabe

Mask Tip Analysis

Mike Sullivan reported on results of EGS studies of the mask tips in the B1 Chambers. He modified the EGS interface program to accept rounded tips, and studied backgrounds in silicon layer 1 for various tip radii and material types.

For the first LEB mask tip, Mike saw little change in backgrounds between an infinitely sharp tip, and one with a 250 micron radius and 100 micron arc length. Both tips were made with copper, which Mike found to be acceptable for this mask. LEB masks 2-4 can also be made from copper, with even rounder tips, if needed for fabrication.

For the HEB mask, Mike had found earlier that the tip material must be high Z, such as gold or tungsten. He looked at three tip radii:

100 micron: 10% increase in photons scattered onto Be vacuum chamber

1000 micron: 67% increase in photons onto vacuum chamber, since the incoming S.R. sees more arc length of tip. The average energy of the photons decreased, since the path through the tip is shorter.

4000 micron: 5X increase compared to sharp tip, with an even lower average energy.

The conclusion of the study is that the tip radius should be less than 1 mm. For fabrication, the target radius will be 100 micron radius, with a high Z metal. The tip itself must be at least 1-2 mm thick (in z-dimension), and 1-2 cm high.

Vertex Vacuum Chamber Processing

Knut Skarpaas updated us on fabrication and processing plans for the beryllium Vertex Vacuum Chamber. The material used for the chamber assembly include layers of plating and epoxy. They stack up as follows:

7 microns gold plating on inside of vacuum chamber

300 angstroms chromium flash

0.032" beryllium vacuum chamber

7 microns Ni plating for corrosion protection

0.0006" epoxy paint (15% strontium chromate)

0.055" water

0.0004" epoxy paint (15% strontium chromate)

7 microns Ni plating

0.020" beryllium water jacket

0.0006" epoxy paint

0.001" epoxy paint second coat

The gold and chromium inner platings will be sputtered on in an argon environment, after final assembly of the chamber. The gold will be sputtered onto all surfaces up to the RF shield fingers on the bellows. Since the assembly cannot be baked out for UHV cleaning, the sputtering will also provide the only cleaning to the vacuum surfaces of the chamber.

The beryllium tubes will be made from S-65 beryllium bar stock. This is die-forged, and our pieces will be made from material containing the least carbon. Brush-Wellman has bar in-stock with 0.02% carbon content, which is 20% of the normal carbon content.

Electroless Nickel plating will be used to help protect the beryllium from corrosion. Speedring (a beryllium fabricator) says that this adheres well and produces a more uniform plating than electrolytic nickel. Since the plating takes place in a bath, the order of plating, and masking regions which must remain unplated is something that will require close attention.

An epoxy paint is used as the first line of defense for corrosion protection. Since this coats the wetted surfaces, the epoxy must have low water absorption (0.5% - 2%). Ideally, it should be radiation hard, taking up to 100 Mrad, lifetime, with no loss of water-proof integrity. It should be heat-resistant to 200°C for baking and welding, and flexible to prevent peel-off failures.

Of these requirements, the radiation hardness is the toughest to meet. Data from CERN shows that 10^9 rads is the upper limit for the rad-hard epoxies, while 1 Mrad is the limit for phenolics. Br127, the paint used in the Cornell CESR beampipe, is made from epoxy and phenolic, so its radiation hardness is somewhere in the middle (and it's a big middle). Knut is actively pursuing other paints which contain epoxy only, or epoxy and polyimide, which is even more rad-hard.

Regarding failure modes, the chamber has multiple paths of protection. It will have two coats of paint, so if there is an undetected pin-hole in one coat, the other one will protect. Also, if the paint peels or detachs from the chamber, the nickel plating serves as the next line of defense.. Finally, if water ever does come in contact with beryllium, the very low carbon content in the beryllium will reduce the liklihood that it corrodes quickly.

The PEP-II Vertex Vacuum Chamber compares favorably with the CESR chamber of comparable design. Technically, the chambers compare as follows:

CESR Be wall thicknesses are thinner. Our thicker walls open up the list of vendors who can fabricate the part.

CESR Be-to-stainless steel brazes are in the water passage, making them more susceptible to corrosion. PEP-II chamber brazes are out of water, and accessible for inspection.

CESR chamber does not have any Ni plating for secondary corrosion protection.

The Br127 epoxy paint was flowed through the CESR chamber, which lets the strontium chromate settle out. This provides the protection in the paint, so resistance to water is reduced. In the PEP-II chamber, the paint is sprayed on before assembly, so it can be inspected.,

The CESR chamber sees far less radiation dose than the PEP-II chamber. Increased radiation dose will affect the integrity of epoxy paint. Knut is working on this now.

These minutes, and agenda for future meetings, are available on the Web at:

http://www.slac.stanford.edu/accel/pepii/near-ir/home.html