To: J.Rifkin, B.Baugh, and S.Cabatic

SLAC MEMORANDUM To: J.Rifkin, B.Baugh, and S.Cabatic From: A.Torres (ext. 4820) Re: Furnaces

Contacts: Chris Pearson, SLAC Klystron Manufacturing, x3382 Earl Hoyt, SLAC personnel, x2295

Multiple Stacking in Furnaces

Multiple stacking of structures in the furnaces are being considered because of the long cycle times of the furnace operations. With the current process, in order to meet production requirements of 15 structures/day, over 15 bonding furnaces, 15 brazing furnaces, and 30 bake out furnaces would need to be acquired for the production factory. In order to reduce the amount of equipment to be purchased for the furnace operations, it is suggested that multiple structures be placed in the furnaces. Brazing/bonding of multiple structures has been observed at CPI on small stacks and other components.

Considerations for Multiple Stacking

1. Steep temperature gradients during temperature ramp-up will negatively effect structure dimensions 2. Use cold wall furnace for clean furnace operations and to make temperature control easier 3. Use heating element in the center of multiple structures to reduce temperature gradient 4. Shielding can also be used to reduce temperature gradients during ramp-up 5. Suggested maximum temperature ramp-up rate = 7degrees/minute

Bonding

The current plan is to bond structures in a hydrogen furnace. Shallow temperature gradients are important during bonding since steep temperature gradients result in warped/bowed structures. Suggestions for further research are to test multiple copper bars using thermocouples to see if it is possible to heat and cool multiple structures axi- symmetrically.

Earl Hoyt also suggests that Jim Hetherington be contacted to find out more information on furnace. Hetherington is a furnace builder, whom Hoyt has worked with in the past. Braze/Bake Out

Brazing multiple structures is a tricky operation because it is difficult to hold the temperature constant over the entire length of the structure since the structure has a complex geometry with the accessories. End to end the temperature must be uniform to within 10 degrees. At the hold/braze temperature, the temperature of the workpiece should be raised high enough so that the braze alloy melts, but low enough so that the copper does not melt. The braze temperature is held for 5 minutes to ensure that all braze alloy at the joints melt properly. With multiple structures, the braze temperature might have to be held for a longer amount of time, thus risking an over-heating of the workpieces.

Another way to reduce furnace operation times would be to combine the braze and bake out steps into one cycle which occurs in the same furnace. The following are two proposed ways of accomplishing this:

1. Braze in vacuum. Since the structure is saturated with hydrogen from the previous bonding step, a partial pressure of hydrogen will naturally occur in a vacuum furnace. The presence of hydrogen during the brazing cycle should improve the quality and the reliability of the brazed joints. After brazing, the furnace temperature will be lowered to that of bakeout temperatures and the remaining hydrogen will be baked out of the structure.

2. Braze in hydrogen. Instead of relying on the hydrogen already saturated in the accelerating structure, the brazing will take place in a hydrogen atmosphere. Brazing in a hydrogen atmosphere is a well- tested process that has already produced high quality braze joints. After brazing in hydrogen, the furnace temperature will be lowered to bakeout temperatures and the chamber will be evacuated and a normal bake out of the hydrogen will occur. This method will take longer than the previous method since more hydrogen will be introduced to the structure and thus take longer to bake out.

The bake out process could easily be a multiple structure process since the temperature gradients are not as critical and the temperature is lower than the bonding and brazing operations. This might suggest that the bake out should occur as a separate process in a different furnace. It was also suggested that the bake out could occur during RF processing, but this is still a topic for much debate. Furnace Vendors:

GM Enterprises Specialize in large industrial furnaces. Atmosphere, gas, & electrical furnace systems. Worked closely with Alpha Braze to set up their equipment. Systems include: Hydrogen Bell Furnaces Conveyor/Belt Furnaces Bottom Load Hydrogen Furnaces

525 Klug Circle Corona, CA 91720-5452 800.348.7474 P 909.340.4646 P 909.340.9090 F Contacts: Brian Grier, Marketing Manager Ed Byrness, Jr., Technical Director of Key Accounts

(Abar) Ipsen International Specialize in vacuum furnace systems. Furnaces used at Brush Wellman and Alpha Braze 800.284.5665 Contact: Carol Nascar, LA Regional Office 818.573.3996

Grieve Specialize in industrial ovens and furnaces Large range of product: belt, clean, atmosphere furnaces

500 Hart Road Round Lake, IL 60073-9980 847.546.8225 P 847.546.9210 F Contact: Carson George (with Hart-Latimer), West Coast Rep. 650.591.2497