Thu Apr 14 14:03:17 2011:  What did we see after the fin diffusion bonding of Ta fins for FEBIAD-HP (w.o#38481-7) ? :

1.  A problem occurred during the FEBIAD HP Ta fin diffusion bonding and was discovered after the evaporator chamber was opened.  The washer was "wielded" on to the left leg of the target (see attachment #1: washer on target leg). 

2. A look inside the evaporator chamber shows that more coating is found on the evaporator lid and the copper base plate (See attachment#2: Inside evap chamber). This coating was previously found when a HP FEBIAD target underwent for the fin diffusion bonding (See Message ID#4: Diffusion bonding of Ta Fins for FEBIAD-HP (w.o#38481-8)).  The evaporator chamber is cleaned with a wiper sheet and EtOH each time before a target is put in;  however the coating is not easily removed and there are always some residual remain. 

3.  The bolt and the washer are deformed.  The top part of the bolt is expended and can no longer fit into the threaded washer  (See attachment#3: Deformed bolt_washer). 

4.  The TIS group was called in to investigate the problem.  It was believed that the problem might have started from the bolts and the washers.  The un-threaded shank bolts and the threaded washers are made for remote handling purposes, so they do not fall apart everywhere in the hotcell.  However, if these bolts and the washers are used for the evaporator, which are bolted on and tighten against the gravity onto the conductors, this may create a gap if the threaded washer is tilted slightly.  (See attachment#4: Gap).  As a result, this causes an uneven pressure contact of the bolt/washer and the target leg.  

Solutions:

Use fully threaded bolts (same material as before - a mild steel) and remove the threads from the washers (See attachment#5: No Gap).

--> To be tested...

 Fri Apr 15 11:59:31 2011: 

The new bolts/washer were cleaned only with simple green and ultrasonic for 5 min.  The bolts and washer should not be cleaned for too long; some remaining grease actually helps prevent them from getting rusty.  The evaporator is wiped with EtOH and  the target is inside.  The bolts and washers are tighten and they are not showing at space like before (See attachment#6: new bolt_washer) .  The evaporator is being pumped down and the target heater will be raised to 820A; we will have to wait until the fin diffusion bonding is complete and the evaporator chamber is vented. 

 Mon Apr 18 12:30:35 2011: 

After a fin diffusion bonding of a HP FEBIAD target, there is no sign of coating in the evaporator.  The bolts/washer and the target look normal. 

--> The problem is solved.