Minutes SNS Accelerator Physics Group Mtg.
1/17/02

Present: J. Wei, J. Galambos, S. Henderson, S. Danilov, J. Holmes, S. Cousineau,
S. Aleksandrov, D. Jeon, S. Kim, M. Doleans, P. Chu, A. Shsishlo,
S. Assadi, T. Pelaia, J. Stovall 


D. Jeon reviewed the low energy collimation scheme: the baseline approach 
includes adding 2 scrapers at the MEBT chopper (x direction) and adopting
a modified MEBT optics (requires 3 new quad power supplies).  DTL collimation 
is not effective, and not reccommended. Also, LEBT collimation is ineffective. 

There is a concern on the transport of the partially chopped LEBT beams.
Sang ho is preparing A 3-D field map for L. Young to use to track partially 
chopped beam from the LEBT through the linac.

J. Galambos discussed the DTL corrector power supply issue. The original
PS bid (300 A) came in high, and reducing to 200 A has minimal impact. 
Two alternatives are possible: 1) using the same type 20A supplies as 
all the other correctors use (linac + ring) and stacking them up to 140 A
if needed, or 2) using unipolar 150 A supplies with reversing switches.
The first alternative appears to be within cost. Estimates for option 2
will be available by next Wed., when a decision will be made.

S. Henderson discussed the backscattered e issue (off the e-catcher at injection).
Even with magnetic mirroring, some backscattered electrons will pass through 
to the foil, or vacuum chamber. More detailed analysis is ongoing.

J. Wei noted that the  ring dipole measurement program has shown three classes 
of transfer functions (10 measured, 6 OK, 2 high, and 1 low by 0.2% compared
to expected 0.01%).  

The extraction kicker rise has tails of ~ 100 nsec bfore and beyond the 5->95% 
range. The trailing rise has a ripple (~7%). The early rise kick gives
~ 1 cm offset. Also there is a concern on the impact on the off-center 
installation of the kicker magnets, and possible impact on inducing
a dipole wobble (on the bunch center which experiences a greater 
impedance effect).This puts more concern on the extraction kicker
imedance issue.

D, Jeon issued a note on the RF reference line stability requirements.
A +- 0.1-deg cavity-to-cavity uncertainty is OK. The RF reference line 
is temperature controlled to compensate for an otherwise 2-3 cm delta-L 
for the expected +- 13 deg F temperature variation (over time).


J. Stovall noted that running SCL cavities at their max. voltage
(+- 10%) and not adhusting the phase to compensate for continuous
focusing results is a fairly well behaved beam (some small longitudinal
growth, but not transverse).

Sasha noted that the LBNL RFQ RF circulator was broken. If it is fixed
next week, he will go.