ALICE : Machine Characterisation
Period 14 : June – November 2012
Beam Stability
Laser beam moved +/- 1000steps both H and V to simulate beam jitter on INJ-1 and its effect on FEL
output (#3023). The FEL was on this shift very much stable.
FEL power (in mW) as a function of laser beam position (schematically):
0
5
5
(V=1500)
0
7
2
(V=500)
1
4
1
(V=-500)
(H=-4000)
(H=-3000)
(H=-2000)
Beam jitter on INJ-1 in #3023 (stable FEL) : ~ 30% peak-to-peak of the full beam movement while laser
changed by +/- 1000steps.
Beam jitter on INJ-1 in #3013 (unstable FEL) : still ~ 30% peak-to-peak. However it is not clear if the
series of INJ-1 images were taken during stable or unstable periods.
Also, the beam motion on CNWG with +/- 1000steps variation of the laser position is only +/- 0.5mm
(from one edge of the CNWG hole to another).
Beam on CNWG in #3023 is very stable (small jitter) while in #3013, the jitter was as high as ~1.5mm
p/p.
Conclusions:
The above data are not entirely conclusive (e.g. too high step change in laser position was used).
The transverse jitter due to laser beam positional instability is certainly a contributing factor but cannot
account fully for intermittent periods of large jitter observed earlier (at least, on 7-10 July).
The observed large jitter is likely to be mainly due to timing instability, be it PI laser or RF.
For some (not understood / unknown) reasons, the two state “large/small jitter” behaviour of the
machine disappeared by itself after ~ 3 days of operation.
NOTE.
The transverse jitter seen on the cathode and INJ-1 cannot account for any sizeable timing jitter due to
variable path length between the buncher and the booster. As a crude estimate:
∆𝑥 2
∆𝑠 ≅
2𝑠0
where s is a path length difference and x is a transverse jitter. Assuming s0=1m and x=2mm, then
s ~ 2x10-3mm.
------------------------Occasional “catastrophic” losses of ER.
Were observed on 15/07/12 while operating in THZ setup (divisor x2). Lasted from a few seconds to
several tens of seconds. Then returned to normal by itself.
Note : the ER in this setup was not “perfect” : LC traces were <70mV over 100us train.
Nothing unusual on RF data was noticed.
A possible link to cryo stability was looked at in #3029. No solid conclusions.
NOTE: the following day, in EMMA setup (single bunch operation) – no instabilities were observed
(that could be judged by the efficiency of injection into EMMA ring).
------------------------
Cathode, PI laser and Gun
22/06. 1st activation
Several FEL/THz shifts. Several days - no beam due to RF fault.
06/07. Re-Cs #1.
FEL operation – all days.
12/07. Re-Cs #2.
FEL/THz/AP ... then EMMA from day 4.
First cathode activation. 22/06/2012.
QE max = 2.9%.
Re-Cs #1. 06/07/2012.
QE max = 2.7%.
Note the hole at approximately the beam position before re-Cs is quite visible.
Left-right non-uniformity but again uniform after re-Cs.
Small hole (near electrostatic centre ?) remains after re-Cs – permanent ?
Re-Cs #2. 12/07/2012.
QEmax = 2.6%.
All features “before/after re-Cs” are the same as in re-Cs #1 but became more pronounced.
Beam loading effects
Beam loading effects appear to be very strong (THZ setup, #3006).
ST1-1 return beam images in THZ setup (#3006)
Left: divisor x5 (16MHz); return beam focussed on ST1-1 using ST4 quads and then steered out
of the hole. T=30us. No beam is seen at 100us and x5 on ST1-1 when it is steered through the
hole.
Right: beam steered through the hole but divisor = x2 (40MHz). T=100us.
----------------------Beam loading effects in FEL setup, #3015.
ST1-1 return beam images in FEL setup (#3015)
Both images at x5.
Left : beam steered out of the hole; T=50us.
Right: beam in the hole ; T=100us.
NOTE: insertion of ST1-1 kills lasing . Why then such a difference between 50us and 100us ???
---------------------------
Dispersion INJ-5
#3045.
Three decimal places in DIP-01 readings.
Q-05 degaussed.
1.4
1.2
Dispersion, m
1
0.8
0.6
0.4
6.5MeV
0.2
0
2.8
3
3.2
3.4
3.6
3.8
4
4.2
I0
Dx = 1.17m.
Does not depend on beam energy hence previous measurements with some tendency to increase with
energy is down to the remnant field of Q-05.
-------------Emittance v buncher power
Measured at 325kV in AP setup (BC=-10/+15). #3042.
Blue dots : Q&D method.
Red dots: Q-02 scan method.
Black dots: earlier measurement at 230kV.
Emittance v Pb : 230kV & 325kV (#2634; #3042)
20
Emit (230kV)
Emit (325kV)
E
Emiitance, um
15
10
5
0
0
5
10
15
20
SQRT(Pb)
25
30
35
40