94,96Kr run June 2009
Target issues
* Target number #399 UC2, VD plasma, cold line
* Beam on target (no converter)
* Ion source gas: 20%-Ne + 20% Xe +
20% Kr + 40% He
*
* 14% ion source efficiency reached for Kr with 0.2 bar buffer-gas pressure
* RFQ cooler operated in cw mode
* Extraction electrode
* Uht(
94Kr: 6.3e6/uC
96Kr: 2-4e5/uC (uncertainty due to technical problems
in our fitting routine)
Experimental issues
* Beam to Miniball
* IS485
Tuning
*
* REXTRAP timing free-running relative to protons pulses
ISOLDE
No
slits, HRS.WG470 in, HRS.FC490=4.1 nA,HRS.FC690=4.2 nA
With
slits, HRS.FC690=1000 pA, HRS.FC748=810 pA,Trap inj plate=750 pA
RFQ
transmission 81%, HRS.FC490 to trap inj plate
75%
Low energy
Stable beam 86Kr
Tperiod=51ms,
Tbeamgate=open, Tbreeding=48.5
ms (almost optimized for 22+)
Trap inj. plate=9.7 pA, RFQ.FC20(22+)=9.8 pA
Trap+EBIS efficiency=4.6%
Stable beam 83Kr
Tperiod=51ms, Tbeamgate=open, Tbreeding=48.5 ms
Trap inj. plate=12 pA, RFQ.FC20(23+)=9.1 pA
Trap+EBIS efficiency=3.3%
Kept Tperiod
at 51 ms also for 96Kr23+ even if not optimum breeding due to:
the short half life of 80 ms
the
recombination /
charge exchange inside the trap
Both radioactive beams
Tperiod=51ms, Tbeamgate=open, Tbreeding=48.5 ms
Efficiency
unknown as large molecular current on the injection plate for 96K
Linac transmission and
energy:
81% RFQ.FC20 to XL65.FC50 for A/q 4, 2.85 MeV/u
73%
RFQ.FC20 to XL65.FC50 for 94Kr22+, 2.85 MeV/u
97%
transmission from XL65.FC50 to Miniball FC
(always present, but this year well
aligned)
Total transmission (radioactive beam)
0.75*0.043*0.73=2.3%
NB!
The yield measurement is done after the RFQ cooler => factor 0.75 taken into
account
Delivered beams
*
94Kr22+, no contamination at Miniball
* Running with 7 pulses out of 15 pulses, 3E13 ppp
* At the REXTRAP injection plate we saw 7 pA; major
fraction from molecule(s) that is broken up in
trap/EBIS as much lower current recorded on RFQ.FC20; trap injection plate
current decreased with a ~20 s half-life after the protons were switched off
* RFQ.FC20=1.5 epA peaks, XL65.FC50=1.1 epA, Miniball FC=1.0 epA
* Miniball measured 1.7E5/s with 1.55 uA proton-beam on target. Would have
expected 3E5/s with stated transmission. The discrepancy was most likely
due to a lower operating target/line temperature than during the target test.
* 96Kr23+, some unknown contamination from the EBIS
* 96Kr22+, large portion of 22Ne5+ in the beam
* 3-4 pA molecular background beam on A=96 on the
trap injection plate; nothing on RFQ.FC20
* Particles rate at Miniball unknown as the
experiment was prematurely terminated
Setting summary
For
*
HRS_SEP_9-06-11_30250V.csv
* RFQ_9-06-11_30200V.csv
* REX_RFQ_9-06-11_30000.csv
REX low energy settings saved as:
* REX-30kV-2009-06-14_HRS_86Kr_Fred.xml
* BTS-30kV-2009-06-14_HRS_86Kr_Fred.xml
* 090614_2320_86Kr22_Fred.csv alternatively 090615_1217_86Kr22_4.2%.csv
REX separator and linac base settings (A/q=4) saved
as:
090615_1551_SEP_Aq4_81%_L65_2.85MeVu.csv
090615_1551_Aq4_81%_L65_2.85MeVu.csv
REX
separator and linac settings optimised
for MB:
090618_1106_Aq4.15_83Kr20+_MB_2.85MeVu.csv
090618_1106_SEP_Aq4.15_83Kr20+_MB_2.85MeVu.csv
Technical problems
* X9GP.MQ30 tripped several times on PS
over-temperature. Increased water flow.
* 9-gap amplifier break downs at 84kW, reduced power by scaling to 23+
*
7-gap1 tripped several times on grid1 over-current
* Air entering the gas line of the ISOLDE target; temporarily reducing the in
source efficiency
* Miniball struggled with unidentified beam
contamination
* Power cut during the delivery of 96Kr23+; REX was restarted and delivered
stable beam within 4 hours but ISOLDE suffered from technical problems with the
vacuum control -> run cancelled.
Comments / Lessons learned
*
The linac scaling was
working well.
* The re-alignment of the Miniball collimator has
made the beam steering much easier and faster. Beam transmission from XL65.FC50
to Miniball FC 100%.
* ‘High current’ 22Ne beam through Miniball
(empty target frame) without scattered beam on the CD and full transmission to
the Miniball FC. This was not the case for the
A/q 4 beam (20-30counts/s on all 4 quadrants) reason unclear.
* The recombination / charge exchange rate inside the REXTRAP is very high for
Kr limiting the storage time to some 50 ms. Apart
from Kr large Ne-peak seen in TOF, nothing else, so it seems to be an
‘energetically forbidden’ charge exchange (see also P. Delahaye, Nucl Phys A746
(2004) 604-607). On the other hand the efficiency seemed to be independent on
the buffer gas pressure and retardation process.
* The total efficiency for Kr is in the order of 5% for short trapping times
(30 ms). For longer times (100 ms) the efficiency is down to 1.5%. The EBIS was
performing correctly, the problem occurred in the trap with an efficiency
ranging from 15% up to 30%.
* To reduce the charge exchange the Ne gas was exchanged for He.
No beam seen on BTS.FC20 though after short tuning. Has to be
tested in a careful manner.