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
* HRS separator
* 14% ion source efficiency reached for Kr with 0.2 bar buffer-gas pressure
* RFQ cooler operated in cw mode
* Extraction electrode 39 mm
* Uht(HRS,CCV)=30.08 kV, Uht(HRS, HT_target)=30.1033 kV, Anode=170 V, Uht(RFQ cooler)=30.20 kV, RFQ extraction=0 V, Uht(trap)=30.00kV, Elec05=170 V

94Kr: 6.3e6/uC
96Kr: 2-4e5/uC (uncertainty due to technical problems in our fitting routine)


Experimental issues
*  Beam to Miniball
*  IS485

Tuning
* Normal extraction from EBIS 
* 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

9 mm lead/tantallum collimator ~4 cm before the MB target frame
                  (always present, but this year well aligned)

    Total transmission (radioactive beam)
             
HRS.FC490_to_trap x Trap_EBIS x Linac 
              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 we used:
* 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.