Results of the argon beam test at Linac3 D. Küchler BE/ABP/HSL Including feedback from R. Scrivens and M. Bodendorfer

Objectives NA61 requires a primary argon beam in 2015 (the setup of the accelerator chain will be done in 2014) Find and optimize settings for source and linac Measure beam parameters (intensity, stability, emittance) Study long term behavior of source and linac

RESULTS

Source behavior Easy to run with argon (gas bottle) Performance well reproducible after stop Short commissioning period (one week should be sufficient) Does not need tuning very often (1-2 times per day)

Intensity (Ar 11+ ) eµA out of the RFQ (FC3) eµA at TRA25 (end of the linac) Lost beam intensity data in Timber due to misunderstanding and misconfiguration.

Stability Overall beam stability OK, but it seems it is less stable compared to the lead beam Often quite some variations within the pulse

Emittance Quadrupole scan in the ITF line (after the tanks) Limited explanatory power due to beam structure (not round nor Gaussian) Beam seems to fill the complete machine acceptance Courtesy of M. Bodendorfer

OPEN ISSUES

Plasma chamber Strong sputtering inside the plasma chamber Metal flakes on the bias electrode Grooves along the plasma loss lines (several tenth of mm deep) Iron in the extracted ion spectrum

Intermediate electrode New design was working properly for the lead run During the argon test the wire was heated by the ion beam and the capton insulation was burned away -> short circuit By retuning the source most of the intensity could be regained but the stability suffered New design for the routing of the wire needed

Beam diagnostic Beam steering due to charge up of the SEM grids Strong negative peaks in Faraday cup measurements do not allow to measure precisely beam impurities and lead to some uncertainty on the actual beam intensity

Vacuum - Source Vacuum leak in the extraction tank in the last week Probably a broken welding due to beam losses The tank has been there for 20 years, that’s why it is difficult to say if it is related to the argon beam Spray sealed to finish experiment Beam direction

Vacuum - linac Ion pumps in LEBT and RFQ suffered due to the gas load Pressure was roughly stable, but the current in the pumps was rising Not all of them have recovered after the run

Radiation For argon ions hitting iron the Coulomb barrier is of the order of 3.2 MeV/u For a full beam loss at the final energy of 4.2 MeV/u roughly 20µSv/h were measured (90cm away from the beam line) The calculated values for the loss point are three times higher Assumption: at 40cm from the beam line one can expect a neutron dose rate of 40-50µSv/h (to be scaled with beam intensity and repetition rate) Material at the end of the linac can now be also activated!

SUMMARY

Conclusions Linac3 can deliver an argon ion beam with roughly 10 times the particle current compared to lead The operation of source and linac is simpler compared to lead, no down time due to oven refills Before the run some modifications are needed

To do I The plasma chamber has now to be considered as a consumable – After only 8 weeks with plasma clear damages visible – Setting up + run will be around 8 months – One should foresee at least 2 chambers (15kCHF each) for the argon run and a scheduled down time of several days to replace it – The present chamber has to be replaced as well (is a repair possible and will it be cheaper?) – Similar considerations have to be applied for the xenon run

To Do II Extraction tank will be replaced by a new, redesigned one, special protection for the intermediate electrode will be build in Spectrometer chamber not yet checked, to be done soon (damages?) Additional pumping group will be installed in the LEBT near to the source Ion pumps may be partially switched of during the run to protect them

To Do III Evaluation of the radiation situation (input from LEIR needed: intensity, pulse length, rep-rate) Definition of limited stay areas needed An improvement of the beam diagnostics in the linac is necessary