LINAC4: RELIABILITY RUN RESULTS AND SOURCE EXTRACTION STUDIES

Linac4, a 160 MeV, 352.2 MHz linear accelerator, has been fully commissioned and will take its place as injector to the CERN chain of accelerators during the long shutdown (LS2) in 2019-2020. In the past year, it has been providing beam during a test run to assess its reliability in view of the connection to the LHC proton injector chain. A target reliability of more than 90 % has been demonstrated during the accumulated nine months of run in 2017 and 2018. The beam quality at 160 MeV is suitable for producing all beams for the CERN physics program of today. Nevertheless, the limited peak current of 30 mA might be a limitation for future high intensity programs. The bottleneck has been identified at the low energy end of the accelerator. Meanwhile, beam extraction and low-energy beam transport studies are ongoing at a dedicated test stand with the goal to reach beam currents from the pre-injector up to 45 mA. We will present the status of the extraction modelling and possible solutions to reach higher beam currents from the RFQ along with results from the reliability run. RELIABILITY AND BEAM QUALITY RUN Linac4 was fully commissioned to 160 MeV in autumn 2016 with 60 % of the nominal current on the dump at the end of the linac [1]. The years 2017 and 2018 were dedicated to achieving the beam quality and reliability necessary for injection into the Proton Synchrotron Booster (PSB). The criteria were set such that the PSB will be able to provide its complete set of beams with pre-LS2 as well as the LIU target parameters. The two most extreme examples are the brightest beam namely 3.4 × 1012 protons per ring in an emittance of 1.7 μm and the highest intensity beam namely 9 × 1012 protons per ring in an emittance of 8 μm. A peak current of 20 mA at the PSB, combined with an increase of the number of the injection turns to the maximum (150 turns per ring) is sufficient to achieve enough intensity to produce the mentioned beams [2] provided that the beam quality requirements given in Tab. 1 are met. The requested target reliability for the linac is more than 95 % [3], a very challenging value to meet for a brand-new accelerator. The linac ran in its almost final configuration for a total of 8 months between 2017 and 2018. Important data has been gathered concerning its reliability, its weak points and the beam quality [3, 4]. The most important achievement of the last run in 2018 was an overall availability of 94.1 % over 10 weeks. The peak beam current at the end of the Linac4 ∗ daniel.noll@cern.ch was routinely 25 mA which allows for 20 % losses in the transfer and capture into the PSB. The beam pulse flatness was achieved by using the chopper (at 3 MeV) to cut the rising edge of the beam, necessary due to the onset of space charge compensation after pre-chopping. This results in the need of a 850 μs beam from the source for a 600 μs beam at 3 MeV, achievable with the present source and RFQ. The transmission from 3 MeV to 160 MeV is approaching 100 % with a beam of 25 mA, whereas the transmission of the pre-injector is 70 % to 80 % at best. ION SOURCE EXTRACTION STUDIES During 2017 and 2018, the extraction system of the ion source was investigated on the Linac4 test stand as a possible origin of the pre-injector performance limitation. This test stand is a copy of the Linac4 low-energy beam transport line with added diagnostics: the pre-chopper hardware between the two solenoids is replaced by a slit-grid emittance meter. Detailed information on the layout is given in [5]. Figure 1 shows the extraction system of the Linac4 ion source as modeled using IBsimu [6]. The beam is extracted through a bore, 6.5 mm in diameter, by the electric field generated by a dual-function puller-dump electrode. Any coextracted electrons are deflected into a cup by the magnetic field produced by two permanent magnets housed within this electrode. The beam is then accelerated to its final energy of 45 keV. An einzel lens additionally focuses the beam to transport it to the first solenoid downstream. More informaTable 1: Target Beam Quality at the PSB Injection Parameter Target value Intensity flatness along the pulse for pulse lengths < 180 μs ±2 % Intensity flatness along the pulse for pulse lengths > 180 μs ±5 % Horizontal/vertical position variation along the pulse ±1 mm Horizontal/vertical injection angle error along the pulse ±0.4 mrad Shot-by-shot current stability ±2 % Transverse norm. rms emittances < 0.4 mm mrad Beam energy 160 MeV Pulse to pulse energy spread 80 keV to 600 keV Nominal chopper operation 65 % at 1 MHz Energy painting ±0.8 MeV 10th Int. Partile Accelerator Conf. IPAC2019, Melbourne, Australia JACoW Publishing ISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-MOPTS096 MOPTS096 1090 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 19 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I MC4: Hadron Accelerators A08 Linear Accelerators