Spiral 2 injector commissioning

The SPIRAL2 injector is composed of two ion sources (p/d and heavy ions up to A/Q=3) followed by a 730 keV/u RFQ. Beam commissioning has started in 2014 in parallel with the superconducting linac and HEBT installations. The RFQ beam commissioning started soon after the first RF conditioning done in October 2015. This paper describes the RFQ beam measurements done on the diagnostic plate for the reference particles (H+, 4He2+ and recently 18O6+) and the difficulties encountered for the RFQ commissioning at the A/Q=3 field level. INTRODUCTION The layout of the SPIRAL2 driver takes into account a wide variety of beams to fulfill the physics requests [1]. It is a high power CW superconducting linac delivering up to 5 mA proton or deuteron beams or 1 mA ion beams for Q/A > 1/3 (Table 1). Our major challenges are to handle the large variety of different beams due to their different characteristics (in terms of particle type, beam currents – from a few μA to a few mA and/or beam energy), a high beam power (200 kW, CW) and to answer correctly to the safety issues, especially with the deuteron beam. Table 1: Beam Specifications Particles H+ D+ ions option A/Q 1 2 3 6 Max I (mA) 5 5 1 1 Max energy (MeV/A) 33 20 15 8.5 Max beam power (kW) 165 200 45 51 We do not have yet the authorization to accelerate deuteron beams or to inject in the LINAC. A test bench, the Diagnostic-plate (D-plate), is used after the RFQ to validate the RFQ performances, to tests various diagnostics and to measure the beam characteristics for the future Linac injection. BEAMS FOR COMMISSIONING Three beams have been selected to demonstrate the injector performances. The 5 mA proton beam requires only 40 kV on the RFQ vane voltage. It is the easiest to produce, requires little RF in the RFQ cavity but is the most difficult to transport in the LEBT because of space charge forces. The 4He2+ was selected in order to mimic the future deuteron beam. It allows to test the A/Q=3 ECR source and LEBT and requires a little higher RFQ vane voltage (80 kV). The ultimate injector performances require an A/Q=3 beam up to 1 mA. The 18O6+ has been selected. The RFQ vane voltage rise to 114 kV (1.65 Kilpatrick). We were not able to achieve stable operation of the RFQ and associated RF system at such set point until recently. 105 kV was easier to achieve and 16O6+ beam (A/Q=2.67) was used as backup for performance measurement. Recent improvement in the cavity conditioning and LLRF tuning allow a much better cavity operation, leading to an availability of about 75% at nominal voltage (114 kV). The merit of this is that the measurement results for both oxygen beams can be compared, and 18O6+ can be measured à 105 and 114 kV. D-PLATE DESCRIPTION The D-Plate is installed in the Medium Energy Beam Transport Line (MEBT, Fig. 1). It allows to measure: • Intensity with Faraday cups, ACCT and DCCT • Transverse profiles with classical multi wire profilers and ionisation gas monitor (MIGR in the text) • H and V transverse emittance with Allison type scanners • Energy with a Time of Flight (TOF) monitor • Phase with the TOF and the 2 BPMs • Longitudinal profile with a Fast Faraday Cup (FFC), and a Beam Extension Monitor (BEM) • Beam position and ellipticity ( − ), with the BPMs. The diagnostics performances are given in [2,3,4] Figure 1: Injector scheme up to Diagnostic Plate. RFQ RF CONDITIONING The RFQ RF conditioning improved during the past few months with a better control of the RF chains and better conditioning methods. At first, the RF conditioning was managed in pulse mode with a Duty Cycle (DC) low enough to avoid copper heating. It allows to burn the dusts and spikes. We were quickly able to reach 125 kV (240 kW), 500 μs/100 ms, ending with operation without sparks. The LLRF is then switched in CW mode, integrating a PLL. In both cases (pulsed, CW with PLL), the cavity frequency tuning is manual using the temperature control. In CW mode, the cavity starts to spark at 105 kV, much lower than the level achieved just before in pulse mode. The copper heating in CW mode increases an outgassing inducing sparks at lower voltage. The RF conditioning is greatly improved increasing the copper temperature up to 55°C with RF for few weeks. Coming back to the normal LLRF operation, unfortunately without PLL during the 29th Linear Accelerator Conf. LINAC2018, Beijing, China JACoW Publishing ISBN: 978-3-95450-194-6 ISSN: 2226-0366 doi:10.18429/JACoW-LINAC2018-THPO047