Commissioning of a high power linac at GANIL: Beam power ramp-up

SPIRAL2 is a high power linear accelerator part of the GANIL laboratory. This superconducting (SC) linac can accelerate cw light ions to heavy ions at energies up to $40\mathrm{MeV}/\mathrm{nucleon}$ and $14.5\mathrm{MeV}/\mathrm{nucleon}$, respectively. In July 2019, the authorization to fully operate the SPIRAL2 SC linac permitted to start the commissioning of the medium energy beam transport line and SC linac. The main milestone of the commissioning in 2020 was to validate the transport and linac design by increasing the duty cycle up to 10% with a proton beam. This paper presents the strategy used by the commissioning team to increase the beam power, as well as the results obtained using different diagnostics. The extrapolation from obtained measurements to 100% duty cycle was also performed; the results are presented.

Figure 1

SPIRAL2 SC linac layout.

Figure 2

Beam diagnostics in the SPIRAL2 SC linac used for the beam power ramp-up.

Figure 3

Phases of the beam power ramp-up for the SPIRAL2 SC linac commissioning.

Figure 4

Beam power ramp-up to 16 kW.

Figure 5

Three rms envelopes from multiparticle simulation for the reference proton beam.

Figure 6

Ellipticity for the 16 kW beam power (blue) and for the reference proton beam (red).

Figure 7

Vertical beam position along the SC linac for the 16 kW beam.

Figure 8

Pressure variation in the warm sections of the SC linac with the MEBT quadrupoles for the reference proton beam (green) and after optimization (orange).

Figure 9

(a) Longitudinal acceptance of the SC linac and (b) measurement of the losses for the HEBT line as a function of rebuncher #3 phase.

Figure 10

BLMs count rate as a function of the voltage of rebuncher #3, 35.6 kV (orange) and 29.6 kV (green).

Figure 11

Pressure variation in the SC linac warm sections as a function of the rebuncher #3 voltage, 35.6 kV (orange) and 29.6 kV (green).

Figure 12

Losses in the HEBT lines a function of the beam peak intensity: (a) with insufficient gain feedback for the cavity fields and (b) for an optimized LLRF control.

Figure 13

Residual perturbation on the accelerating field of cavity #1 in the low $\beta$ section with a 4.1 mA proton beam with and without feedforward (FF).

Figure 14

Transmission in the SC linac (red) and beam power (blue) during the beam power ramp-up to 16 kW.

Figure 15

Beam position in the vertical plane along the SC linac for 16 kW (blue), 10 kW (violet), and 2 kW (green).

Figure 16

(a) Count rate for each BLM at 16 kW (blue), 10 kW (violet), and 2 kW (green) beam power and (b) beam density in the longitudinal plane of a proton beam. Dark lines are the ${\varphi }_s$ and $-2{\varphi }_s$ limits.

Figure 17

Horizontal particle density of a $5\mathrm{mA}$ ${{}^{2}H}^{+}$ beam for the reference simulation with $2.5\times {10}^9$ particles.

Figure 18

Distribution of the losses in the linac brought back to the exit of the RFQ.

Figure 19

BLMs count rate for the proton beam at 16 kW (blue), extrapolation to 100% duty cycle (green), and BLMs thresholds (red).

Figure 20

Pressure variations at 16 kW (blue), 10 kW (violet), and 2 kW (green).

Figure 21

Beam view of SAFARI. The colors represent the temperature rise in each thermocouple at 16 kW, light blue colors lower temperatures and red colors higher temperatures.