Achievement of high-current continuous-wave deuteron injector for linear IFMIF prototype accelerator (LIPAc)

The Linear IFMIF (International Fusion Materials Irradiation Facility) Prototype Accelerator (LIPAc) is aiming at demonstrating the low-energy section of a 40 MeV/125 mA IFMIF deuteron accelerator up to 9 MeV with a full beam current in continuous-wave operation. For such a high-power beam, the LIPAc injector is required to produce a beam current of 140 mA and 100 keV D+ beams. The injector commissioning to reach high beam current has been progressing at high duty cycle operation, then stable operation for many hours at a beam condition of 150 mA total extracted current. This result demonstrates that the LIPAc injector can meet the required performance for nominal 125 mA long pulse deuteron beam acceleration.


Introduction
The International Fusion Materials Irradiation Facility (IFMIF) is a fusion neutron source based on two parallel deuteron accelerators (40 MeV, 2 × 125 mA) and a liquid lithium target for the study of structural materials for future fusion Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.reactors.Currently, the Linear IFMIF Prototype Accelerator (LIPAc) is being commissioned in Rokkasho, Japan, under the Broader Approach (BA) agreement between Japan and the EU [1].The objective of LIPAc is to demonstrate the low-energy section of the IFMIF deuteron accelerator with a beam current of 125 mA and 9 MeV in Continuous-Wave (CW) operation.The LIPAc consists of an injector, a Radio Frequency Quadrupole (RFQ) accelerator, a Medium Energy Beam Transport line (MEBT), the first section of the Superconducting RF (SRF) linac with the same characteristics as the ones of the IFMIF accelerator, a High Energy Beam Transport line (HEBT), and a Beam Dump (BD).For such a high-power deuteron accelerator, the LIPAc injector is required to provide a stable deuteron beam of 100 keV, 140 mA with low emittance (⩽0.25 π mm mrad) to obtain 125 mA and 5 MeV CW beam by the RFQ.In 2019, the RFQ successfully accelerated a 125 mA deuteron pulsed beam at low duty cycle (pulse width of 1 ms, 1 Hz) [2].In this phase, the LIPAc injector fully satisfied the requirements at low duty cycle operation, achieving a total extracted current of 160 mA being matched into the RFQ with good RFQ transmission (>90%) [3].Since then, construction of the LIPAc has progressed and beam commissioning in the current Phase B+ has started in 2021 [4].One of the primary objectives of Phase B+ beam commissioning is to validate the RFQ performance at high duty cycle up to CW operation at nominal 125 mA D + beams.In parallel, the LIPAc injector commissioning to establish a stable operation at high duty cycle with extracting a beam current above 150 mA is progressing.In this paper, the results of the LIPAc injector CW campaigns are described.

Overview of the LIPAc injector
The LIPAc injector consists of an Electron Cyclotron Resonance (ECR) ion source and a Low Energy Beam Transport line (LEBT) [5].It was designed and built by CEA-Saclay based on the SILHI.Figure 1 shows the schematic layout of the injector.The LIPAc injector requirements are summarized in table 1.In order to avoid activation of the accelerator components due to beam loss in the early stage of beam commissioning, it is also necessary to provide a 50 keV, 70 mA proton beam (the half energy and the half current, but the same perveance with respect to the nominal D + beam) so that the tuning method can be established before starting the deuteron beam acceleration.

Ion source
The ECR ion source consists of a plasma chamber with two magnetic coils and the 2.45 GHz magnetron (1.2 kW maximum).Since D 2 + and D 3 + are also produced, the ion source is required to extract a total current of 150 mA or more.The beam extraction system of the ion source consists of five electrodes: Plasma Electrode (PE), Intermediate Electrode (IE), first Ground Electrode (GE1), Repeller Electrode (RE), and second Ground Electrode (GE2).The PE is an electrode that can be replaced with a different aperture from Φ6 mm to Φ12 mm.When a proton beam or low beam current is required for the LIPAc beam commissioning, the source can be adapted by replacing the electrode with a smaller extraction aperture.The IE is used to optimize the beam divergence angle by adjusting the applied high voltage (from 15 kV to 30 kV).The repeller electrode is used to prevent the electrons produced in the LEBT to be attracted and accelerated towards the plasma chamber by applying −4.5 kV.

LEBT
In the LEBT, two solenoids (SOL1 and SOL2) are used to focus the beam while transporting it to meet the RFQ injection  conditions.The total length from the PE to the RFQ is 2.05 m.
To minimize the total length a pair of steerers (ST, horizontal and vertical) is installed at the middle of each solenoid.The chopper consisting of two parallel electrodes fed by a highvoltage power supply (max.+10 kV) can also be inserted between the two solenoids.It allows making beam pulses of tens or hundreds of microseconds with sharp rise and fall time to avoid breaking interceptive diagnostics, e.g. the Secondary Electron Emission grid (SEM grid), installed at the HEBT.When the chopper is used, the chopped beam is intercepted by the injection cone.Not possible to use the chopper at high duty cycle operation since the injection cone cannot accept so much power.

Diagnostics
For the characterization of the beam extracted from the ion source, several diagnostics are installed along the LEBT [6].The Allison scanner type Emittance Measurement Unit (EMU) is installed vertically between the two solenoids.Therefore, the emittance values presented in this paper are in the y-y ′ plane.Since this EMU is an interceptive device, the measurement results are reliable only for low duty cycle operation (10% or less).When the beam is extracted at high duty cycle, the light emitted from the interaction of the beam with the residual gas is observed with a CMOS camera to measure the beam profile at the same location as the EMU. Figure 2 shows the vertical beam profile for different SOL1 current values observed with a CMOS camera.The ion species fraction is also an important characteristic and can be measured using Doppler-shifted spectroscopy [7].The movable Faraday Cup (FC) is located just after the EMU (upstream of the second solenoid) and allows the injector to be commissioned without injecting the beam into the RFQ.

Injector CW commissioning campaigns
The LIPAc injector is required to establish operating conditions that simultaneously satisfies the requirements of high current and low emittance in CW operation.During the previous injector commissioning performed in 2015-2016 with a PE of Φ12 mm aperture, the nominal beam current was extracted successfully with low emittance [8].However, many discharges limited the beam stability at high duty cycle operation.Since it is difficult to theoretically predict a stable operating region with low discharge frequency that satisfies the requirements, several PEs with different extraction apertures have been fabricated in advance, and the relationship between beam current and emittance was systematically investigated by sequentially changing the electrodes from smaller to larger diameters.In order to determine the optimal electrode, we have performed injector CW commissioning campaigns with Φ9, Φ10, Φ11, Φ12, and Φ11.5 mm apertures.

Experimental results with the Φ9 and 10 mm PE
A first injector CW commissioning has been performed in 2019 with Φ9 mm PE.And in the same year, the commissioning was also conducted using Φ10 mm PE.The primary goal of these commissioning was to reach CW up to the maximum beam current that could be extracted with these PEs.In addition, the emittance was measured with EMU at low duty cycle for various beam currents to see if it meets the LIPAc requirements.For high duty cycle operation, the beam profile, species fraction, and beam current at the FC were monitored to analyze whether the beam parameters remained stable from low duty cycle to CW and over several hours of CW operation.Although the beam current was lower than the LIPAc requirement, as expected with these PEs, a stable process to reach CW from low duty cycle was established.With Φ9 mm PE, it was possible to achieve CW for a total extracted current of up to 100 mA.With Φ10 mm PE, the total extracted current for stable CW operation increased to 130 mA.

Experimental results with the Φ11 mm PE
In 2022, the injector CW commissioning with Φ11 mm PE started, aiming at stable CW operation with higher than 150 mA total extracted current and precise beam characterization.After one month of conditioning from the Φ11 mm PE installation, the emittance as a function of the IE voltage was measured for different beam currents.The IE voltage corresponds to the potential difference between the PE and IE.The results of the measurements are presented in figure 2. These measurements were taken at 5% duty cycle.Emittance increased with the extracted current due to higher space charge.Reducing the IE voltage down to 15 kV improved the emittance.It should be noted that the tuning range of the potential difference in the first extracting gap was limited to [15][16][17][18][19][20][21][22][23][24][25] kV in order to prevent discharges during high duty cycle operation.With Φ11 mm PE, a total extracted current of 150 mA has been achieved in stable CW operation.Emittance measurements in y-y ′ plane have been performed at 5% duty cycle as shown in figure 3. The result shows that the normalized rms emittance was of 0.24 π mm mrad, which is within the LIPAc requirements.Species fraction measurements performed at CW with Doppler shift analyzer showed that the D + fraction was of 91%.This means that Φ11 mm PE can be used for the injection into the RFQ of up to 137 mA of D + beam current up to CW.In addition, continuous CW long run operation was performed during 11 h with this beam condition as shown in figure 4.During this long run operation, the beam profile with a CMOS camera and the beam intensity on the FC were recorded to confirm the beam stability.The beam stability was evaluated from the beam current measured on the FC.Typical result gave 2.6% rms noise as shown in figure 5.This is at the same level as the required value.So far Φ11 mm PE is considered a reliable aperture diameter for stable extraction of high D + beam current in CW operation.

Experimental results with the Φ12 mm PE
CW commissioning with Φ12 mm PE was then carried out in 2022.It was in fact expected that this set-up could provide higher beam currents than the ones obtained with smaller PE apertures, and still with the required emittance.Thanks to a series of CW commissioning experiences, it was possible to achieve CW operation at high beam currents with the Φ12 mm PE this time.It was possible to go up to a total extracted current of 150-160 mA in CW.At 160 mA, the emittance was however measured to be higher than the one obtained with the Φ11 mm PE.Typical emittance measurement for 160 mA at 5% duty cycle is shown in figure 6.In this case the emittance was of 0.30 π mm mrad.From beam stability measurement, the deviation was derived and was of 9.4% rms as shown in figure 7.Although a high beam current could be achieved during the commissioning with Φ12 mm PE, it was difficult to adjust the beam parameters to maintain a stable CW operation with low emittance.

Experimental results with the Φ11.5 mm PE
It has been confirmed that the LIPAc requirements can be met with Φ11 mm PE, but injector commissioning is still progressing to explore a potential working point between Φ11 mm and Φ12 mm to achieve a D + current of >140 mA at the RFQ entrance (i.e. a total extracted current of >155 mA).With the installation of a Φ11.5 mm PE, total extracted current above 150 mA in CW could be achieved.Note that the time that could be spent on CW operation was shorter than for other aperture PEs due to the experimental schedule.Emittance measurement has been performed at 5% duty cycle for extracted current of 155 mA as shown in figure 8.The measured emittance value was of 0.19 π mm mrad.On the other hand, the frequency of discharges during this commissioning was slightly higher than what was obtained with the Φ11 mm PE.Beam stability in CW operation was analyzed from beam current measurement at FC, and the results showed a deviation of 6.5% rms as illustrated in figure 9.Although the long-term stability needs to be confirmed with this aperture, it is a very promising candidate.

Conclusion
The performance required for the LIPAc injector system was demonstrated during CW commissioning campaigns.We have simultaneously achieved the high current, high quality and stability required for the 125 mA CW deuteron beam acceleration.The next step is to make fine adjustments of the injector parameters while injecting the deuteron beam into the RFQ so that the nominal beam current of 125 mA can be transported to the BD in the high duty beam commissioning currently underway.

Figure 2 .
Figure 2. Normalized rms emittance vs intermediate electrode voltage at various total extracted currents with 11 mm diameter aperture PE.

Figure 3 .
Figure 3. Phase space distribution in y-y ′ plane at 150 mA total extracted current with Φ11 mm PE.

Figure 4 .
Figure 4. Trend of the total extracted current during the injector CW long run operation.

Figure 5 .
Figure 5. Beam intensity stability analyzed from 10 s of acquisition in CW operation with Φ11 mm PE.

Figure 6 .
Figure 6.Phase space distribution in y-y ′ plane at 160 mA total extracted current with Φ12 mm PE.

Figure 7 .
Figure 7. Beam intensity stability analyzed from 10 s of acquisition in CW operation with Φ12 mm PE.

Figure 8 .
Figure 8. Phase space distribution in y-y ′ plane at 155 mA total extracted current with Φ11.5 mm PE.

Figure 9 .
Figure 9. Beam intensity stability analyzed from 10 s of acquisition in CW operation with Φ11.5 mm PE.