RF-driven H− sources of long life-time and new LEBT developed at CSNS

An RF-driven ion source has been put into commissioning on the China Spallation Neutron Source (CSNS) accelerator since September 2021. In the last two run cycles, the ion source has operated for 310 and 323 days respectively, with an availability of almost 100%. To fully meet the requirements of the CSNS project Phase-II (CSNS-II), the beam intensity from the linac should be enhanced to above 40mA, and the transverse emittance should be minimized to suppress beam loss during acceleration and transportation. A new test bench consisting of an ion source and a LEBT has been constructed to carry out these optimization and research. The featured functions of the LEBT are associated with proton elimination and electrostatic beam chopping. This report covers the operation status and development of the RF-driven H− source and the new LEBT.


Introduction
The China Spallation Neutron Source project Phase II (CSNS-II) aims to deliver a proton beam with a power of 500 kW, requiring a peak beam current of at least 40 mA from the linear accelerator.Taking into account of the beam loss in low energy beam transport (LEBT) and in the radio-frequency quadrupole (RFQ), more than 50 mA H − ion beam is needed from the ion source.To meet this requirement, an external antenna RF-driven negative hydrogen ion source has been developed.
The structure and main features of the ion sources are described in other papers [1,2].Fig. 1 shows the cross-sectional view of the ion source.Silicon-nitride (Si 3 N 4 ) ceramic is used for plasma chamber, which is cooled by the antenna and segmented copper block around the chamber.Si 3 N 4 has a thermal shock resistance up to 800 and thermal conductivity around 30 W/(m•K).The plasma chamber is able to withstand an average RF power of 1500 Watt according our test and simulation [1].To synchronize the plasma pulse with the timing of the whole accelerator, a glow discharge igniter is used to trigger the main plasma on time.The principle of the igniter is introduced in Reference [3].

Performance in last two run cycles
The ion source generated its first beam in 2019.After two years of optimization and cesiation study, the ion source started to commission on CSNS accelerator since September of 2021.Up to July 15 th of 2023, it has worked for last two run cycles with major maintenance interval of 315 and 323 days respectively.Typical operational parameters of the ion source for experimental study and accelerator service are listed in Table 1.As the start point of the whole accelerator, the ion source is always ready to deliver the H − ion beam, either for neutron service or for machine study.Fig. 2 shows the curves of the beam power on the spallation target and the RF-power of ion source in last two run cycles.The beam power on the spallation target ramps from 100 kW up to 140 kW.Since the beam current produced by the ion source is much larger than required, the beam is throttled to around 11∼12 mA by a collimator before RFQ.The beam current is regulated by the solenoid current in low energy beam transport (LEBT) to stabilize the beam power of the accelerator.In the run cycle of 2021-2022, the ion source accumulated a service time of 315 days.In the next run cycle, the service time is extended to 323 days.No component is changed in the summer maintenance between the two run cycles.Only the inner wall of the plasma chamber is cleaned and the extractor electrodes are rinsed.The routine maintenance was done when the whole accelerator was stopped for inspection, usually on Mondays.The routine maintenance includes gas bottle replacement, 50 kV insulation cleaning, gas-water line inspection, hydrogen gas purifier change, and plasma emission spectra check.To make sure a stable operation in next run cycle, the ion source is disassembled and cleaned in the summer shut-down time of 2022 and 2023.So it makes no sense to measure the life-time intentionally.The proved life-time of more than 310 days is among the long life-time sources applied to the accelerators around the world [4].Cesium vapor is fed from the tail of plasma chamber, as shown in Fig. 1.To drive the cesium vapor to the converter electrode efficiently, an evaporator made of alumina cylinder is used to cover the cold area of the plasma chamber and decrease the cesium deposition on it.The cesium evaporator is unattached to the plasma chamber.And it is warmed up mainly by the radiation of plasma to a temperature of at least 100 according to our estimation.Typically 2 to 4 grams cesium is initially loaded in the reservoir.In the run cycle of 2021-2022, it consumes up to 0.38 g of cesium because of the saturation of the hydrogen gas purifier [2].In the next run cycle, the consumption is lower down to 0.97gram.

Low energy beam transport
In the machine study, the RFQ can accelerate about 35 mA H − beam with a transmission of 93.5%, not enough for 500 kW beam power on target required by CSNS-II.One of the reasons is the emittance growth in low energy beam transport (LEBT) , verified by our measurement and beam optic simulation [5].The LEBT currently used in accelerator tunnel is initially designed for the penning ion source [6], as shown in Fig. 3.It has a length of 1650 mm from the ion source extracting aperture to the RFQ entrance.The H − beam is focused and regulated by three solenoids to match the phase space acceptance of RFQ.The diameter of the LEBT is 138mm, close to the inner diameter of the solenoids.For a current of 37.5mA, the measured transverse emittances are 0.355π•mm•mrad in horizontal and 0.306 π•mm•mrad in vertical after optimized.The factors for emittance growth and the uncertainties of the measurement are analyzed in [5].To suppress the emittance growth and promote the beam transmission rate, a test bench including an RF-driven H − ion source and a new LEBT (LEBT-II) is constructed and tested in lab.LEBT-II has two solenoids, as shown in Fig. 4.Each solenoid has an axial length of 127 mm and inner diameter of 64 mm.The distance from extracting aperture of the ion source to the inlet of the RFQ is 800 mm.Two turbo pumps of 2000 L/s are installed to the ion source section to pump most of the hydrogen diffused from the ion source.Another two pumps of 350 L/s are installed on the diagnostic chamber and chopper chamber respectively.With this vacuum design the transmission coefficient of the H − ion beam is about 92%.The emittance optimizations for in the extracting and LEBT section are still on going.
Superconducting cavities (SSC) will be used in CSNS-II to accelerate the H − beam to 300 MeV in the linac [7].The associated proton beam halo produced by residual gas stripping and intra-beam stripping could heat up and quench the SCC.To remove the associated proton beam produced in LEBT section, a bending magnet is installed behind the diagnostic chamber.It deflects the H − and proton beam by 1.8°into opposite direction Therefore, the whole LEBT is mechanically beveled by 1.8°relative to the RFQ axis.According to our beam tracking simulation, the associated proton beam produced before passing the bending magnet is deflected out of the phase space acceptance of the RFQ.Merit from the differential pumping of the vacuum system, the back ground pressure drops rapidly along the LEBT.Consequently the associated proton beam produced after the bending magnet contributes less than 8% of the total proton ions.A pair of electrostatic chopper is used to chop the beam, enabling the beam injection to the synchrotron accelerator [8].The electric field produced by the chopper destroys the accumulation of the H + 2 ion cloud nearby.Consequently space charge compensation [9] can not be built up around the chopper, resulting a rapid growth of the emittance of H − beam.So the length of chopper electrodes is reduced from 52 mm to 27mm in the new LEBT.The effectiveness of this modification will be test experimentally.

Summary and outlook
An RF-driven H − ion source with external antenna is developed in CSNS.It has a silicon nitride chamber.In the last two run cycles of the accelerator, it have operated for 310 and 323 days respectively, with availability of almost 100%.To further optimize the performance of the ion source and the new LEBT, a test bench is constructed, aiming to meet the requirement of CSNS-II.The new LEBT has a two-solenoid structure, enabling the elimination of the associated proton beam.An electrostatic chopper with shorter electrodes is also designed for the new test bench.

Figure 1 .
Figure 1.Cross-sectional view of the RF-driven negative hydrogen ion source developed at CSNS.

Figure 2 .
Figure 2. Status of the RF-driven H − ion source in last two run cycles.The blue curve shows the RF power of the ion source, and the red curve shows the beam power on the spallation target.The ion source delivers beam to the accelerator either for neutron production or for machine study.The ion source was powered off and kept in vacuum during the Chinese New Year holidays. l

Figure 3 .
Figure 3. LEBT used in the accelerator tunnel, which is initially designed for the penning ion source test.

Figure 4 .
Figure 4. Structure of the LEBT under test developed for CSNS-II.

Table 1 .
Typical operation parameters of the RF-driven H − ion source for experimental study and accelerator service.