A new proton injector based on PKU-type 2.45GHz PMECR ion source for BNCT facility

A first fully domestically Accelerator-based boron neutron capture therapy demonstration device(AB-BNCT) located in Yangtze River Delta region is being build recently. The project is held by “Xi’an Jiaotong University-Huzhou Neutron Science Laboratory” joint lab established by Xi’an Jiaotong University and Huzhou Industrial Group. The AB-BNCT facility requires a proton beam of 30 mA@40 keV at RFQ entrance, with its beam duty cycle between 0.5%-100%, and its normalized root mean square emittance less than 0.2 π.mm.mrad. Ion source group of Peking University (PKU) is in charge of the proton ion source and its low energy beam transportation section (LEBT). A PKU type compact permanent magnet 2.45 GHz ECR ion source (PKU-Type PMECR) and a two solenoid LEBT is under development for this purpose. So far, the PMECR ion source conditioning is complete and all parameters are better than required for this facility. The details will be presented in the present paper.


Introduction
Cancer treatment can be catalogued in three ways: surgery, chemotherapy and radiation therapy.Radiation therapy includes ion therapy (proton therapy, heavier ions therapy) and boron neutron capture therapy (BNCT).Cancer cells within the tumor region are destroyed by depositing ions' energy within the Bragg peak with ion therapy.BNCT uses the suitable propensity of the non-radioactive nuclide boron-10 to capture thermal neutrons, which results in the prompt nuclear reaction 10B(n,α)7Li that creates a high degree of localized damage to the tumour cell.The products of this reaction have high linear energy transfer characteristics (α particle, ≈150 keV µm −1 , 7 Li ion, ≈175 keV µm −1 ).The path lengths of these particles are in the range of 4 -10 µm, and their energy deposition is limited to the diameter of a single cell.Therefore it is possible to selectively irradiate cancer cells that have already taken up a sufficient amount of 10B while sparing normal healthy cells.And it is currently seen to be a viable therapy option for various cancers [1].The Accelerator-based Boron Neutron Capture Therapy (AB-BNCT) is being established worldwide as a future modality to start an era of in-hospital facilities [2].Compared with reactor neutron sources, AB-BNCT has the advantage of better beamline performance, higher neutron quality, low construction cost, high safety and easy maintenance.At Yangtze River Delta region inside China, an AB-BNCT that has a combined acceleration structure of RFQ and Cross-bar H-mode DTL (CH-DTL) will be built by "Xi'an Jiaotong University-Huzhou Neutron Science Laboratory" joint lab.In this design, a 40 keV CW proton will be accelerated to 1.8 MeV by one RFQ cavity and up to 2.2 MeV to 3.0 MeV by a CH-DTL cavity.The AB-BNCT facility requires a proton beam of 30 mA@40 keV at RFQ entrance, with its beam duty cycle between 0.5%-100%, and its normalized root mean square emittance less than 0.2 π.mm.mrad.The parameters of the beam to be delivered to the RFQ are listed in Table I.In this framework, PKU is responsible for the design and implementation of both the 30 mA@40 keV cw/pulsed proton source and the associated low-energy beam transport through the LEBT line.This paper mainly focuses on the proton ion source and LEBT design that is charged by PKU ion source group.In Sec.II, a brief review of the PKU standard PMECR ion source and LEBTs, together with the specific design of the proton injector, will be presented.In Sec.III, the commissioning results of this ECR ion source will be given.A summary and work of the next step will come up at the end of this paper.

Proton injector setup
To meet the requirement of this AB-BNCT facility, a PKU standard 2.45GHz ECR ion source (PKU-type PMECR) and a compact two-solenoid LEBT will be designed for this project.The design are mainly consulting the experience gained during the development of D⁺ ion beam injector for PKUNIFTY and He⁺ ion beam injector for C-RFQ accelerator at PKU years ago.

PKU standard PMECR Ion Source and LEBT
Study on high current permanent magnet 2.45 GHz microwave driven ion source at PKU can trace back to 1980's [3].Several types of particle ion sources based on 2.45 GHz microwave have been developed here.There are standard ion sources [4,5], miniaturized ion source [6], H2 + /H3 + Cluster ion source [7], H -ion source [8], low charged ion source [9] and electron source based on 2.45GHz ECR and surface plasma source (SPS e-gun) [10].Among the above particle sources, most of them are all permanent magnet type with ceramic microwave matching method except for the SPS e-gun.The size of the source body of PKU standard 2.45GHz ECR ion source is around Φ100 mm × 100 mm with plasma chamber size of Φ40 mm × 50mm.Up to now, this source has produced 130 mA H⁺ beam, 83 mA D⁺ beam, 65 mA He⁺, 70 mA O⁺, and 84 mA N⁺.Launched at the beginning of 2015, a long-term CW 50mA@50keV proton beam operation was done in June 2016, a 300 hour running record with no beam-off, no spark, no beam drop, no interrupting action was obtained with this standard PKU PMECR ion source [5].Several copies of this sources have been utilized to deliver beams for PKUNIFTY [11], C-RFQ [12], SRFRQ [13], DWA [14], Proton therapy facilities [15].To date, those operating PMECR ion sources require no maintenance.Ion source group of PKU have developed magnetostatic type LEBTs for PKUNIFTY [11] and C-RFQ [12], electrostatic type ones for SFRFQ [13] and DWA accelerator [14], and proton therapy facility [15].

The structure of AB-BNCT proton injector
This AB-BNCT proton injector consists of a PKU standard 2.45GHz ECR ion source (PKU-type PMECR) and a compact two-solenoid LEBT.In this design we will keep some structures used for D + and He + injectors, including the embed structures by inserting the ion source body into the extraction system and the whole source into the first diagnostic box, carrying out space charge compensation between two solenoids, collimating heavy ions before RFQ entrance, and isolating vacuum between ion source, LEBT and RFQ, etc.The proton ion source is a PKU standard type PMECR one.A 45kV well water-cooled three electrode extraction system was build for this source.More details of this source can be found in ref. [5].It will be operated in both CW mode and pulse mode under RFQ requirement by varying microwave power operation parameters.To obtain a narrow pulsed beam whose pulse length shorter than 0.1ms, a chopper after the second solenoid is used.
LEBT comprises of two solenoids and three drift sections.At low energy, few tens of milliamperes of high-intensity beam transport has to be carefully studied in order to fulfill the beam characteristics required by RFQ.The beam dynamics is mainly dominated by nonlinear space charge effects.When the beam interacts with residual gas, electrons and secondary ions are produced.In transit gaps with no magnetic or electric field influences the particle trajectories, electrons are trapped in the beam and positive ions are repelled toward the walls.As a result, a reduction of the space charge effects is achieved.Theoretical analysis indicates that the space charge compensation is greatly reduced in the LEBT solenoids where both secondary ions and electrons are confined in the beam.And in the fringe field of solenoids, electrons are attracted toward the solenoid center, whereas the secondary ions tend to be repelled towards the pipe.
To minimize such emittance growth, the length of the LEBT especilly the length of solenoids should be reduced as much as possible.Taking the beam focusing, space charge compensation (SCC) and the minimization of beam emittance growth into account, the total length of solenoids is limited to 200 mm.In drift 1, an extraction system, a movable Faraday cup and a vacuum valve will be installed.In drift 2, two steering coils and one SCC gas inlet port will be mounted between two solenoids.Ar gas will be used for space charge compensation.After the second solenoids, there are a heavy-ion collimation cone, a beam chopper, a chopped beam absorption, an ACCT and an electron trap (E-trap).To maintain the vacuum for this injector, two sets of 1200 l/s molecular pumps will be mounted in drift 1 and drift 3 section.A 12 kV bias voltage is applied on the chopper to deflect the beam when a short pulse beam is needed.The total length of LEBT is 1160 mm.The first drift section that starts from the source emission aperture to the first solenoid is 280 mm.Following are solenoid I: 200 mm, drift II: 200 mm, solenoid II: 200 mm and drift III: 210 mm.In drift 3, a 30 mm space will be occupied by RFQ entrance flange.
Fig. 1 is a screenshot of this proton injector.At the left side there is a microwave system with a homemade HV break wave guide to limit the high voltage region.A 4-phase aperture, a flange for test and an emittance measurement unit (EMU) displayed in Fig. 1 are accessories for the injector test.

Commissioning arrangement
The injector commissioning will be carried out in two steps, ion source qualification and LEBT testing.Ion source qualification was done on a PKU ion source test bench.Parameters of the PMECR ion source such as beam intensity, beam distribution, its emittance and H + fraction will be given on this test bench.RFQ acceptance tests of this injector will be performed with the help of a four-quadrant diaphragm and an EMU.The four-quadrant diaphragm with a 4 mm aperture that equals to the diameter of RFQ electrodes, will be installed at the end of LEBT to simulate the entrance of RFQ facility.Each segment of it is independent and isolated.Cooperated with two steering magnets, it is also used to align the beam.The EMU is a slit-grid emittance measurement unit.It has the ability to measure the beam current, beam profile, its emittance, and its twiss parameter.

The proton ECR Ion Source Performance
The PMECR ion source was qualified in early February this year.To meet the requirements, the PMECR ion source was tested in both CW mode and pulsed mode.For the pulsed mode, the repetition frequency of microwave power covers from 1 Hz to 500 Hz and the duty factor varies from 0.5% to 99%.The results show that there is not too much difference when repetition frequency changes.Since most of the time the AB-BNCT facility will be operating at around 200 Hz, the results presented here are mainly at this frequency.The data listed in Table 2 shows some results obtained at different duty factors when repetition frequency is set at 200 Hz.As shown in table 2, a 50 mA proton beam at 40 keV can be easily produced with this PMECR ion source, H + factors are larger than 80%, their normalized RMS emittances are between 0.09 ~ 0.11mmmrad.When the duty factor is changed from 10% to CW, the gas consumption increases from 0.6 sccm to 3.0 sccm.Fig. 2 is an emmitance diagram of 200 Hz 70% mode 56.6 mA proton beam.There were no sparks appeared during the PMECR ion source test.

Summary and Future Plan
The proton PMECR performance commissioning was finished.The commissioning results show that this PMECR ion source has fulfilled the requirements of the AB-BNCT in all aspects that including beam intensity, ion species fraction, emittance and stability.The installation of the entire proton injector, including the LEBT test facilities plotted in figure 1, is complete.After 1.5 hour pumping in the first time, the background vacuum at three ports from ion source to LEBT end are 1.3×10 -5 Pa, 6.3×10 -5 Pa and 4.7×10 -5 Pa, respectively.Beam transport tests that include Twiss parameter at the RFQ entrance, mismatching degree and raise/full edge will start when all power supplies are ready for operation.

Figure 1 .
Figure 1.A screenshot of this proton injector, front view.

Table 1 .
Beam parameters at the entrance of RFQ.

Table 2
Some results obtained with this PMECR ion source at 40 keV.