AISHa: an ECRIS for nuclear-physics, new clinical protocols and material experiments

The Advanced Ion Source for Hadrontherapy (AISHa) is an ECR ion source operating at 18 GHz, developed with the aim to produce multiply charged ion beams with low ripple, high stability and reproducibility. Due to its unique peculiarity, it is the most suitable choice for medical applications, but also for nuclear-physics and material experiments. Two sources have been realized: the first at INFN-LNS and the second at the Centro Nazionale di Adroterapia Oncologica (CNAO). The first one, fully commissioned at INFN-LNS, will be used as test-bench for development of new beams and several R&D activities are planned within the IONS experiment to increase plasma confinement and to refine techniques of non-invasive plasma diagnostics to correlate plasma and beam parameters. The second one recently produced the first beam and it will allow to increase the CNAO potential in the field of experimental and industrial research, with the long-term goal of introducing new ionic species into clinical practice such as helium, oxygen and later also iron and lithium, useful for bio-spatial research. In this work, the key peculiarities and the experimental results of the two Aisha ion sources will be presented.


Introduction
Electron cyclotron resonance ion sources (ECRIS) are widely used to generate high intensity -highly charged ion beams to feed particle accelerators [1].They are also the workhorse for hadrontherapy centers, offering reliability, commercial availability and low power consumption [2].The Advanced Ion Source for Hadrontherapy (AISHa) is a compact ECRIS, conceived in 2014 by the INFN-LNS (Istituto Nazionale di Fisica Nucleare -Laboratori Nazionali del Sud) ion source team, developed to allow the generation of high brightness multiply charged ion beams for hadrontherapy, industrial and research applications.The AISHa magnetic field is produced by a set of independently energized superconducting coils and by a Halbach-type hexapole magnet.The superconducting coils produce the axial confining field with a maximum field of ∼2.65 T while the normal-conducting hexapole generates a radial field with a maximum value of ∼1.3 T close to the lateral chamber walls.The hybrid set-up allows achieving high values of magnetic field in a cost-effective way.Electron cyclotron resonance between free electrons moving in the magnetic field and microwaves in the 17.3-18.4GHz band allows the electromagnetic energy to be transferred to plasma [1].Magnetic confinement ensures plasma lifetime to be larger enough (up to several hundreds µs) to guarantee multiple ionization.The AISHa test-bench was installed at the INFN-LNS in late 2016 while commissioning with gaseous ion beams has been carried out in the following years.Results of the AISHa commissioning at INFN-LNS together with further information on technological aspects can be found in Refs.[3,4].Due to its peculiarity, a copy of AISHa ion source, hereinafter named AISHa-CNAO, has been chosen as the third ion source of the Centro Nazionale di Adroterapia Oncologica (CNAO) facility.It is currently waiting to start the commissioning in the framework of the INSpIRIT (INnovative accelerator facility with Sources Ions for Research and radiation hardness studies with IndusTrial and clinical applications) project.AISHa-CNAO will operate with the aim of introducing new ion species such as helium, lithium and oxygen into clinical practice [5].In perspective, also F e 19+ ion beams will be used for bio-spatial research.Helium and lithium ion beams are of growing interest for hadrontherapy due to their lower lateral dose distribution with respect to protons.Oxygen is also considered because of its energy deposition characteristics [6].AISHa-CNAO differs only slightly from its INFN-LNS prototype.It has an improved hexapole containment chamber, a new titanium bias disk with improved injection cooling system, a new electrode handling system in the extraction column and an improved cooling system in the standard oven [5].

Experimental results
The AISHa commissioning at the INFN-LNS test-bench has been carried out with gaseous ion beams of interest for clinical and research applications: hydrogen, helium, oxygen, carbon and argon beams.The results include the production of more than 5000 eµA He 2+ , up to 1400 eµA O 6+ , 200 eµA O 7+ , 520 eµA C 4+ and 50 eµA Ar 14+ .Fig. 2 shows some of the most significant charge state distributions obtained during the beam commissioning phase [3,4].Commissioning of metal ion beams (including iron and lithium) is also foreseen in late summer 2023, after the installation of a dedicated oven together with the boosting of the microwave injection system by means of a dedicated 21 GHz -1.5 kW microwave generator.
Until today, CNAO has used two identical commercial ECRIS to provide proton and carbon beams for patient treatment [7].The installation of AISHa-CNAO was started on July 2022 with the assembly of the ion source and ancillary equipment at the INFN-Pavia section.On September 2022, the deployment into the synchrotron room has been performed during dedicated time slots allocated for synchrotron ordinary maintenance.A first ignition, useful for the purposes of the project milestone, was carried out in November 2022.Fig. 3 shows the charge state distribution of the first helium beam extracted by the AISHa-CNAO ion source [5].

Figure 1 (
a) shows a picture of the AISHa ion source installed at the LNS test-bench while Fig. 1 (b) shows a picture of AISHa-CNAO after installation at the CNAO accelerator facility.

Figure 1 .
Figure 1.On the left, a picture of the AISHa ion source installed at the LNS test-bench.On the right, a picture of the AISHa-CNAO after installation at the CNAO accelerator facility.

Figure 3 .
Figure 3. First charge state distribution of the Helium beam extracted from the AISHa Cnao.

Table 1
lists the requirements of the INSpIRIT project in terms of beam intensities.Although the results of the beam commissioning on AISHa-CNAO are really preliminary, they already satisfy the INSpIRIT requirements regarding helium beam intensity.

Table 1 .
Requirements of the INSpIRIT project.