Design and commissioning of a versatile surface ion source

A compact and versatile ion source was designed and commissioned to be used with the new low energy ion beam facilities at TRIUMF. The ion source is based on the principle of surface ionization, in which ions are generated from a neutral atom by impact with a solid surface. The design of the ion source considered several factors, including the type of ions, intensity, energy and emittance of the extracted beams from the source. Commercially available alkali materials with an ionizer are used in the source; the ionizer is integrated with the in house design of extraction and transport system. The source performance was characterized using diagnostics techniques, and its key parameters, such as beam current, emittance, and beam species, were measured. The results showed that the compact surface ion source is able to achieve high ionization efficiency and stable operation, and is suitable for use in commissioning and operation of various ion beam facilities in particle accelerator laboratories.


Introduction
A new rare isotope beam (RIB) facility, ARIEL is being commissioned at TRIUMF [1].The facility includes a 200 m long electrostatic beam transport line with a high resolution separator (HRS), and an electron beam ion source (EBIS) based charge-breeding facility with a RFQ cooler/buncher and Nier-spectrometer [2,3].In order to commission all these low energy beam transport systems, readily available stable ion beams from an off-line ion source are required.The basic beam requirements for the off-line ion source are listed in table 1. Surface ionization is a widely used technique for producing alkali beams [4].For this purpose a surface ionization based alkaline ion source has been chosen.In a surface ionization process low work function elements such as alkali elements are ionized by contact with a surface of high work function material such as Tantalum, Tungsten, Iridium, Rhenium, etc [5].From the Saha-Langmuir equation, it is well known that a high ionization efficiencies can be achieved by using an ionizer materials with a high work function at high temperature to ionize elements with a low ionization energy [6].This technique has a wide range of applications, including mass spectrometry, ion implantation, and plasma processing.The surface ion source is ideally suited to our requirements (see table 1) because of a high degree of beam purity.In this case an additional mass separator is not essential for commissioning the low energy beam facilities.Also the extracted ion beam from this source has relatively low emittance and low energy spread (fraction of an electron volt), which meet the strict beam requirements imposed by the HRS, requiring an energy spread of less than 0.5 eV with an emittance of 3.0 µm in the case of high resolving power about 20000 [7].However, a surface ion sources using a hot cavity for ionization (e.g.surface ion sources for rare isotope production [8]) tend to be large and complex, making them difficult to use in compact or portable systems.
An off-line ion source, known as ARIEL test ion source (ATIS), has been designed [9].It is simple to construct and operate.The ATIS is capable of producing stable beams of alkali ions, ranging from heavier to lighter ones, with energies up to 60 keV.In this work, the primary design considerations and beam dynamics calculations are presented with initial beam commissioning results.

Figure 1.
A cross-sectional layout view of the ARIEL test ion source (ATIS) includes the associated beam transport section with electrostatic quadrupoles (Q1, Q2, and Q3), a beam diagnostics profile monitor (PM3), and Faraday cups (FC3).

Design considerations
In the past few decades surface ion sources have been well developed [10].A commercially available (HeatWave Labs, Inc. [11]) compact ion source (ionizer/emitter assembly) for given alkaline elements was considered.A cross-sectional layout view of the ATIS and its transport section is presented in Fig. 1.The source employed in the ATIS (as shown in Fig. 1) has a diameter of 6.0 mm.It consists of a rhenium filament in an alumina insulator assembly and a porous tungsten plug into which the alkali elements has been fused.The filament is heated by passing a current through it which in turn heats the alumina and hence the tungsten ionizer plug.This causes alkaline ions to be released from the hot surface.The required beam intensity is mainly controlled by controlling the temperature of the source, i.e. controlling the ionization efficiency according to the Saha-Langmuir equation.About 90% ionization efficiency and more can be achieved for alkali elements other than Na and Li with a tungsten ionizer at 1000 • C, whereas ionization efficiency for Na and Li will be around 0.4% and 0.04%, respectively.In this operating regime the expected beam current through a 3 mm aperture size could be up to 3 mA [11], which is well within the required beam current specified in table 1.
Ions are initialized assuming a Maxwell-Boltzmann energy distribution at the ionizer temperature, which is around 1000 • C. The beam emittance due to the thermal temperature can be calculated for a given initial temperature and the radius of ion emission aperture by [12] 4ε with R is the aperture radius of the source electrode, k is the Boltzmann constant, T is the thermal temperature of the ion, E is the kinetic energy of the ion.
Figure 2 shows the calculated beam emittance due to the thermal temperature of extracted ion beams at various acceleration voltages with an extraction aperture radius of 3 mm at 1500 • C. It is well within the required beam emittances given in table 1. x 2RMS (cm) y 2RMS (cm) Beam energy (100 keV) Figure 3.
The calculated beam envelope (positive scaling for x and negative scaling for y) of a 30 keV 85 Rb 1+ ion beam through the ATIS and its beam transport section.

Beam dynamics simulations
The in-house built ion extraction system comprises an "accel-accel" type three-electrode extraction system.The first electrode at the source emitter has a 3.0 mm diameter to achieve a beam emittance of less than 12 µm (refer to table 1 and Fig. 2).The opening aperture in the extraction cone (the 2nd electrode) has a 6.0 mm diameter and is positioned 15.0 mm downstream from the source aperture.The third electrode, which serves as the ground electrode, features an aperture diameter of 30.0 mm.The separation distance between the extraction and ground electrodes is 70.0 mm.The beam transport section of the ATIS beamline is composed of three electrostatic quadrupoles (Q1, Q2, and Q3) designed to match the beam for downstream low-energy beam transport.Furthermore, the beamline is equipped with a Faraday cup (FC3) and a beam profile monitor (PM3) for measuring beam intensity and transverse profiles.The beam profile monitor utilizes three metallic wires capable of measuring the beam profile in the horizontal plane (x), the vertical plane (y), and correlated positions (xy) between the x and y planes.
The extraction and transport system of the ion source was designed and optimized using the finite element analysis code OPERA [13] and the particle tracking code GPT [14].Extraction and transport simulations for the ATIS were performed with an acceleration voltage up to 60 kV and an initial ion emission energy of 0.12 eV.Space charge effects were not considered in the simulations in order to model typical beam currents of 1 nA from the source.
The calculated beam envelope of the 30 keV 85 Rb 1+ beams through the ATIS extraction and transport system up to the location of the profile monitor (PM3) is illustrated in Fig. 3. Figures 4 and 5 display the calculated spatial and phase space distributions at the location of PM3.These beam phase space parameters are in accordance with the specifications outlined in Table 1.

Beam commissioning
The source (ionizer/emitter) in the ATIS ion source requires replacement to accommodate the specific alkaline ion beams needed.For the initial beam commissioning, the ATIS was equipped with a Cs source to generate 133 Cs 1+ beams.The ionizer/emitter, extraction system and the high-voltage (HV) bias systems for the source were conditioned under a vacuum pressure of approximately 10 −8 torr by applying the source bias voltage up to 60 kV.Initially, a 30 keV Cs 1+ beam with a beam intensity of approximately 2.7 nA was extracted, with the heater current set at 1.6 A and the extraction voltage at 50 V, i.e., the potential difference between the emitter and the 2nd electrode in the extraction sytem.To achieve a higher beam intensity of several tens of nanoamperes (nA) or higher, the ion source can be operated with a higher heater current of up to 4.0 A and an extraction voltage of up to 2.0 kV.Subsequently, beam extraction and transport of Na 1+ , K 1+ , and Rb 1+ with beam energies up to 60 keV are conducted.Currently, an Rb source has been installed to produce beams of 85 Rb 1+ and 87 Rb 1+ .The measured mass-to-charge (m/q) spectrum shows about 27.0% for 87 Rb 1+ , 70.0% for 85 Rb 1+ , 2.9% for 39 K 1+ and less than 0.1% for other alkaline species.In this work, results for 30 keV Rb 1+ are presented to serve as a benchmark for beam dynamics simulations.
Figure 6 shows the measured beam current at various ionizer heater currents with a set extraction voltage of 300 V; these are perfectly adequate for the required beam current, as indicated in the table 1.

Summary and outlook
The ARIEL test ion source (ATIS) and its associated beamlines have been successfully commissioned and meet the beam requirements.Stable beams from the ATIS are currently being used for commissioning other ion beam facilities, such as high resolution separator (HRS), charge breeder (EBIS), RFQ cooler/buncher, and ARIEL rare isotope beam transport systems at TRIUMF.The measured current varying with extraction voltage presents a further opportunity for an in-depth exploration of the potential effects on the beam current in the near future.

Figure 4 .
Figure 4.The calculated spatial distribution of a 30 keV 85 Rb 1+ ion beam at the location of the profile monitor (PM3).

Figure 5 .
Figure 5.The calculated phase space distribution in the horizontal plane (a) and in the vertical plane (b) of a 30 keV 85 Rb 1+ ion beam at the location of the profile monitor (PM3).

Figure 6 .Figure 7 .Figure 8 .
Figure 6.The measured beam current of 30 keV Rb 1+ beams from the ATIS at the location of Faraday cup (FC3) at various ionizer heater currents with a given extraction voltage of 300 V.

Figure 9 .
Figure 9. Reconstructed spatial density profile at the location of PM3 using 1D measured profiles in Fig. 8 and the MENT algorithm with tomography reconstruction methods.

Figure 10 .Figure 11 .
Figure 10.The measured phase space in the horizontal (x) plane is obtained for a 30 keV Rb 1+ beam transported from the ATIS at the entrance of the ARIEL high resolution mass separator, 20 meters downstream from the ATIS ion source.

Table 1 .
Required beam from the ARIEL test ion source.