Development of a novel Internal Radio-frequency Ion Source for Cyclotrons (IRISC)

IRISC (Internal Radio-frequency Ion Source for Cyclotrons) is an innovative design of an internal ion source, primarily for producing H− ions, in which 2.45 GHz radio-frequency power is supplied to its electrodes for achieving electron emission. Theoretical work indicates that lower electrode sputtering and higher extraction efficiency of H− may be achieved as compared to its standard cold-cathode direct current (DC) Penning Ionization Gauge (PIG) counterparts. If achieved, IRISC would arise as a ground-breaking ion source with significant importance for compact cyclotrons. IRISC dimensions have been matched to CIEMAT’s AMIT cold-cathode DC PIG ion source so that, in the future, side-by-side testing can be carried out and, eventually, both types can be used in AMIT cyclotrons. This contribution presents IRISC key design aspects, outcome from simulations, its dedicated test bench and first low-power test results from a prototype.


Introduction
As of today, hospitals ranging from small to mid-size having on-site accelerator facilities are rare, mainly due to their size and cost.As a consequence, interest in compact cyclotrons for producing medical isotopes has risen in recent years.In this context, internal H -ion sources play an important role in supplying small accelerators with enough beam current for their applications.Moreover, easy-to-use and low-maintenance systems are key in order to, on the one hand, maximize the availability of the system, and, on the other hand, minimize the number of repair and replacement operations, the number of trained personnel needed and the radiation dose received by them while performing these tasks.
A widely used type of internal ion source, which has been known for decades, is the DC (Direct Current) operated cold-cathode Penning Ionization Gauge (PIG) ion source.However, this kind of sources has the following drawbacks: -Because plasma is sustained through ion bombardment, cathodes suffer from sputtering, which leads to performance degradation over time and the need for periodic replacement of cathodes.-The high electric fields required to operate such ion sources tend to produce high-energy electrons, which may destroy H-ions through several processes, such as collisional electron detachment, mutual neutralization, and associative and non-associative detachment [1].
In an attempt to overcome these drawbacks, CIEMAT, in collaboration with GE and Cyclomed, has developed a Radio Frequency (RF) driven internal ion source, operated at 2.45 GHz.By using RF power,   we aim to generate a capacitive discharge, operating in collisionless heating regime, in which electrons gain energy inside the plasma-wall region, thus reducing the plasma potential.Presumably, this lower potential seen by ions should prevent electrode sputtering [2].Furthermore, we intend to operate IRISC within the 1 -10 eV range, considered as the lower-end of the electron temperature range of conventional PIG sources, which vary from few eV to hundreds of eV [3] [4] [5].Hence, we might be able to maintain a high-temperature region that supports the excitation of H2 at higher vibrational levels [6] [7] (figure 1), and a low-temperature region where H-ions are produced by dissociative attachment of vibrationally excited H2 (figure 2).Although this remains to be experimentally validated, it could enhance H-production efficiency, which in turn may result in a reduced need of H2 gas supply, thus resulting in a better vacuum and efficiency at the corresponding cyclotron.In addition, as in the AMIT PIG ion source [8], the plasma expansion gap between the arc column and the extraction slit is maintained to increase the low-temperature region that allows higher H-production [8].
In this paper, we present the design, simulations, test bench and first low-power measurements and results from an IRISC (Internal Radio-frequency Ion Source for Cyclotrons) prototype.

Design and simulations
IRISC ion sources consists of a vacuum flange, two RF cables, gas and water pipes, and a hollow cylinder (chimney) placed between two water-cooled copper bodies.Inside each of these bodies there is a gas inlet, an RF cavity and a replaceable electrode.

Dimensions
IRISC dimensions were required to match the ones from CIEMAT's AMIT DC cold-cathode PIG so that, in the future, side-by-side testing can be carried out and, eventually, both types can be used in superconducting AMIT cyclotrons for radioisotope production: -Chimney: benchmarking against an AMIT ion source (figure 3.a) should give us a good estimate of how IRISC performs.For this reason, both the dimensions of the extraction slit and the position of the plasma column inside the chimney must be the same in both designs, since these parameters are related to the extracted beam current.At a later stage, they will be optimized for IRISC operation conditions.The slit is 6 mm high, 0.2 mm wide, on a 0.1 mm thick wall [8].-Copper bodies: they have the same outer dimensions as the ones in AMIT PIG ion sources, but instead of a replaceable cathode connected to a DC circuit, IRISC bodies have a λ/4 RF cavity, a window for RF coupling and a replaceable electrode (figures 3.b, 3.c and 3.d).
-Vacuum flange: all feedthroughs needed for operating an IRISC unit were selected and arranged to fit in a 75 mm x 72 mm vacuum flange (figure 3.b).-Clearance between cables and pipes: a 32 mm x 340 mm clearance is left so that particles trajectories in AMIT cyclotrons are unaffected throughout their acceleration process.This clearance is smaller between copper bodies, corresponding to the first few turns (figure 3.b).

RF design
The two most commonly used topologies for injecting RF power into plasma chambers are: waveguides ending in an open circuit and resonant cavities.We have decided to use a resonant cavity topology since its RF reflected power is significantly lower than the one at open circuit ended waveguides, thus reducing undesired RF emissions.
Given the size of IRISC and the relation between cavity length and resonant frequency (f0 = c / λ), the selected operating frequency is 2.45 GHz (2.4 -2.5 GHz band) resulting in cavities 3.06 cm long, which largely fit in the previously mentioned copper bodies.
Finally, we evaluated which of the three main RF coupling methods [9] was best for our design: -Antenna (electric) coupling: it was ruled out due to the very small gap between the antenna and central conductor needed for coupling, giving rise to very strong electric fields.-Loop (magnetic) coupling: given the size of IRISC sources, this coupling method was ruled out due to its fabrication and tuning complexity.-Slot (magnetic) coupling: this was the selected coupling method since no tuning is required and fabrication is not as complex as in loop coupling.Two sizes of coupling windows were designed, the larger for tantalum electrodes and the smaller for copper ones.(figure 3.c).

Replaceable electrodes
As stated previously, one of the main goals of IRISC is to have from very low to no electrode sputtering, which would lead to somewhat maintenance-free internal H -sources.For this reason, several sets of electrodes have been manufactured, whose degradation and wear in both qualitative (deformation, burn marks) and quantitative (weigh and thickness) terms would be periodically assessed.Furthermore, in α1 α2 α1<α2 order to test how different materials perform, half of the total number of electrodes manufactured are made of copper whereas the other half are made of tantalum (figure 3.d).

Chimney inserts
The position of the plasma column generated inside the plasma chamber of IRISC units can be modified by using inserts of different dimensions.The trajectories of electrons entering the plasma chamber from the replaceable electrodes depend on the geometry of the inserts mounted.This will allow us to generate plasma at different distances from the chimney walls (expansion region) and extraction slit.

Simulations and design validation
In order to make sure all different parameters were within expected ranges, thermal, plasma and RF simulations were performed throughout the design phase.On the one hand, the at all parts of IRISC units during operation was calculated by means of Finite Element Method thermal simulations.
Even though these simulations show a difference of up to 280 C between tantalum (325 C) and copper (45 C) electrodes at their tips, when operated at 1 kV, these are well within safe limits.On the other hand, validation and refinement of IRISC's dimensions was achieved through RF simulations.The resonant frequency of the IRISC final design falls within the pre-selected range, whereas power consumption at 1 kV remains well below the 250 W maximum value.PCu = 68 W; PTa = 148 W. Finally, results from Molflow+ simulations show that the chimney inner walls are at 0.25 Pa when plasma is present.

Test bench and first results
A dedicated Ion Source Test bench (IST) for IRISC prototypes has been designed, manufactured and assembled, which consists of the following subsystems: -Vacuum system: one primary pump, one turbo pump, one low-vacuum gauge, one high-vacuum gauge, controllers and a custom-made vacuum chamber, which is a copper-coated SS316 chamber, acting as a 60 MHz resonant cavity for extracting ions from the plasma chamber.-Extraction system: in-house low-level circuit, 250 W RF amplifier, copper parts, puller electrodes (four different geometries), two beam probes (for measuring the extraction current of both H + and H -ion species) and a picoammeter.-Gas injection system: a pressure regulator, manual precision valves, electric valves, a flow controller and a leak detection device.-Electromagnet: a custom-made water-cooled 0.84 T @ 300 A magnet and a 300 A, 50 V current source.Magnetic field lines run parallel to the chimney for radial confinement of ions.-Cooling system: a recirculating chiller, a water manifold, a water resistivity sensor and flowmeters.-RF system for electrode heating: two in-house RF circuits, which can supply up to 250 W each in the 2.3 -2.5 GHz range.-Optical Emission Spectroscopy (OES) system: spectrometer and fiber optic elements.
-Monitoring, safety and control: PLC, PC application developed in-house and safety devices.
The described IST enables us to control the operating parameters (gas flow, magnetic field, RF power, relative RF phase, extraction field and cooling temperature), the operating strategies (injection of gas through either or both copper bodies, RF power applied to either or both electrodes) and to test different electrode materials, puller geometries, chimney inserts, etc. while monitoring the plasma properties through its emission lines or the extracted current of both H + and H -ion species.Furthermore, electrode wear vs ion source operation (measured in Amp * hour) will be assessed.
As of today, the following tests and measurements have been carried out: -Reflection coefficient (S11), coupling factor (β), quality factor (Q0) and resonant frequency (fres) measurements of the first IRISC prototype outside the IST.Three different configurations were tested: copper electrodes and small coupling windows; copper electrodes and large coupling windows, and tantalum electrodes and large coupling windows (figure 4.a).-S11 measurements of the IRISC prototype water-cooled, in vacuum and with two copper replaceable electrodes and small coupling windows mounted (figure 4.b).-Plasma ignition achieved by ramping up the RF injected through both electrodes while injecting 1 sccm of H2 through both gas inlets (0.5 sccm through each of them).Plasma ignition took place when 70 W were injected through each electrode.

Conclusions and future work
A first IRISC ion source unit was successfully designed, manufactured and assembled.Its S11 parameters are in good agreement with simulations performed during the study phase.Moreover, the Ion Source Test bench (IST) was fully commissioned and enabled us to generate and sustain plasma in the manufactured IRISC prototype.
In the following months, we will work on characterizing the plasma through Optical Emission Spectroscopy (OES) and analyze its response to different parameters.Once the plasma characterization campaign is completed, we will try to maximize the extracted ion current by varying the operation parameters.Moreover, replaceable electrode wearing will be assessed periodically.At a later stage, beam emittance will be analyzed too.
In the longer term, the aforementioned different geometries, materials, operation strategies and, eventually, gas mixtures will be tested and analyzed.

Figure 3 .
Figure 3. 3D model of the AMIT PIG ion source (a), 3D model of IRISC (b), 3D model of IRISC large and small RF couplers (c), Picture of Cu and Ta electrodes mounted onto their corresponding RF couplers (d).

Figure 4 .
Figure 4. S11 measurements of three different configurations at ambient pressure and temperature (a), S11 measurements of a Cu electrode and small RF coupler in operating conditions (water-cooled and at vacuum pressure) (b).