FEBIAD ionization development via a web-app for multidimensional characterization

The ISAC-FEBIAD is an electron impact ion source typically used to ionize radioactive molecules or isotopes of elements beyond the reach of either surface or laser ion sources. The FEBIAD’s key tuning parameters are the cathode temperature defining the number of electrons created; the anode voltage establishing the electron energy; and the magnetic field controlling the electron density inside the anode volume. However, these parameters are typically scanned in a small and limited range when optimizing the source. Recent investigations have shown the need to explore the entire range of operational values accessible by the power supplies, not only due to the intrinsic variations from source to source but also to operate the source at optimal settings. To address this, a scanning algorithm has been implemented as a web interface thanks to the High-Level-Application (HLA) infrastructure available at TRIUMF. The ion beam intensity during both offline and online commissioning of the web app are presented here as contour plots. The optimal settings found for stable 20Ne are confirmed as the optimal settings for radioactive 18Ne. The main takeaway, however, is that the optimal ion source parameters differ between singly-charged, doubly-charged, and molecular species. This development demonstrate and facilitate the need for element and charge state-specific parameter optimization. Additionally, the results highlight the possibility of parameter optimization to enhance the ratio of the species of interest to co-ionized contamination.


Introduction
The Forced Electron Beam Induced Arc Discharge (FEBIAD) ion source [1,2] is an electron impact ion source typically used to ionize radioactive molecules or isotopes of elements beyond the reach of either surface or laser ion sources.The FEBIAD consists of a cathode that is resistively heated up to about 2000 • C from which thermionic electrons are emitted (see Fig. 1).A voltage difference between the cathode and grid accelerates the electrons into the anode volume where the presence of a magnetic field-tunable via an electromagnetic coil-confines the electron beam.The neutrals to be ionized are produced via nuclear reactions between a driver beam and a target material.The reaction products diffuse out of the target material and effuse via a transferline to the source for ionization and extraction towards a dipole magnet for mass separation, in what is called the Isotope Online Separation (ISOL) method [3,4].Due to the limited number of neutrals produced, highly ionization-efficient ion sources are paramount.
In a typical FEBIAD operation, a fixed DC current heats the cathode to emit electrons, and the anode and electromagnetic coil are set at specific values (Table 1).Moreover, with a known leak rate injection, the ionization efficiency of the injected species can be measured.The total ion source current and the mass-separated ion of interest current are recorded as part of a standard FEBIAD characterization.Typically, a standard data record set consists of vacuum chamber pressure, electron current, heating power(s), and species measured.During a development campaign, this data set was either recorded on a spreadsheet or as a time series generated by the control system employed (EPICS [5]).Multi-parameter optimization was previously hindered by the time used to manually sweep parameters, which only allowed measuring between 25 to 50 data points per day.This paradigm prevented proper characterization of the complex parameter space of the FEBIAD due to the time constraints of the experimental apparatus.To overcome this, a web-app has been developed to operate FEBIADs both offline at the ISAC test stand and online at either one of the ISAC ISOL stations via TRIUMF's High-Level Applications (HLA) framework [6,7].The framework allows automatic and systematic measurements while also plotting the live data for relevant trends, and exporting data in a user-friendly structure.0 to 100 30 5

Material and methods
The HLA task-force at TRIUMF has developed a framework [7,6] that simplifies the implementation of new web apps, with the goal of reducing overhead and improving reliability of the complex series of accelerators.Primary avenues pursued in HLAs include automation, model-coupled tuning, and machine learning.Automation of repetitive tasks frees personnel at all different levels, including both Operators and Accelerator Physicists, to focus on other important tasks.A dedicated HLA server handles the interaction with EPICS using the python library pyEPICS and collected data is saved to an HLA database which includes meta-data about beam properties, target type, and more, for easy searching and filtering at a later time.The ion sources are characterized at the offline ISAC test stand [8] where beam properties such as ion current, emittance, and ionization efficiency [9] can be quantified on stable species.The injected leak rate used throughout the offline experiments was ≈1×10 13 pps of argon, which is an equivalent neutral particle current of 1600 nA that can be compared directly to the ion current readings.The test stand is controlled via EPICS which offers a graphical interface to vary the desired parmeters via interactive sliders (see Fig. 2).The readbacks, however, are only displayed around the sliders or plotted as a time series.
As a proof-of-principle, the ISAC test stand was controlled via a Jupyter notebook [10] connected to the HLA server, and an extensive experimental campaign for the FEBIAD different settings and geometries was used to improve the numerical ionization models of this ion source [9,11].The data set is recorded in the background, and relevant measurements are displayed as scatter or contour plots and live data analysis is performed (see Fig. 2).The number of data points taken in a single day of operation has increased ten-fold with the customized data acquisition upgrade.Additionally, the measurement time at each data point is consistent throughout the experimental campaign.In this case, and due to the negligible signal variation once a set of parameters is defined, the time average window for the signal can be set as low as 2 s.A typical anode voltage and coil current scan shows the location of maximum ion current intensity and, consequently, the ionization efficiency for a known leak rate used.The online implementation has been successful (see Fig. 3), and all operational personnel can perform a scan thanks to the friendly user interface and safety policies implemented by the HLA task-force.The app is based on a python package called Flask, a micro-framework for web applications.The app sends a series of instructions to the HLA server and receives the desired readbacks.The ion current intensity can be recorded in any of the Faraday cups along the beamline and a heatmap is displayed from which the parameter settings corresponding to the maximum ion current are extracted.Recently, the beta counts from the ISAC yield station [12] were used as the measuring device to quantify radioactive species.

Results and discussion
The offline scans included several isotopes but only some are presented here for brevity as they encompass the main trends for different species.For a cathode temperature of 1700 • C, 40 Ar + presents an island of maximum efficiency near the lower left corner of the plot while the contours above 200 V continue vertically, indicating a weak coil current dependency.A similar dependency is observed for 40 Ar ++ but the maximum efficiency is shifted to a voltage value ≈50 V higher with respect of 40 Ar + .The different parameter combination changes the electron energy that in   turn changes the value of a specific process cross-section for a specific species [13], for example, argon double ionization (Fig. 4b) or H 2 O molecular breakup (Fig. 4d).The first online results for neon show how a rather quick scan (121 data points) presents a maximum for stable 20 Ne + at 200 V and 80 A (see Fig. 5).A suboptimal point at 150 V and 10 A was chosen as the current measured was less than half from the optimal value.Yield measurements were taken for 18 Ne + at the optimal (cyan point) and suboptimal (gray point) settings.The yields share the same intensity ratio as the stable isotope and confirm that optimal settings should be consistent across a single element at one charge state (see Fig. 6).Due to the variability in manufacturing, each FEBIAD might present different optimal operational parameters.The new scanning process additionally generates a database that will enable the quantification of trends over multiple FEBIADs.The development ultimately demonstrate the need for element and charge state-specific parameter optimization, but more importantly, enables such optimization for the species of interest.

Conclusions and outlook
The multidimensional operation of the FEBIAD can now be measured systematically and plotted automatically during both offline and online operations.Offline analysis has become crucial before operating the ion source online as it is now possible to tune and optimize a stable ion beam, from which an optimal operation is expected for radioactive species of the same element.Online runs can benefit from the different behaviors observed for doubly charged ions and molecular species to, for instance, either move the ion of interest to a more favorable mass-overcharge, or break molecules that generate isobaric contamination.The developments presented here, requiring just a couple of hours for setup, enable effective online FEBIAD optimization and potentially enhance the yield delivered to the experiments.
In the near term, beam emittance measurements are foreseen with the same methodology to quantify the beam quality as a function of FEBIAD parameters, as well as the cathode heating current for fully automated FEBIAD parametrization.Moreover, with the presence of isobaric contamination, a similar contour plot could be presented showing the ratio of two species as measured in a multi-reflection time-of-flight mass spectrometer.Ultimately, continuing yield measurements at several combinations would provide more insight into the interplay of the FEBIAD source parameters and the ion beam intensity and purity delivered to an experiment.

Figure 1 :
Figure 1: Cross-section of the ISAC-FEBIAD ion source.The distance between cathode and grid is ≈1 mm.

Figure 2 :
Figure 2: Top left window shows the EPICS software and the sliders to manually vary the FEBIAD ion source parameters.Bottom left, the time series plot.Right image, the Jupyter notebook with an ion current contour plot.

Figure 3 :
Figure 3: Typical scan showing a 20 Ne ++ measurement, where 1600 data points were measured automatically in just under two hours.The web app uses python bindings to connect with EPICS.

Figure 4 :
Figure 4: Ion current as a function of anode voltage and coil current at a cathode temperature 1700 • C (290 A heating current).Identical parameter combinations provide different contours for either singly charged (a), doubly charged (b), or molecular species(c-d).Gray cross indicates typical parameters used and the cyan dot is the maximum for the species of interest, in this case 40 Ar + .The contours change for different temperatures and the multidimensional data can further inform the numerical models developed[13,14,15].

Figure 5 :
Figure 5: FEBIAD scan for 20 Ne + .Cyan dot indicates the maximum, while gray dot shows the location of a mid-low value for comparison.

Figure 6 :
Figure 6: A relatively quick scan finds a factor ≈2 increase for neon.The yield measurements were taken at the parameters shown in Fig 5.