New multispecies compact ion source and efficient experimental proton beam characterization

Author: Victor Etxebarria, professor, Dept. Electricidad y Electrónica. Fac. Ciencia y Tecnología, Universidad del País Vasco-UPV/EHU

Any particle accelerator needs a reliable electron or ion source as a first, humble but essential and critical component. The world’s most powerful particle accelerator today -the Large Hadron Collider- extracts its protons from a single bottle of Hydrogen gas. Ion sources produce beams for a large variety of different scientific experiments, industrial processes and medical applications.

The IZPILab Beam Laboratory from UPV/EHU has designed and built a new generation compact multispecies ion source ¹, conceived for low current applications, including broad science, industry and biomedicine. The ion source can be operated with gases other than Hydrogen, for instance for Helium, Nitrogen or any other elemental gas for ion production. No major changes would be needed other than the use of an appropriate gas mass flow controller.

A large variety of technologies had to be integrated in the ion source, and made to work in harmony: mechanical, electromagnetic, vacuum, radio frequency, high voltage and control. Once the ion source was fully assembled and the individual components were tested separately, beam extraction experiments were carried out in order to validate the operability of the complete ion source. Figure 1 shows a photograph of the ion source when it was first assembled to work.

The basic components of the ion source shown in the photograph -with one of the protective panels removed- include (1) The DC break designed to be transparent to the radio frequency power, but opaque to DC High voltage, (2) The gas inlet tube from the Hydrogen gas bottle shown in the foreground of the photo, (3) Permanent magnet structure around the plasma chamber, (4) High voltage stand-offs (5) Alumina isolator, (6) Port aligner, (7) Turbomolecular pump, (8) Pressure sensor, (9) Gas mass flow controller, (10) P43 scintillator screen.

By means of an electrostatic field, the proton beam was extracted and several types of experiments were carried out in order to characterize the beam. First, in order to verify the beam alignment, a P43 phosphorescent screen was used to look at both the position and the size of the beam spot. The misalignment of the beam was corrected by adjusting a vacuum port aligner placed between the alumina isolator and the pumping vessel.

If the intensity as a function of position of the image is analyzed, we can see that the intensity follows a gaussian distribution. This is shown in Figure 2, where the measured data (semi-transparent) are laid over the 2D gaussian that best fits the data, where a very reasonable fit is achieved (R²=0.93).

With the beam centered on the furthermost port down beam, the phosphorescent screen was replaced with a Faraday cup connected to a galvanometer to measure the proton current extracted. A very usable 4.3 mA proton beam at 6 keV was first extracted from our ion source, and higher energy and current can easily be obtained for the beam by appropriately regulating the electrostatic field and the plasma density, respectively.

The emittance of the extracted beam was also measured to complete the characterization of our ion source. The proton optics is much more difficult than the photon optics once that we get a beam of charged particles. Horizontal and vertical divergence of the protons versus their position for the 6 keV beam we extracted was measured by means of a pepperpot, which consists of a plate with a rectangular array of 0.45 mm holes spaced 1.27 mm between centers. The images of the proton beam on the scintillator through the pepperpot let us measure the emittance of the extracted beam. Very remarkable maximum 0.065p mm mrad emittance of our ion source compare positively with other more complex, expensive and bulky existing ion sources.

The proton beam generated by means of the IZPILab ion source -designed, built and tested at UPV/EHU- has excellent low emittance and a small beam spot size, suitable for a number of applications where low currents are needed, including biomedical and industrial uses. Because of its compactness, simplified magnetic structure, low power requirements, efficiency, quality of beam and straightforward operation, the presented source compares favorably to other traditional ion source designs, which are typically conceived for large-scale particle accelerators or large scientific facilities. It should be noted that traditional bulky source designs may not be as convenient nor as cost-effective as ours for industrial or medical applications in smaller environments.