A. G. Drentje

Techniques and mechanisms applied in electron cyclotron resonance sources for highly charged ions

Electron cyclotron resonance ion sources are delivering beams of highly charged ions for a wide range of applications in many laboratories. For more than two decades, the development of these ion sources has been to a large extent an intuitive and experimental enterprise. Much effort has been spent in theoretical work, but a consistent description still is not available. From experimental activities, scaling rules have been formulated, which have successfully been used for the construction of more powerful devices. Special techniques like the coating of the plasma chamber walls, usage of secondary electron emission materials, installation of a biased probe or disk, and mixing the supply gas with other gas species, are generally being incorporated for improving the output of highly charged ions. Various ideas to understand these mechanisms have been brought up, again without consistent description. In experiments, the effect of the techniques with respect to physical parameters, i.e., reducing the plasma potential and/or lowering the ion temperature, has been demonstrated. In a recent study, the requirement of charge neutrality in the fluxes from the plasma to the walls of the plasma chamber has been evaluated; this shows that the occurrence of Simon currents in the conducting walls plays an important role in determining the value of the self-adjusting plasma potential. Most of the special techniques do affect the Simon currents, and therefore the plasma potential, thus the confinement. The effect of the gas mixing technique is mainly (but not exclusively) to decrease the ion temperature, and by that to increase the confinement. The present state of understanding the various techniques will be reviewed.

Development of a compact electron-cyclotron-resonance ion source for high-energy carbon-ion therapy

Ion sources for medical facilities should have characteristics of easy maintenance, low electric power consumption, good stability, and long operation time without problems (one year or longer). For this, a 10GHz compact electron-cyclotron-resonance ion source with all-permanent magnets (Kei2 source) was developed. The maximum mirror magnetic fields on the beam axis are 0.59T at the extraction side and 0.87T at the gas-injection side, while the minimum B strength is 0.25T. These parameters have been optimized for the production of C4+ based on the experience at the 10GHz NIRS-ECR ion source and a previous prototype compact source (Kei source). The Kei2 source has a diameter of 320mm and a length of 295mm. The beam intensity of C4+ was obtained to be 530μA under an extraction voltage of 40kV. The beam stability was better than 6% at C4+ of 280μA during 90h with no adjustment of the operation parameters. The details of the design and beam tests of the source are described in this paper.

Simon short circuit effect in ECRIS

The plasma confinement within an electron cyclotron resonance ion source significantly influences the charge state distribution and hence the performance of the source. The different axial and radial diffusion processes govern the confinement time. In many experiments it has been shown that negatively biasing the end plate in the injection region improves the charge state distribution. In a few x-ray and vacuum ultraviolet spectroscopy experiments to clarify the mechanism it is observed that the biasing improves the confinement of the plasma. It is estimated that the effect cannot be explained solely by secondary electron emission from the plate into the plasma. We propose that by biasing, the overall balance between radial ion losses and axial electron losses will change, resulting in a different diffusional mode of the entire plasma. Hence, the plasma potential and the average charge state of ions in the plasma are significantly influenced. Usually, the ion flux is dominating radial diffusion while the ...

Review on heavy ion radiotherapy facilities and related ion sources (invited)

Heavy ion radiotherapy awakens worldwide interest recently. The clinical results obtained by the Heavy Ion Medical Accelerator in Chiba at the National Institute of Radiological Sciences in Japan have clearly demonstrated the advantages of carbon ion radiotherapy. Presently, there are four facilities for heavy ion radiotherapy in operation, and several new facilities are under construction or being planned. The most common requests for ion sources are a long lifetime and good stability and reproducibility. Sufficient intensity has been achieved by electron cyclotron resonance ion sources at the present facilities.

Role of low charge state ions in electron cyclotron resonance ion source plasmas

We have tried to shed more light on the possible mechanisms of the beneficial effect of gas mixing for highly charged ion production, in particular on the presence of an ion cooling effect. For the first time a method was applied to deduct the ion temperature from all measured ionic currents. The ion temperature values derived showed a clear decreasing trend in conjunction with mass changes of the gas mixture, consisting of “pure” argon (no mixing gas), argon plus natural oxygen, argon plus isotopic 18O, or argon plus 22Ne. Each of the applied mixing gases gives a higher charged ion (HCI) output (highest for 18O) as well as a lower ion current for the singly charged beam-particle output. The relative high particle fraction (about 40%) of singly charged nonbeam particles is an indication that effective plasma ion cooling is possible. Although the differences in ion temperature calculated are small, the effect is likely substantial, since the ion confinement has a dependency with (Ti)−5/2. Hydrogen currents...

Anomalous charge state distribution in ECRIS for oxygen isotopes

The performance of the KVI ECRIS‐2 for a mixture of 18O, 17O, and 16O isotopes is−as expected−very much the same as for natural oxygen gas, i.e., the sum of the currents of given charge states of the isotopes equals that of the same charge state of the natural oxygen. However, it is found that the ratios of currents 18Oq+/17Oq+ and 18Oq+/16Oq+ increase with q. New measurements on ECRIS‐1 (Minimafios) at KVI confirm this anomalous charge state distribution. This effect is also present when nitrogen or argon gas is mixed into the plasma. However, when applying helium as a mixing gas, the anomaly disappears. An explanation in terms of ion cooling (here transfer of kinetic energy to, in particular, the light species) seems valid.

Anomalous oxygen isotopic charge state distribution in ECRIS: New evidence

The highly performing Electron Cyclotron Resonance Ion Source CAPRICE in Grenoble was operated with a mixture of three oxygen isotopes. The summed currents per charge state show a distribution almost identical to that of natural oxygen. However, the distributions per isotope are distinctly and systematically different in the sense that the heaviest isotopic distribution is most prominently peaked at high charge states. This anomalous effect reduces or disappears if helium is added to the isotopic mixture. The addition of either natural helium or isotopic 3He has no significantly differing result. Presently no quantitative model is available to satisfactorily explain the measurements.

Enhanced highly charged ion production using a metal-dielectric liner in the KVI 14 GHz ECR ion source

Forming on an aluminum surface a dielectric layer of alumina (aluminum oxide) in order to create a metal-dielectric (MD) structure increases the secondary-electron emission properties. The idea of using this material as a MD (Al–Al2O3) cylindrical liner inside an ECR ion source was previously tested in the 14 GHz ECRIS of IKF (Frankfurt/Main, Germany). The purpose of the present experiment was to observe the effect of such a MD liner on the high charge state operation of the KVI 14 GHz ECRIS, in particular in comparison to the technique of gas mixing. Measurements were made both with and without the MD liner, with pure argon and with an argon plus oxygen mixture. In the case of pure argon, the source with the MD liner is running remarkably stable. The high charge state ion beam currents are by far higher than those obtained in the situation where the source was operated with pure argon but without the MD liner. With MD liner, some low intensity oxygen peaks were clearly present in the spectra, implying th...

Experiments with biased cylinder in electron cyclotron resonance ion source (plenary)

The shape of the magnetic field of an electron cyclotron resonance ion source (ECRIS) gives rise to different particle fluxes (losses) from the plasma to the end plates (mainly diffusion of electrons), and to the side walls (mainly ions). The electron fluxes to the injection end plate can be reduced by negatively biasing this plate (or so-called biased disk); this appeared to be successful to improve the output of highly charged ions (HCIs). In the present experiment it was demonstrated that by positively biasing the side walls, the HCI production can be improved as well. Electrons leaking from the ECRIS plasma to the side wall find their way easiest at the “pole” areas (of the hexapole magnet) because these are the areas where fieldlines pass the plasma chamber. A special construction was made to study the effect of biasing two different regions: (1) the areas between the six poles, and (2) the parts corresponding to the poles. Measurements show that method (2) gives better performance of the source. The...

The compact electron cyclotron resonance ion source KeiGM for the carbon ion therapy facility at Gunma University

A high-energy carbon-ion radiotherapy facility is under construction at Gunma University Heavy Ion Medical Centre (GHMC). Its design was based on a study of the heavy ion radiotherapy at the National Institute of Radiological Sciences (NIRS) in order to reduce the size and construction cost of the facility. A compact electron cyclotron resonance ion source (ECRIS) for Gunma University, called KeiGM, was installed in 2008. It is almost a copy of the prototype ECRIS Kei2 which was developed by NIRS; meanwhile this prototype produced over 1 e mA of C(4+) using C(2)H(2) gas (660 W and 40 kV). The beam intensity of C(4+) was 600 e microA with CH(4) gas (250 W and 30 kV). The beam intensity satisfies the required value of 300 e microA.