Proton ion source development for neutron sources

The decreasing number of research reactors decreases the number of the available neutron sources. To overcome this problem, accelerator based neutron sources are being developed worldwide. There are many different types of these neutron sources (concerning neutron flux, final beam energy, etc.), but the principle is the same: microwave radiation excited proton/deuterium ion source with a beam current of one to a few hundred mA, beam energy of 30-100keV (it is further accelerated in the second/third stages of acceleration) and beam emittance of below 1 π mm mrad. The development of such a source started at the beginning of 2021 in Centre for Energy Research, Fusion Plasma Physics Laboratory.

The basis of this kind of ion source is a resonant cavity excited by microwave radiation. Protons are extracted electrostatically from this chamber. The parameters of the hydrogen discharge is defined by the excited microwave mode, the microwave energy and the volume of the hydrogen flow (at a given magnetic field). The density and its distribution of the forming plasma will limit the extractable ion current. The aim is to produce protons with 35keV beam energy and 20mA ion current (with appropriate beam parameters), which will work – among others – at the compact neutron source in Martonvásár. 

The cavity is applied with holes at different places making possible to observe the plasma. This is possible either with one or more endoscope camera or with fibres. To design, install and measure with such an observation system will be the task of the student. The extractable ion current has to be defined at different plasma modes/densities (using a Faraday cup) and the connection between the plasma density distribution and the extractable ion current. The results have to be compared with the literature.

Since the 2nd acceleration stage needs a beam frequency of ~40Hz, it is necessary to check whether it is possible to switch the plasma off on such a time scale, or whether the extracted beam must be deflected, or the extracting voltage must be modulated with this frequency. The latter is the standard solution for several working proton sources. It will be the student's responsibility to participate in the necessary electronic development of such a high voltage chopper system.

The Faraday cup operating in our laboratory is also suitable for measuring the current distribution of the ion beam. With this the beam emittance (and current distribution) can also be measured with the help of a properly designed mesh. The planning, implementation and evaluation of this measurement will be the responsibility of the student.

A strongly cohesive beam must be shot into the 2nd accelerator stage, so a magnetic lens is also needed. The implementation of this solenoid is in progress, the student will be part of this development and participate in the measurement. The magnetic lens must be designed with such limiters that the molecular ions (H₂+ and H₃+) in the beam (present in a small percentage) no longer reach the 2nd accelerator stage. For this we currently use the CPO simulation code to study the beam propagation. It will be the responsibility of the student to use the code and compare the results of the simulation and measurements.