Electron cyclotron resonance ion sources – physics, technology and future challenges

The performance of Electron Cyclotron Resonance Ion Sources (ECRIS), producing high charge state ions from a great variety of elements, has improved dramatically over the past decades, thus enabling significant advances in accelerator-based nuclear physics. The orderof-magnitude performance leaps of ECR ion sources, e.g. from 15 μA of extracted O in 1974 [1] to 4700 μA in 2016 [2], result from improvements to the magnetic plasma confinement, increases in the microwave heating frequency and techniques to stabilize the plasma at high densities. The design of modern ECR ion sources is based on semi-empirical scaling laws, suggesting most importantly that the extracted current at the peak of the ion charge state distribution scales with the microwave frequency squared [3], i.e. Ipeak RF. (1) Direct comparison of 1 – 3 generation ECR ion sources operating at frequencies below 10 GHz, at 10– 18 GHz and above 18–28 GHz, respectively, is complicated by the fact that the peak of the charge state distribution also shifts with the frequency. However, it is evident that increasing the microwave frequency improves the extracted currents of highly charge ions. Several explanations for the frequency scaling have been proposed. These include for example damping of the microwave electric field at plasma densities exceeding the critical (cut-off) value and so-called RF pitch angle scattering limiting the electron density, both scaling with the microwave frequency squared. The magnetic field of an ECRIS is a superposition of solenoid and sextupole fields resulting to a minimum-B structure serving several purposes. (i) The field configuration provides a closed resonance surface fulfilling the ECR-condition RF = ce = eBECR/ me, (2) where ce is the electron gyrofrequency, which depends on the magnetic field strength B and electron energy through the relativistic -factor (e and me are the elementary charge and electron mass, respectively). The resonant interaction with the microwave electric field assures that a sufficient number of electrons gain energies required for ionization of high charge state ions. The magnetic field fulfilling the resonance condition for non-relativistic electrons can be written conveniently as BECR [T] = fRF [GHz]/28. (3) (ii) The minimum-B configuration results to appropriate electron confinement. The development of ECR ion sources over the past decades has led to the formulation of the following scaling laws [4] for the magnetic field: Binj/BECR = 4 (4) Brad/BECR = 2 (5) Bext 0.9Brad (6) Bmin 0.4Brad, (7) where Binj, Brad, Bext and Bmin are the magnetic field at the injection, radial wall of the plasma chamber, extraction and B-minimum, respectively. (iii) The minimum-B configuration suppresses magnetohydrodynamic instabilities. The scaling laws in Eqs. (1) and (4)–(7) set a practical limit for ECRIS technology relying on roomtemperature (RT) technology if the desired power consumption for generating the magnetic field is < 1 MW. In RT-ECRISs the solenoid field is created by electromagnetic coils while the radial sextupole field is formed by an array of permanent magnets. Figure 1 shows the dependence of the maximum solenoid field at the injection (Binj) and the sextupole field at the chamber wall fulfilling the scaling laws on the microwave frequency. Commonly applied frequencies are highlighted by the dots. The practical RT-limits of both field components, set by the power consumption and permanent magnet remanence / coer-civity, are depicted by horizontal lines crossing the data curves at 18 GHz, which is deemed as the upper frequency limit for 2 generation RT-ECRISs.