Design studies for an advanced ECR ion source

Abstract

Magnet design codes, plasma dispersion solvers, and particle-in-cell (PIC) simulation codes have been used to arrive at the first step in the design of an advanced ion source based on electron cyclotron resonance (ECR) technology. The advanced concept design uses a minimum-B magnetic mirror geometry which consists of a multicusp magnetic field to assist in confining the plasma radially, a flat central field for tuning to the ECR resonant condition, and specially tailored mirror fields in the end zones to confine the plasma in the axial direction. The magnetic field is designed to achieve an axially symmetric plasma ‘‘volume’’ with constant mod-B, which extends over the length of the central field region. This design, which strongly contrasts with ‘‘surface’’ ECR zones characteristic of conventional ECR ion sources, results in dramatic increases in the absorption of rf power, thereby increasing the electron temperature and ‘‘hot’’ electron population within the ionization volume of the source. The creation of a volume rather than a surface ECR zone is, therefore, commensurate with the generation of higher beam intensities, higher charge states, and a higher degree of ionization. A summary of the results of these studies is presented in this report.

ECR ion source developments at the Oak Ridge National Laboratory

“…An alternative to the multiple-frequency technique has been proposed (3)(4)(5) that employs a new magnetic field configuration which has an extended central flat region tuned to be in resonance with single-frequency microwave radiation. Because of the extended resonant plasma volume, significantly more RF power can be coupled into the plasma, resulting in heating of electrons over a much larger volume than is possible in conventional ECR ion sources.…”

Future prospects for ECR plasma generators with improved charge state distributions

“…The ECR zones in any ECR source are limited to regions of the ionization volume where the magnetic field meets the resonance condition, given by w,, = B e l m = wd (1) where a,, is the electron-cyclotron resonant frequency, a, is the resonant frequency of the microwave power source, e is the electron charge and m the mass of the electron. Whenever the microwave frequency is tuned to the electron-cyclotron frequency, electrons can be resonantly excited and thereby given sufficient energy to cause ionization within an evacuated volume.…”

“…They can be increased either by tailoring the magnetic field to achieve a large, uniformly distributed ECR plasma volume'*2 (spatial domain) or by using broadband microwave radiation techniques in combination with conventional minimum-B magnetic field configurations (frequency domain). These techniques include: (1) a tailored magnetic field configuration in combination with single-frequency microwave radiation to create a large uniformly distributed ECR "volume" and; (2) the use of broadband-frequency domain techniques (variable-frequency, broadband frequency, or multiple-discrete-frequency microwave radiation), derived from standard…”

Computational design studies for an ion extraction system for a “volume-type” ECR ion source

“…[l-5]. PBGUNS [3] was used in the present study to simulate extraction of mA intensity levels of multi-charged ion beams from a new concept [6] all-permanent-magnet ECR ion source under development at the Oak Ridge National Laboratory [7]. A more detailed account of this work can be found in Ref.…”