Electron Cyclotron Resonance Ion Sources

Home › Technology › Electron Cyclotron Resonance Ion Sources

Background

Particle accelerators — such as the cyclotron — use electromagnetic fields to accelerate elements. To be accelerated, atoms need to become electrically charged (ionized) by having some of their electrons removed. At NSCL, the removal of electrons from neutral atoms is accomplished with Electron Cyclotron Resonance (ECR) ion sources. Ions from the ECR sources are sent down a beam pipe and injected into the cyclotron for acceleration.

Expanded Description

In an ECR source, vapor of the desired element is held in a specially designed magnetic field — a “magnetic bottle” — long enough for the elemental atoms to be ionized in collisions with electrons, which are kept in motion by microwaves. The magnetic bottle is shaped by circular coils at its top and bottom and a hexapole magnet (three “north poles” alternating with three “south poles”) around the sides. The transmitters for the microwaves that heat the electrons run at several times the frequency of a household microwave oven.

At NSCL, we have three ECR ion sources:


Schematic diagram of an Electron Cyclotron Resonance ion source. more
  • The Superconducting-ECR (SC-ECR) is built with superconducting magnets that are held at a temperature of -452 °F (4 Kelvin) by contact with liquid helium.
  • The Advanced Room TEMperature Ion Source (ARTEMIS) is built with normal conductor circular coils and permanent magnets arranged to make a hexapole.
  • The Superconducting Source for Ions (SuSI), the newest NSCL ion source, also uses chilled superconducting magnets to produce an enhanced magnetic field. SuSI’s maximum magnetic field is around 4 Telsa, more than three times that of earlier generation ECR ion sources. SuSI, to be commissioned in late 2007 or early 2008, will increase available ion currents for nuclear physics experiments at NSCL.

Here’s how an ECR ion source works: When gases are used as ion sources, gases (such as helium, nitrogen, oxygen and so on) are simply injected directly into the magnetic bottle. Metals also can be used to produce ions. In such cases, a very tiny oven is used to heat metals to a gas or vapor, which enters the ion source at the top of the magnetic bottle.


NSCL ion source group leader adjusts SuSI. more

No matter the source, the ions now in the magnetic bottle must be extracted. This is achieved by applying a high voltage (up to 30 kilovolts) that forcibly pulls the ions from the bottle, from which point they are sent on to the cyclotron.

The ion sources are important because, in order to do an experiment in a reasonable amount of time, the accelerator complex should provide enough ions at the desired energy. The quality of the ion beam is important because only ions with right “flight trajectory” can be injected in the cyclotrons and accelerated to the desired energy.

The charge state of ions needed for experiments varies. For the lighter elements, such as oxygen, it might be 3+. For the heavier elements—uranium, for example—the charge state needed might be 32+.

Earlier in the laboratory’s history, while the K500 and K1200 cyclotrons ran independently, the emphasis was on producing ions in very high charge states at relatively low intensity (up to 90 microamperes). NSCL’s mode of operation since 2001 — the laboratory now operates the Coupled Cyclotron Facility, with the two cyclotrons linked together—requires ions with lower charge states but higher intensities (up to 400 microamperes).

Next-generation rare isotope science will require ion sources, like SuSI, capable of producing ion currents with increased current and brightness. SuSI, designed and built entirely at NSCL, delivers on both counts.