Tutorials

NEC is the world's leader in the manufacture of electrostatic ion beam accelerator systems. Our systems consist of three main subsystems: the ion source, the Pelletron® accelerator, and the analytical endstation.

Ion Sources

RF Charge Exchange Ion Source

The purpose of the ion source is to produce either positive or negative ions from neutral atoms. Positive ion sources are placed inside the tank of a single-ended accelerator; negative ion sources inject the ion beam into the tank of a tandem accelerator. Different types of sources may be used depending on the mass and charge of the desired ion.

An RF (radio frequency) ion source produces positive ions. A gas or gas mixture is bled into a quartz bottle; an RF oscillator connected to the quartz bottle dissociates the neutral gas. A voltage difference (usually about 2-6 kV) is used to push the ions out of the chamber through the exit aperture, making a continuous beam. To produce a negative beam, the positive beam is immediately injected into a rubidium charge exchange cell.

A Duoplasmatron ion source can produce either a positive or a negative ion beam depending on the placement of the intermediate (zwischen) electrode.

SNICS-II Ion Source
The Source of Negative Ions by Cesium Sputtering (SNICS) produces a negative ion beam. Cesium vapor comes from the cesium oven into an enclosed area between the cooled cathode and the heated ionizing surface. Some of the cesium condenses on the front of the cathode and some of the cesium is ionized by the hot surface. The ionized cesium accelerates towards the cathode, sputtering particles from the cathode through the condensed cesium layer. Some materials will preferentially sputter neutral or positive particles which pick up electrons as they pass through the condensed cesium layer, producing negative ions.

The Pelletron Accelerator

The principal founder of the National Electrostatics Corp., Professor Raymond G. Herb, was the first person to make the Van de Graaff electrostatic generator practical for use as an MeV ion beam accelerator in the early 1930's. The basic concepts of the pressurized generator with a reliable charging system, positive ion source integrated into the high voltage terminal, graded accelerating tube and column were developed by Prof. Herb, his students and colleagues at the University of Wisconsin.

In the early 1960's, the basic Pelletron inductive charging system was invented by Dr. James A. Ferry (President of National Electrostatics) in association with Prof. Herb. In 1965, with a reliable Pelletron charging system and the unique NEC all metal and ceramic accelerating tube well in hand, the decision was made to form National Electrostatics to produce the Pelletron accelerators.

NEC Pelletrons, whether S-series or U-series, come in two configurations, tandem and single-ended. In both cases, ion beams (both positive and negative) are accelerated by an electrostatic field, i.e. a potential drop, resulting in a true DC beam.

In single-ended machines, one end has the high-voltage terminal containing an ion (e.g. see Positive Ion Beam Sources) or electron source, and the other end is at ground, where the ion/electron beam emerges from the Pelletron with an energy roughly equivalent to the terminal voltage. Reversible polarity, single- ended Pelletrons are available for producing positive ions or electron beams from a single system.

By contrast, tandem accelerators have both ends at ground with the high- voltage terminal in the middle. Tandems are always charged positively, so that the negatively-charged injected ions (e.g. see Negative Ion Beam Sources) accelerate to the terminal. Inside the terminal is a stripper, which uses a gas (usually N2 or Ar) or a thin carbon foil to collisionally remove electrons from the incoming negative ions. The now positively-charged ions experience a second boost of acceleration (hence the name "tandem" accelerator) as they exit the terminal and travel down the acceleration tube to ground at the high- energy end of the machine. The primary benefits of the tandem configuration are:

  1. Much higher beam energy for a given terminal voltage -- in most cases the final beam energy is approximately (q + 1) times the terminal voltage, where q is the charge state to which the ions are stripped in the terminal.

  2. With the ion source(s) external to the pressure vessel, maintenance requiring entry into the tank and letting the acceleration tubes up to atmosphere is minimized.

For orders, inquiries, comments and more information about our products, please contact our sales department at nec@pelletron.com

NEC Homepage About NEC What's New at NEC Links to Pelletrons®

Ion Beam Systems Vacuum Components Ion Beam Components

National Electrostatics Corporation
Last revised May 31, 2002 by Tim Davis, nec@pelletron.com
Comments and suggestions about this website are welcome.
URL: http://www.pelletron.com/tutor.htm