1. Field of the Invention
This invention relates to a new, compact, and improved negative ion source and injector system, including preaccelerator and beam transport innovations. More particularly, it relates to methods and apparatus for efficiently excluding electrons from the negative ion beam and creating a high-angle convergent beam for injection into other devices including accelerator stages.
2. Description of the Background
Energetic neutral beams are difficult to create but are necessary for many applications. For example, charged particles cannot be injected through the large magnetic fields surrounding many high energy accelerators, for example the proton synchrotron, and other devices because the charged particles would be deflected from their path by the magnetic field. None-the-less, particles must be charged so that a voltage gradient can be used to extract them from the plasma. This means that particles used in high energy devices are likely to go through several stages prior to injection in the device. They are initially electrically charged so they can be extracted from the plasma gas that contains the negative ion. The charged particles are accelerated through many stages, then the beam has its charge changed just prior to injection into the high energy device.
It is desirable that as many electrons as possible are eliminated from the beam immediately upon exit from the ion source. The undesired high energy electrons, found in negative ion beams, are currently separated from a negative hydrogen ion beam by using an ExB electron extractor which diverts them to collector electrodes. Because of the magnetic field surrounding them, permanent magnets could not be placed near the exit aperture of the ion source without compromising the efficiency of the source. Placing the magnets downstream requires a considerable amount of power and is a substantial power drain on a system.
One approach for the production of negative ions and suppression of accompanying electrons is disclosed in US. Pat. No. 4,486,665. The approach described therein filters high-energy electrons by dividing the ion source chamber into two zones, an ionizing zone and an extraction zone. Excess electrons are suppressed by placing a plasma grid adjacent to the extraction zone of the ionization vessel that is positively-biased with respect to the anode. The positively biased grid suppresses electrons that would be dragged along with the positive ions which originate in the ionizing zone and pass through the magnetic filter. Additionally, electrons are suppressed from the output by providing a magnetic field, aligned with respect to the electric field of the extractor, to provide ExB drift to the electrons. There is some difficulty with that system, however. The magnets described in that patent are located interior to the ion source. Those magnets interfere with negative ion extraction, reducing the number of negative ions extracted by approximately a factor of 2. In addition, fewer electrons are successfully diverted from the negative ion beam than with the present invention. Further, when a radio frequency source is used to generate the negative ions, the magnet field resulting from the magnets as used in US. Pat. No. 4,486,665 interfere with the production of negative ions within the ion source volume. In order to create a magnetic field to deflect electrons from the extracted beam without interfering with ion production and extraction for the invention described in that patent, the magnets would have be located far enough down the beam transport line that the magnetic field would not penetrate the ion source vessel. This would lengthen the beam transport dimensions and interfere with the geometry of the electrodes used to focus the beam. If the magnetic field does enter the ion source vessel, negative ion production becomes less efficient. Lengthening the beam transport dimensions requires added power consumption and control circuitry.
It is important to keep the gas pressure low in the accelerator column. If gas pressure rises too high, collisions between negative ions and contaminants will cause electrons to be stripped from the ion prematurely.
There are several examples of devices that require energetic neutral beams created from negative ion particles. For example, large magnetic-confinement fusion energy devices such as tokamac accelerators, require 1 MeV neutral beams for injection. In order for the extracted beam to be injected into the cyclotron storage and acceleration ring without being defocused when crossing the electromagnetic fields, the beam charge must be changed and negative ions are more easily changed than positive ions. High energy cyclotron accelerators also require charge changed beams in order for the beam to be extracted from the cyclotron, booster ring. (L. W. Alvarez, Rev. Sci. Instr. 22(1951)705). Another application of the invention is for energetic particle beams that must be neutral in charge to transit large distances in space. The superconductor supercollider (SSC) is similarly in need of a high energy particle beam that could be created from a high quality negative ion source.