This invention relates to high energy relativistic beams. More particularly it is related to systems and methods for sensing the direction and velocity of neutral particle beams. Still more particularly, this invention relates to a system and method which utilizes the Doppler shift in the wavelengths of the emissions from the high energy atoms in the beam. It also relates to beam steering methods utilizing such information.
A neutral particle beam has the advantage that it exhibits straight line propagation in a vacuum completely independent of the distortions and deflections that would be induced by local electric and magnetic fields in an electrically charged particle beam. Such neutral beams have a variety of applications. Conventional radars and lasers are normally limited to having only surface effects on objects, but neutral particle beams are capable of penetrating and probing deeply into the body mass of the object and be hence used for object identification and discriminate between similar looking objects. Potential commercial and scientific applications include heating targets for controlled fusion and developing a neutral particle beam microscope. The microscope will not be subject to the limitations of the electric and magnetic distortions inevitably present in the electromagnetic lenses of scanning ion and electron microscopes. Theoretically the neutral particle beam microscope could operate with much high resolution and depth of penetration.
The promise of the neutral particle beam cannot be realized, however, until the direction of the beam can be precisely sensed and controlled. Indeed precise practical beam sensing methods must precede the achievement of precise beam scanning techniques. Several methods of sensing the direction of a neutral particle beam have been proposed, but all of them suffer from one or more serious difficulties. Proposed methods have included pinhole, shadow wire, laser resonance fluorescence, and other less well defined concepts. Both the pinhole and shadow wire methods are inherently intrusive and may present difficulties when high currents and quasi continuous beam operations are involved.
Non-intrusive or non-beam interrupting methods use photons or electrons to sense the beam. One method is laser resonance fluorescence. There are many laser resonance fluorescence (LRF) variations, but all involve the insertion of laser light into the beam at a special angle (the magic angle) to detect the photon fluorescence (or the ionization electrons) emitted by the interaction of laser photons and beam atoms. The magic angle is defined as cos .theta.=.beta. in the Doppler formula set forth below (formula one).
The laser fluorescence method is extremely sensitive to the direction of the beam relative to the laser light direction reference. Unfortunately, most detection schemes must observe this fluorescence in the presence of a stronger background due to the spontaneous photo emission of excited neutral hydrogen atoms. These atoms emit at the same frequency band as the relaxation fluorescence induced by the laser light.