Method of correcting eddy current magnetic fields in particle accelerator vacuum chambers

Abstract

A method for correcting magnetic field aberrations produced by eddy currents induced in a particle accelerator vacuum chamber housing is provided wherein correction windings are attached to selected positions on the housing and the windings are energized by transformer action from secondary coils, which coils are inductively coupled to the poles of electro-magnets that are powered to confine the charged particle beam within a desired orbit as the charged particles are accelerated through the vacuum chamber by a particle-driving rf field. The power inductively coupled to the secondary coils varies as a function of variations in the power supplied by the particle-accelerating rf field to a beam of particles accelerated through the vacuum chamber, so the current in the energized correction coils is effective to cancel eddy current flux fields that would otherwise be induced in the vacuum chamber by power variations in the particle beam.

We claim: 
     
       1. A method of correcting undesired magnetic field aberrations that are produced by eddy currents induced in a metallic vacuum chamber of an accelerator by magnetic fields that are produced by energization of particle beam-positioning electro-magnets which are operably mounted adjacent to the vacuum chamber for confining electrically charged particles in stable orbits while the charged particles that enter and exit the vacuum chamber through apertures in its walls are driven by an rf accelerating field, comprising, (a) attaching correction windings to the outside surface of the vacuum chamber at predetermined locations thereon,   (b) providing secondary coils for supplying electric power to said correction windings, and providing a variable resistor,   (c) electrically connecting said secondary coils in series with the correction windings, with the variable resistor operably positioned in the series circuit between the secondary coils and the correction windings,   (d) providing means for supplying electric power to the secondary coils as a function of a predetermined proportion of the electric power in a beam of charged particles that is accelerated through said vacuum chamber, so that as the electric power in the charged particle beam line varies, the electric power in the secondary coils varies proportionately and,   (e) adjusting the variable resistor thereby to cause the resultant current in the correction windings to generate a magnetic flux that effectively cancels eddy current induced aberration flux fields that are induced in the vacuum chamber by the beam confining magnets while the charged particles are accelerated through the chamber.   
     
     
       2. A method as defined in claim 1 wherein said means for supplying electric power to the secondary coils comprises providing an external power supply that is operably connected to deliver electric power to the secondary coils. 
     
     
       3. A method as defined in claim 1 wherein said means for supplying electric power to the secondary coils comprises positioning the secondary coils in inductive relationship to one of the electro-magnets that is energized to confine charged particles within an orbit through the vacuum chamber, whereby electric current is induced by transformer action into said secondary coils in proportion to the particle accelerating flux driving the charged particles through the vacuum chamber in a path to which the particle beam is confined by the beam-positioning electro-magnets. 
     
     
       4. A method as defined in claim 1 including, after step c, (c-1) measuring the magnetic field flux B produced at the horizontal mid-plane of the vacuum chamber for a given rise rate of the particle accelerating magnetic flux, and comparing the field flux B with the eddy current induced magnetic flux, thereby to determine the appropriate adjustment of the variable resistor that is needed for effecting step (e) of the method.   
     
     
       5. A method as defined in claim 1 wherein the predetermined locations of the correction windings are ascertained empirically by temporarily securing the correction windings to various arbitrarily selected positions on the outer surfaces of the vacuum chamber, then energizing the correction windings to produce a current that essentially cancels the eddy current flux produced in the vacuum chamber walls when they are positioned at optimum locations, and then securing the correction windings in their optimum locations on the outer surfaces of the vacuum chamber. 
     
     
       6. A method as defined in claim 1 wherein the desired final locations of the correction windings are determined by computed theoretical analysis and then the windings are attached to the vacuum chamber outer walls in said final locations.