Apparatus and method relating to charged particles

A method of acting upon charged particles differently in dependence upon different values of at least one parameter of the particles, said parameter being selected from the group comprising mass, energy, and charged state, of a particle, said method comprising:

• providing a beam of charged particles in the field of a magnetic pole array, the beam having at any given position across the beam an initial general direction of propagation of the beam and having an elongate cross-section perpendicular to the initial general direction of propagation of the beam;
• the array of magnetic poles having a virtual reference surface for providing a reference for the steps of the method, said virtual reference surface being a virtual surface selected from the group comprising (I) a surface defined as extending in said direction of elongation of said beam cross-section and also containing said initial direction of propagation of said beam at any position across the beam, and (II) a surface defined as extending in said direction of elongation of said beam cross-section and also parallel to said initial general direction of propagation of the beam at any position across the beam;
• the virtual reference surface passing through the array with at least one magnetic pole on each side of the reference surface;
• the magnetic poles being elongate poles extending in a direction of elongation of the array which lies in the virtual reference surface and which is transverse to the initial general direction of propagation of the beam, and said magnetic poles being constructed and arranged in such a manner that, in a plane perpendicular to said direction of elongation of said array, said magnetic poles have a physical geometry which is constant along said direction of elongation of the array and have a magnetic field distribution which is constant along said direction of elongation of the array;
• arranging the said charged particles to move in the field of the magnetic pole array with an initial general direction of movement which is selected from the group comprising (I) a direction which is in said reference surface and which is other than said direction of elongation of said magnetic pole array, and (II) a direction which is substantially parallel to said reference surface and which is other than said direction of elongation of said magnetic pole array, said initial direction being selected from the group comprising an initial direction of particles entering into the field of the magnetic pole array, and an initial direction of particles originating in the field of the magnetic pole array;
• producing parameter dependent dispersion of the charged particles moving in the magnetic pole array by producing parameter dependent focusing action in a plane perpendicular to the virtual reference surface by dispersing and focusing different particles of the beam in dependence upon different values of said parameters for different particles;
• and
• selecting required particles from the beam by the said parameter dependent focusing action.

Apparatus for acting upon charged particles differently in dependence upon different values of at least one parameter of the particles, said parameter being selected from the group comprising mass, energy, and charged state, of a particle, said apparatus comprising:

• an array of magnetic poles constructed and arranged to give parameter dependent dispersion of charged particles moving in the magnetic pole array whereby parameter dependent selection of charged particles may be achieved;
• said array of magnetic poles having a virtual reference surface providing a reference for the arrangement of components of the apparatus, said virtual reference surface passing through said array of magnetic poles with at least one magnetic pole on each side of said reference surface;
• a beam source constructed and arranged to provide a beam of charged particles in the field of said magnetic pole array, said beam having at any given position across the beam an initial general direction of propagation of the beam and having an elongate cross-section perpendicular to said initial general direction of propagation;
• said virtual reference surface being a virtual surface selected from the group comprising (I) a surface defined as extending in said direction of elongation of said beam cross-section and also containing said initial direction of propagation of said beam at any position across the beam, and (II) a surface defined as extending in said direction of elongation of said beam cross-section and also parallel to said initial general direction of propagation of the beam at any position across the beam;
• and
• resolving structure constructed and arranged to select required particles from said beam by said parameter dependent dispersion;
• said magnetic poles being elongate poles extending in a rectilinear direction of elongation which lies in said virtual reference surface and which is transverse to said initial general direction of propagation of said beam;
• said beam source being constructed and arranged in such a manner that said charged particles of said beam moving in the field of the magnetic pole array have an initial general direction of movement which is selected from the group comprising (I) a direction which is in said reference surface and which is other than said direction of elongation of said magnetic pole array, and (II) a direction which is substantially parallel to said reference surface and which is other than said direction of elongation of said magnetic pole array, said initial direction being selected from the group comprising an initial direction of particles entering into the field of the magnetic pole array, and an initial direction of particles originating in the field of the magnetic pole array;
• said magnetic poles being constructed and arranged in such a manner that, in a plane perpendicular to said direction of elongation of said array, said magnetic poles have a physical geometry which is constant along said direction of elongation of the array and have a magnetic field distribution which is constant along said direction of elongation of the array, and said magnetic poles being constructed and arranged to give parameter dependent focusing action in a plane perpendicular to said reference surface to give selection of particles by parameter dependent dispersion and focusing of different particles of said beam in dependence upon different values of said parameters;
• said array of magnetic poles being such as to provide between physically opposed poles at least one transverse field region of magnetic field with a magnetic field direction transverse to the virtual reference surface, said at least one transverse field region of magnetic field having boundary regions comprising entry and exit regions associated with individual opposed poles in the array and leading to and exiting from the transverse field region of magnetic field respectively, the boundary regions providing curved magnetic fields which are curved in a plane perpendicular to the virtual reference surface,
• said regions of magnetic field being such that the beams of charged particles of the beam moving in said transverse field region have a curved motion imposed thereon by the transverse field region of magnetic field, and pass through the curved field of at least one boundary region at an angle to the normal to the boundary region so as to provide parameter dependent focusing of said beam of charged particles.

Apparatus for selection of particles of at least one beam of charged particles in dependence upon a parameter including at least one of mass, energy and charged state of the particles, the apparatus comprising:

• an array of elongate magnetic poles extending longitudinally in a direction of elongation of the array,
• the array having a reference surface extending in the direction of elongation of the array of magnetic poles and passing through the array with at least one magnetic pole on each side of the reference surface,
• the magnetic poles having a configuration in a plane perpendicular to the direction of elongation such as to give parameter dependent focusing action of the at least one beam passing through the magnetic pole array with a general direction of propagation that is at least one of in and substantially parallel to the reference surface, and other than a direction of elongation of the magnetic pole array,
• the configuration of the magnetic pole array being such as to produce focusing action towards a cross-over point, the position of which is parameter dependent;
• a resolving structure provided at or near the cross-over point, the resolving structure including an aperture through which selected particles may be transmitted;
• and
• a transmission limiting structure positioned in at least one plane transverse to the general direction of propagation of a focused beam through the resolving structure, to prevent transmission of particles following trajectories beyond a range obstructed by the resolving structure.