Industrial material processing electron linear accelerator

An electron linear accelerator for use in industrial material processing comprising:

• an elongated, L-band, resonant, electron accelerator structure defining a linear electron flow path and having an electron injection end and an electron exit end, means at said injection end for producing and delivering one or more streams of electrons to said electron injection end of said accelerator structure during pulses of predetermined duration and of predetermined repetition rate, said accelerator structure being comprised of a plurality of axially coupled microwave cavities constructed to resonate in a .pi./2 mode and including:
• a graded-.beta. capture section at said injection end of said accelerator structure for receiving and accelerating electrons in said one or more streams of electrons;
• a .beta..apprxeq.1 section exit section at the end of said accelerator structure remote from said capture section for discharging accelerated streams of electrons from said accelerator structure;
• and
• an rf coupling section intermediate said capture section and said exit section for coupling rf energy into said accelerator structure;
• an rf system including an rf source for converting electrical power to rf power and a transmission conduit for delivering rf power to said coupling section of said accelerator structure, said rf source being a klystron having:
• a collector maintained at ground potential,
• a cathode maintained at a high, constant negative potential of predetermined magnitude;
• a modulated anode maintained at an intermediate voltage while the klystron is conducting current and amplifying the rf pulse;
• a first power supply for maintaining the potential of said cathode;
• and
• a second, separate programmable power supply for controlling the voltage of the modulated anode, said programmable power supply being responsive to a control signal from said control means to apply a predetermined voltage to said modulated anode;
• said control means being operable to determine the operating efficiency of said accelerator and to incrementally change the potential applied to said modulated anode in a first direction when said operating efficiency improves and in the opposite direction when said operating efficiency deteriorates;
• said rf transmission conduit including a microwave window assembly for sealingly separating the interior of said conduit ,from interior of said accelerator structure while permitting the transfer of rf power from said conduit to said accelerator structure, said conduit including a microwave elbow for connecting a coupling cavity of said coupling section with said conduit, said microwave window assembly being positioned within said elbow such that electrons and x-rays originating from within said accelerator structure cannot travel by line-of-sight to said microwave window assembly;
• means disposed at said exit end of said accelerator structure for receiving said one or more streams of electrons and scanning said streams of electrons over a predetermined product area;
• control means for controlling said scanning means and synchronously energizing said stream producing means and said rf source during said pulses at said repetition rate;
• said stream producing means including a Wehnelt controlled electron gun having:
• an anode plate having a central aperture;
• a dispenser cathode for emitting electrons;
• a Wehnelt focusing-electrode assembly for focusing electrons emitted by the cathode through said aperture of said anode plate and into said capture section of said accelerator;
• a resistive heater associated with said cathode for heating said cathode;
• and
• means controlled by said control means for energizing said heater during said pulses to cause said cathode to emit electrons during said pulse;
• said control means being responsive to a signal representative of the resistance of said heater to cause said means for energizing said heater to deliver only sufficient energy to said heater to maintain the resistance of said heater at a predetermined value;
• a housing for axially securing said electron gun to said capture section of said accelerator structure, a first port for connection to a gun ion pump, a second port for connection to a getter vacuum pump for maintaining the pressure within said housing and said accelerator structure to a predetermined level;
• said control means being further operable to maintain the voltage of said cathode at a first predetermined nominal value between pulses and at a second predetermined value during pulses, and for adjusting the voltage on said Wehnelt focusing-electrode so as to provide an electron beam current of a predetermined magnitude during full power operation;
• further including means for measuring the electrical current of the electron beam from said accelerator structure a beam line for transporting said electron beam, a portion of said beam line being connected to but electrically insulated from the balance of said beam line, an axial gap in said beam line portion, a tubular member extending across said gap, said measuring means including a beam current toroid concentrically disposed about said beam line portion, an electrical conductor axially extending between said toroid and said beam line portion and having opposed ends for connection to a pulsed current source, the current flowing through said conductor being representative of the current of said electron beam, said control means being responsive to the magnitude of said beam current to adjust said means for producing said stream of electrons so as to maintain said beam current at a predetermined value, said beam line portion including an annular flange at each end thereof for connection to similar flanges at adjacent ends of the balance of said beam line, an electrically insulating gasket interposed between each said flange and its adjacent flange, each said gasket including a pair of gasket elements separated by a radiation resistant polyimide film joined to said gasket elements by a layer of heat-cured glue;
• and
• a feedback control system for maintaining said beam line current within predetermined limits, said feedback control system including a pulse generator for generating reference current pulses synchronized and coincident with beam current pulses to be measured, said reference pulses being of the opposite polarity to that of said beam line current so that the current in the first mentioned conductor is the differential between the beam line current and the current of said reference pulses, said control system outputting a control signal to said pulse generator tending to reduce said differential to zero.