A d. c. charged particle accelerator comprising: accelerator electrodes (10, 11, 12, 13) including end electrodes (10, 11) and at least N-1 intermediate electrodes (12, 13), said accelerator electrodes (10, 11, 12, 13) defining at least N acceleration gaps between adjacent pairs of said electrodes, where N is at least three; and regulated d.c. high voltage power supply apparatus having N pairs of output lines (29, 30, 31, 32, 33, 34, 63, 64, 65, 66, 67, 68) connected to respective adjacent pairs of said accelerator electrodes (10, 11, 12, 13) defining said N acceleration gaps, said regulated d.c. high voltage power supply apparatus operative to provide N independently regulated high voltage d.c. output voltages on said N pairs of output lines (29, 30, 31, 32, 33, 34, 63, 64, 65, 66, 67, 68) to provide independent gap voltages across said N acceleration gaps, said N regulated high voltage d.c. output voltages being electrically isolated from each other; wherein said regulated d. c. high voltage power supply apparatus comprises: N independent d. c. voltage generators (26, 27, 28, 60, 61, 62), each of said voltage generators (26, 27, 28, 60, 61, 62) having a respective one of said N pairs of output lines (29, 30, 31, 32, 33, 34, 63, 64, 65, 66, 67, 68) and being arranged to independently generate a respective one of said N regulated high voltage d. c. output voltages from input electric power delivered to said voltage generator (26, 27, 28, 60, 61, 62); and d. c. isolating power delivery apparatus arranged to deliver said input electric power to said N voltage generators (26, 27, 28, 60, 61, 62) and configured to maintain d. c. isolation between said voltage generators; wherein said N independent d. c. voltage generators comprise N independent d. c. high voltage power supply units (60, 61, 62) providing said regulated high voltage d. c. outputs from unregulated input electric power; wherein said d. c. isolating power delivery apparatus comprises N alternators (72, 73, 74) to deliver a. c. power as said unregulated input electric power to respective said d. c. high voltage power supply units (60, 61, 62); wherein each of said alternator (72, 73, 74) comprises a stator (75, 76, 77) having at least one stator winding and a rotor carrying at least one rotor magnet (78, 79, 80) to produce a rotating magnetic field in said stator (75, 76, 77) to induce alternating currents in said stator winding to provide said a. c. power; wherein said stators (75, 76, 77) of at least two of said alternators (72, 73, 74) are axially aligned; and said d. c. isolating power delivery apparatus comprises at least one common rotor shaft carrying, axially spaced along said shaft, said rotors (78, 79, 80) of said at least two alternators (72, 73, 74), and a motor connected to rotate said common rotor shaft. accelerator electrodes (10, 11, 12, 13) including end electrodes (10, 11) and at least N-1 intermediate electrodes (12, 13), said accelerator electrodes (10, 11, 12, 13) defining at least N acceleration gaps between adjacent pairs of said electrodes, where N is at least three; and regulated d.c. high voltage power supply apparatus having N pairs of output lines (29, 30, 31, 32, 33, 34, 63, 64, 65, 66, 67, 68) connected to respective adjacent pairs of said accelerator electrodes (10, 11, 12, 13) defining said N acceleration gaps, said regulated d.c. high voltage power supply apparatus operative to provide N independently regulated high voltage d.c. output voltages on said N pairs of output lines (29, 30, 31, 32, 33, 34, 63, 64, 65, 66, 67, 68) to provide independent gap voltages across said N acceleration gaps, said N regulated high voltage d.c. output voltages being electrically isolated from each other; N independent d. c. voltage generators (26, 27, 28, 60, 61, 62), each of said voltage generators (26, 27, 28, 60, 61, 62) having a respective one of said N pairs of output lines (29, 30, 31, 32, 33, 34, 63, 64, 65, 66, 67, 68) and being arranged to independently generate a respective one of said N regulated high voltage d. c. output voltages from input electric power delivered to said voltage generator (26, 27, 28, 60, 61, 62); and d. c. isolating power delivery apparatus arranged to deliver said input electric power to said N voltage generators (26, 27, 28, 60, 61, 62) and configured to maintain d. c. isolation between said voltage generators;