A synchrocyclotron (10; 502; 604) comprising: a particle source (90, 700) to provide pulses of ionized plasma to a cavity; a voltage source to provide a radio frequency voltage to the cavity to accelerate particles from the plasma column outwardly; and an extraction channel to receive a beam of particles from the cavity for output from the synchrocyclotron, wherein the voltage source comprises a first dee and a second dee (710, 711; 100, 102), wherein at least one of the first dee or the second dee has a direct current bias voltage applied thereto to reduce effects of multi-pactoring and thereby increase an intensity of the beam of particles; wherein one of a), b), c), or d), a) wherein the voltage source is configured to vary the radio frequency voltage in order to control the intensity of the beam of particles; b) wherein the particle source is configured to selectively output the pulses of the ionized plasma in order to control the intensity of the beam of particles; c) wherein the voltage source is configurable to vary a slope of the radio frequency voltage in order to control the intensity of the beam of particles; and d) wherein the particle source is configured to provide the pulses of the ionized plasma at specific frequencies proximate to a decrease from a maximum frequency of the radio frequency voltage to a minimum frequency of the radio frequency voltage during a sweep of the radio frequency voltage between the maximum frequency and the minimum frequency. a particle source (90, 700) to provide pulses of ionized plasma to a cavity; a voltage source to provide a radio frequency voltage to the cavity to accelerate particles from the plasma column outwardly; and an extraction channel to receive a beam of particles from the cavity for output from the synchrocyclotron, wherein the voltage source comprises a first dee and a second dee (710, 711; 100, 102), wherein at least one of the first dee or the second dee has a direct current bias voltage applied thereto to reduce effects of multi-pactoring and thereby increase an intensity of the beam of particles; wherein one of a), b), c), or d), a) wherein the voltage source is configured to vary the radio frequency voltage in order to control the intensity of the beam of particles; b) wherein the particle source is configured to selectively output the pulses of the ionized plasma in order to control the intensity of the beam of particles; c) wherein the voltage source is configurable to vary a slope of the radio frequency voltage in order to control the intensity of the beam of particles; and d) wherein the particle source is configured to provide the pulses of the ionized plasma at specific frequencies proximate to a decrease from a maximum frequency of the radio frequency voltage to a minimum frequency of the radio frequency voltage during a sweep of the radio frequency voltage between the maximum frequency and the minimum frequency. a) wherein the voltage source is configured to vary the radio frequency voltage in order to control the intensity of the beam of particles; b) wherein the particle source is configured to selectively output the pulses of the ionized plasma in order to control the intensity of the beam of particles; c) wherein the voltage source is configurable to vary a slope of the radio frequency voltage in order to control the intensity of the beam of particles; and d) wherein the particle source is configured to provide the pulses of the ionized plasma at specific frequencies proximate to a decrease from a maximum frequency of the radio frequency voltage to a minimum frequency of the radio frequency voltage during a sweep of the radio frequency voltage between the maximum frequency and the minimum frequency.