Radiotherapeutic apparatus

Radiotherapeutic apparatus comprising a patient support, magnetic coils disposed around the patient support for creating a magnetic filed therewithin, a radiation source producing a beam of radiation directed toward the patient support and mounted on a rotatable source thereby to rotate the radiation source around the patient support. A slip ring for conveying electrical power to the radiation source. A substantially airtight chamber surrounding and enclosing at least the slip ring, the patient support lying outside the substantially airtight chamber. In another aspect a power supply for conveying electrical power to the radiation source comprising, in sequence, a step-up transformer, a slip ring, and a step down transformer.