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Linac Klystron (Cross-Platform)

The microwave power amplifier that drives the accelerator waveguide on most clinical medical linear accelerators above ~6 MV. Klystrons amplify a low-power RF input signal (delivered from a small-signal RF source) into the multi-megawatt RF pulses required to accelerate electrons to therapeutic energies in the accelerator waveguide. Magnetrons (oscillator tubes) are the alternative on lower-energy linacs and a few mid-energy designs, but klystrons dominate the high-energy (15+ MV) and dual-energy clinical-linac segment.

Klystron technology is largely vendor-agnostic at the component level — Varian, Elekta, Accuray, and Siemens linacs all use klystrons from a small set of microwave-tube specialists (CPI / EEV / Thales-vendored). OEM service organizations rebadge klystron assemblies with system-specific part numbers, but the underlying tube technology is shared infrastructure. Refurb / aftermarket supply has consolidated as the linac installed base has aged.

Fits

Klystron-equipped linac platforms (representative):

The OEM-specific entry Clinac iX klystron documents the Varian Clinac iX implementation in detail.

Failure modes

  • End-of-life vacuum loss — gradual outgassing inside the klystron envelope. Manifests as beam-current drift, modulation instability, and eventual interlock trips.
  • Cathode aging — the electron-source cathode emits less current over thousands of hours of operation. Increased filament-heating current is required to maintain output, and eventually the cathode no longer sustains specification.
  • Window failures — RF-output windows can fracture under thermal stress.
  • Magnet-coil issues (klystrons use focusing magnets to maintain electron-beam confinement through the tube).
  • Insulation breakdown in the high-voltage modulator interface.

Diagnosis

  • Daily QA beam-output checks — output drift correlated with klystron parameters is the canonical signal.
  • Klystron-current trending in the service log over months — predictive of end-of-life.
  • Modulator pulse-stability monitoring.
  • Filament-heater current — rising filament current to maintain emission predicts cathode end-of-life.

Replacement path

  • Major service event. Klystron replacement involves vault entry, modulator-cabinet access, RF-network disconnection, vacuum-system handling, and a substantial commissioning suite afterward (beam-energy verification, beam-symmetry, output linearity, dosimetric recommissioning).
  • Multi-day downtime is typical.
  • Aftermarket / refurb klystron supply exists for mature platforms (Clinac-era) but is thinning; OEM-new supply remains available for current platforms.

Field notes

  • Klystron lifetime is highly site-dependent — high-volume IMRT / VMAT clinics burn klystrons faster than low-volume conventional clinics. Tube-hour and pulse-count tracking is the canonical wear indicator.
  • Refurb-linac due-diligence — klystron age + most-recent-replacement-date + cumulative tube-hours.
  • Magnetron vs klystron platforms age differently — magnetron failure modes are similar in symptom but the components are smaller, cheaper, and shorter-lived per unit. See Clinac iX magnetron.

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