The hydrogen thyratron is a high peak power electrical switch that uses hydrogen gas as the switching medium. Switching is achieved by a transfer from insulating neutral gas to conducting ionised gas. Exploiting this basic principle, the hydrogen thyratron is designed to operate at high voltage with high peak current pulses and high repetition rates.
The hydrogen thyratron’s unique capabilities make it the ideal switch for pulsing high peak power microwave sources such as magnetrons an klystrons. Additionally, the thyratron is electrically robust. A compelling example is the glass thyratron pictured on the left. The intense glow arises from a peak current of 25 kA passing through the thyratron at 2 Hz. This peak current is more then 20 times the level typically found in a medical linac modulator.
The schematic shows the basic structure underlying thyratron designs. The cathode, grid and anode structures are contained in a vacuum-tight envelope of glass or ceramic sealed to the electrode and heater connections. The envelope is filled with hydrogen at about 66 Pa (0.5 torr). The Paschen curve defines the breakdown voltage in gases as a function of pressure and distance and leads to grid/anode structures separated by 3 mm inside the envelope and by 75 mm outside the envelope. The internal electrode structures are designed to optimise high voltage hold-off, triggering, current pulse rise time and recovery.
The thyratron uses a thermionic cathode heated by a radiant tungsten filament to an operating temperature chosen to ensure good electron emission with low evaporation of the active barium surface. The cathode structure is then able to sustain pulsed current densities in the hydrogen plasma of greater than 100 A/cm2. The hydrogen gas is supplied by a titanium reservoir system. The reservoir replaces hydrogen lost during switching and keeps the hydrogen measure in the thyratron in equilibrium, depending on the reservoir temperature.
e2v’s thyratron designs are based on a sound understanding of plasma theory, experimental verification and extensive, in-house experience to ensure good performance and reliability.
The line-type modulator is the archetypal pulse power circuit, designed to achieve high peak power by compressing the timescale of electrical energy release. In a typical linac, when 20 kW is drawn steadily on the charging side, 5 MW pulses are produced at the load. The thyratron is at the heart of the modulator, controlling the release of the pulse energy and initiating the subsequent charging cycle.
Thyratron switching cycle
The process of switching in a hydrogen thyratron can be broken into four phases: voltage hold-off, commutation, conduction and recovery.
Thyratron high voltage structures are designed in accordance with the Paschen curve and field emission constraints to give reliable performance at 25 kV and above in line-type modulators.
Thyratron commutation is initiated by fast rising trigger pulses to the grid(s) which fill the cathode/grid region with plasma. Apertures in the grids admit the plasma to the grid/anode region and the influence of the anode field. Electrons accelerate across the anode gap creating a conducting plasma path between anode and cathode. The anode voltage collapses in about 20 ns and full conduction proceeds.
Current is carried between cathode and anode with a potential drop of the order of 100 V at a peak current of 1000 A. This low voltage drop results from the shielding effect of the positive ions in the plasma which overcome space-charge limitations.
The hydrogen plasma remaining in the thyratron at the end of the current pulse begins to recover back to neutral hydrogen with a time constant of about 7 microseconds. Recovery of the anode gap is usually complete in about 20 microseconds and the thyratron is ready for the next cycle.