Introduction of Hydrogen Thyratrons

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Introduction of Hydrogen Thyratrons

Hydrogen Thyratron performance and application 

Hydrogen thyratrons use hydrogen or deuterium as arc discharge dielectric, which have high working voltage , high pulse current , quick and steady fire lighting, low trigger voltage, high efficiency, long life ,light weigh , small size , convenience useful such advantages. Hydrogen thyratrons are widely used in scientific research, military, medical treatment and industry such fields , which include laser instruments, radar, pulse modulator , medical beeline accelerator , tommy bar protection and other electric instruments and equipments.


Structure of Hydrogen Thyratron

Hydrogen Thyratron made up by grid, cathode, anode, hydrogen reservior and ceramic envelope. The basic structure is shown as the drawing. Sometimes Tetrode is developed with a preliminary ignition electrode for a better ignition characteristic and reduces the delayed time of ignition. Grids need to be piled up and made double gaps or more gaps hydrogen thyratron to improve working voltage when working voltage surpasses certain value. Cathode structure of vane style increases the area of transmit, and cathode hot screen reduces heating power to make the temperature equal and at the same time to prevent the material of cathode spattering on the grid. The designs of anode and grid are good for withstanding voltage and giving out heat. Hydrogen reservoir is a special component of hydrogen thyratron. It is a metal envelope, which has heating thread and filled with the powder of
titanium and hydrogen compound, and also there are holes on the wall of tube. When heating thread is electrified and heated, the powder of titanium and hydrogen compound will be decompounded and give out hydrogen, then hydrogen diffuses into hydrogen thyratron through the hole and ensures pd value on the left side of the curve. After the temperature becoming cool, hydrogen is sucked in by holes and forms the powder of titanium and hydrogen compound again and then make the tube at high vacuum state.

Operation principle of Hydrogen Thyratron

Operation process of hydrogen thyratron is a process which gas changes into high conductive state from isolated high voltage state, transforms the storage energy during pulse time into strong power pulse output at the moment of the pulse. The whole process is divided into three phases. When the grid is not added trigger pulse, the isolated high voltage between anode and cathode is at isolated state. After cathode heating thread and hydrogen reservoir are electrified, cathode reaches the temperature of heat emission, the cathode emission electrons get together between cathode and grid. 

Phase 1 --- Grid ignition process

When grid adds trigger pulse, the higher grid voltage , the bigger the grid current . The space between grid and cathode begins to produce ionization when grid voltage rises up to the ionization situation, grid current continues to increase. When grid current increases to grid lighting current, grid begins ignition, grid current increases obviously and grid voltage falls quickly, the space between grid and cathode begins to discharge and forms plasma. 

Phase 2--- Discharge developing process from grid to anode

The higher grid current continues, the bigger plasma between grid and cathode increase quickly and begin to diffuse. Electron diffused around the grid hole moves to anode through the grid hole under anode electric field, which arouses gas ionization between grid and anode and discharge develops from grid to anode.

Phase 3---Hi-Pot process of the whole tube (discharge process from anode to cathode)

After the space between grid and anode discharges, current of anode increases rapidly and voltage of anode falls quickly, then the tube enters into Hi-Pot discharge process. Because most of hydronium in the plasma shield the minus electric field of grid, grid voltage makes no difference on cathode current, so grid is out of control, it also is not provided with the capability of break current. When anode voltage is too low to keep discharging, the discharge will stop, the tube inner appear ionization process. At this time anode current reduce zero , anode voltage also rises to original value ,anode and cathode also resume the state of high voltage isolation , after ionization , grid will resume control function , then repeat the process.

Main electric parameters

  • Heating time: The shortest time to reach temperatures which hydrogen thyratron can work satisfactorily.
  • Peak forward anode voltage: The maximum instantaneous positive voltage applied to the anode vs cathode.
  • Peak inverse anode voltage: The maximum inverse voltage applied to the anode vs cathode.
  • Peak anode current: The highest instantaneous current of anode pulse.
  • Average anode current: The average current of anode pulse.
  • Anode pulse duration: The time interval between the instant at which the value of the parameter equals 70% of the pulse amplitude, unless otherwise stated.
  • The maximum pulse power: The maximum output power of hydrogen thyratron is a half of peak forward anode voltage multiply pulse current
  • Average output power: The maximum average output power of hydrogen thyratron is a half of the highest anode voltage multiply the highest average current.
  • Waste coefficient of anode power: Pulse current, anode voltage multiply pulse repeat frequency, which means the biggest heat waste that tube allowed.
  • Grid ignition and start-up performance: When trigger pulse range is width enough, grid and cathode ignition time will shorten and get a steady value with trigger pulse slope increasing, so the bigger the trigger pulse slope and, the shorter the ignition time is. When anode voltage is at a certain value, discharge grid current from grid to anode must be higher than start-up current

Typical application circuitry 

Hydrogen thyratron is used as pulse modulator in radar, accelerator etc, typical circuitry as the following. Modulator starts to work, hydrogen thyratron keeps close state. High voltage power charges PFN to a rated value through charge inductance and charge diode. When grid is added to trigger pulse, hydrogen thyratron will be made. PFN discharges through hydrogen thyratron and pulse transformer, pulse transformer outputs a certain pulse width and repeats frequency torque pulse. Pulse width depends on PFN, repeats frequency depends on repeat frequency of trigger pulse.
The grounded grid thyratron is one kind of especial hydrogen thyratron, which has the hydrogen thyratron and the spark gaps advantages. The grounded grid thyratron can be applied under low impedance circuitry, and can get a needed current of short pulse width and high peak current. Structure of the grounded grid thyratron is the same as hydrogen thyratron, including cathode, anode and grid etc. Grid is used as discharge cathode in the grounded grid thyratron, so the grid is made of a special material of metal. The grid flange connects the earth, and the cathode adds negative trigger pulse.

Model Selection and Use Notes for Hydrogen Thyratrons:

User can choose suitable hydrogen thyratrons according to different purpose. User should retain appropriate margin for the parameters, all the parameters can not reach to limit value at the same time. 

Preparatory Work

  • Check the package, no damage or inversion
  • Open the package and take out the clean glove. User can not touch the ceramic housing of the hydrogen thyratron with hands.
  • Take out the sponge and thyratron carefully.
  • Check the thyratron and confirm if there is any pollution, oxidation, ceramic crack on the thyratron surface. The color and length of the lead should meet with the specification. 

Installation and Cooling 

  • Hydrogen thyratron must keep vertical (anode upward) or horizontal direction and can not be turned over or powder/particle on the cathode will enter into the anode, and lead to unnormal work, even arcing and damage;
  • When installing the hydrogen thyratron, user need to properly connect the electric poles and soft leads to the circuit base on the outline;
  • When hydrogen thyratron needs cooling, user should adopt the cooling method as prescribed. If adopting air cooling, the wind direction must bring into correspondence with vertical direction.

Preheating

  • Preheating: It means the required shortest heating time for thyratron realizing normal operation;
  • Before the high pressure, increase the heating voltage to rated value and preheat for enough time 
  • After applying the heating voltage, increase it slowly to the rated value. 

Notes for Hydrogen Thyratrons

  • The main parameters can not reach the limit value at the same time.
  • Choose the suitable cooling method based on the hydrogen thyratrons, specification, or it will get high temperature and might not work properly and even be damaged.
  • Don’t touch the hydrogen thyratron immediately after stopping operation, or user might get scalded.
  • User should avoid filament subcooling or overheating, since hydrogen thyratrons will be damaged instantly under subcooling operation and it won’t be recovered.
  • Unauthorized modification on the hydrogen thyratron is forbidden.
  • Take protection measures to avoid the harm of X-Ray.