The Perkin Elmer thyratrons are high energy switches capable of operation up to 20 kA and 75 kV. A wide range of standard thyratrons are offered, all constructed of rugged ceramic and metal parts. These tubes are typically used in applications such as gas laser, radar, and other modulator applications. Five basic sizes from I to 41/2 inches in diameter define the thyratron types in production at Perkin Elmer.


Design Features

The basic thyratron is a three electrode, low pressure gas filled vacuum tube, with a thermionically emitting cathode (See Figure 8). It is a triggerable, closing only electronic switch. This basic design is used in the vast majority of applications. However, additional features have been added in order to enhance performance in certain situations.

Auxiliary Grid (TETRODE)

Basic thyratrons usually require several hundred nanoseconds to switch "on" (time measured from a standard fixed level on the rise of the trigger pulse until a fixed level is reached on the rise of the current discharge waveform). Addition of an auxiliary grid can substantially reduce the switch-on delay time (Figure 9). In actual usage, the switch-on delay time is inversely related to the current being driven in the auxiliary grid-cathode circuit until a current level is reached where little or no further reduction of the turn on delay time is realized.

The auxiliary grid may be run in the "DC primed," "pulsed," or "combination" modes. The most common mode is the "DC primed" mode. In this mode, a DC voltage is applied between the auxiliary grid (+) and the cathode (chassis ground). There is usually a series resistor to limit the DC current in the auxiliary grid circuit. When the auxiliary grid is operated in the "pulsed" mode, a positive (with respect to the cathode) pulse is applied to the auxiliary grid which draws a current that can be up to an order of magnitude greater than would be the case for the "DC primed" example. The pulse is applied to the auxiliary grid about I microsecond prior to pulsing the control grid. The auxiliary grid pulse may end at any time after the control grid pulse has been on for at least I microsecond.

Operating the auxiliary grid in the "combination" mode combines both "DC primed" and "pulsed" modes. In all cases, gas is ionized between the cathode and the auxiliary grid when the control grid is pulsed. Therefore, the time required to ionize the gas and establish electron flow to the auxiliary grid level is already past, leaving only the time required to spread the plasma from the auxiliary grid level through the control grid level. The total time from application of the control grid pulse to the start of main discharge current flow is substantially reduced.

Liquid Cooling

For several reasons, it is desirable for the tube's electrodes to achieve at least certain minimum temperatures during operation. However, where a high anode heating factor and/or high RMS current is required in the application, excessive heat may develop. In these cases liquid cooling the thyratron may allow operation at those otherwise unacceptable conditions. If liquid cooling is not possible or sufficient, the next larger tube size is recommended. Two methods of liquid cooling the tube are in general use, immersion and circulation. In some cases, the desired performance may be achieved by simply immersing the tube in a coolant bath or inlet temperature not to exceed 30°C. In demanding situations it is desirable to circulate the liquid toward certain tube "hot spots" to achieve a higher degree of cooling. To this end, Perkin Elmer offers thyratrons with cooling pipes attached to the anode and/or grid area to allow direct circulation of liquid for cooling.

Hollow Anode

Basic thyratrons are much like normal vacuum tubes in that they have a heated cathode and a relatively cool flat, solid anode. The flat, solid anode is not a good electron emitter and therefore the tube will act like a diode when a reverse voltage (i.e., negative with respect to the cathode) is applied to the anode (it will not conduct in the reverse direction). This characteristic is desirable in most instances. However, in some under-damped pulse circuits, the reverse (negative) voltage on the flat, solid anode becomes high enough to cause conduction for one or more reverse half-cycles of the discharge current waveform. In those instances, anode damage is caused by cathode spot formation. A cathode spot is a very localized molten spot that is high enough in temperature to emit electrons. Unfortunately, the temperature of this spot is usually high enough to liquefy the anode material and evaporate it from the anode surface. Not only does this result in the loss of anode material, but it usually results in deposition of some of that material on the ceramic insulators of the tube. When the insulators become locally conductive near the very high electric field space that exists between the anode and the grid, arcing from anode to grid through that locally conductive layer usually results and further leads to deteriorating hold off performance of the tube.

To improve electron emission of the anode during reverse half cycles, holes are made in the anode's flat face that lead to a cavity behind that face. A significant number of electrons enter those holes on the forward half cycle of the tube's conduction. These electrons are available to contribute to reverse current flow during the reverse half-cycles of the discharge waveform since many have not yet had time to become trapped within the metal walls of the anode. Perkin Elmer hollow anode thyratrons are usually rated to conduct in the reverse direction up to 40% of the peak current that was conducted on the immediately previous positive half cycle of the current discharge waveform. Higher reverse current levels are achievable at the expense of some damage to the anode. However, since the electron emission predominates from cathode spots that are formed within the holes or cavity of the anode, the metal vapor byproducts of those spots do not coat the ceramic insulator. Hollow anode thyratrons do "wear" at a higher rate as the magnitude of reverse current increases, but they do not generally exhibit the rapidly deteriorating hold off capacity of solid anode tubes.

Grounded Grid

In applications with extremely high peak current and rate of rise of current, a grounded grid thyratron is the best choice. The thermionic cathode of the grounded grid thyratron only has to inject enough electrons into the space between the grid baffle and the grid itself to start the hollow cathode process that is described in most "pseudospark" switch literature. Almost all of the electrons that are involved in the grid-anode discharge are generated at cathode spots in the hollow cathode area that is located in the grid slots and at the back surface of the grid away from the grid face (the large flat area that directly faces the anode). Proper design of the slot area, grid material, and spacing of the grid from the anode will allow extremely high peak currents at extremely short rise times while minimizing grid wear. Currently grounded grid thyratrons all have had solid, flat anodes. Anode and grid wear due to cathode spots has been the life-limiting factor in the grounded grid tube. See the individual tube's data sheet for more specific information.

Low Inductance

Perkin Elmer's low inductance tubes are designed to fit into very tight discharge loop circuits. Of necessity, this generally means that they have an overall seated height that is significantly shorter than the length of a tube that has similar electrical parameters but one expected to operate in a circuit with more inductance.

In essence, the low inductance tubes are just "normal" thyratrons that have been severely repackaged to fit into the smallest height circuit that is reasonably possible.

Because of the significant overall height reduction in the thyratrons, the length of the high voltage stand-off ceramics may not be sufficient for operation at normal ambient atmospheric conditions. Under normal conditions, 10 kV per inch may be applied across the ceramics. Industry standard de-rating data should be consulted for the voltage that may be applied to the ceramic under other conditions. Most users of low inductance tubes usually find it necessary to operate those tubes in pressurized alternative gases or dielectric liquids.

Current Derating Curve

Perkin Elmer data sheets will generally list the maximum peak current capability of the individual thyratron under a section titled "Absolute Ratings." That peak current is usually specified at a reasonably short duration current pulse width such as 250 nsec and it generally remains the maximum peak current rating for pulses shorter than the pulse width that is listed. However, as the pulse width is increased in time, the peak current limit must be reduced. The formula for rating the peak current capability of a thyratron at wider pulse widths is as follows:

i_bt = i_b3(3/t_p)^1/2

where i_b3 is the peak current rating at the pulse width listed for the absolute maximum limit; t_p is the pulse width of interest (in µS); i_bt is the peak current rating at the pulse width of interest.

Thyratron Lines

1 inch thyratrons: The typical thyratron in this line is able to switch 100 A at up to 8 kV. "Low power" pulsers and Pockels cell drivers are typical of the applications for these tubes.

1 1/2 inch thyratrons: The typical thyratron in this line is able to switch 350 A at up to 16 kV. Many mechanical and a few electrical variations of the basic tube design are readily available. Typical applications are in Pockels cell drivers, radar transmitters, and portable gas lasers.

Product availability is subject to change without notice. For current pricing and availability, please contact your local Richardson sales office.

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