Microwave vacuum tube device employing grid-modulated cold...

Details Microwave vacuum tube device employing grid-modulated cold... Microwave vacuum tube device employing grid-modulated cold...

2000-08-04

2001-10-02

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Cathode ray tube circuits

Combined cathode ray tube and circuit element structure

C315S005180, C313S045000

Reexamination Certificate

active

06297592

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to microwave vacuum tube devices, in particular tubes modulated by a proximately positioned grid structure, i.e. gridded tubes.
2. Discussion of the Related Art
Microwave vacuum tube devices, such as power amplifiers, are essential components of many modern microwave systems including telecommunications, radar, electronic warfare and navigation systems. While semiconductor microwave amplifiers are available, they generally lack the power capabilities required by most microwave systems. Microwave vacuum tube amplifiers, in contrast, can provide higher microwave power by orders of magnitude. The higher power levels of vacuum tube devices are the result of the fact that electrons can travel at a much higher velocity in a vacuum with much less energy losses than in a solid semiconductor material. The higher speed of electrons permits a use of the larger structure with the same transit time. A larger structure, in turn, permits a greater power output, often required for efficient operations.
Microwave tube devices typically operate by introducing a beam of electrons into a region where the beam interacts with an input signal, and deriving an output signal from the thus-modulated beam. See, e.g., A. S. Gilmour, Jr., Microwave Tubes, Artech House, 1986, 191-313. Microwave tube devices include gridded tubes (e.g., triodes, tetrodes, and klystrodes), klystrons, traveling wave tubes, crossed-field amplifiers and gyrotrons. All require a source of emitted electrons. For example, a conventional klystrode
10
is shown in FIG.
1
. The klystrode contains 5 main elements—a cathode
12
, a grid
14
, an anode
16
, a tail pipe
18
, and a collector
20
. The whole tube is optionally placed in a uniform magnetic field for beam control. In operation, a RF voltage is applied between the cathode
12
and grid
14
by one of several possible circuit arrangements. For example, it is possible for the cathode to be capacitively coupled to the grid or inductively coupled with a coupling loop into an RF cavity containing the grid structure. The grid
14
regulates the potential profile in the region adjacent the cathode, and is thereby able to control the emission from the cathode.
The resulting density-modulated (bunched) electron beam
22
is accelerated toward the apertured anode
16
at a high potential. The beam
22
passes by a gap
19
, called the output gap, in the resonant RF cavity and induces an oscillating voltage and current in the cavity. RF power is coupled from the cavity by an appropriate technique, such as inserting a coupling loop into the RF field within the cavity. Finally, most of the beam passes through the tail pipe
18
into the collector
20
. By depressing the potential of the collector
20
, some of the dc beam power can be recovered to enhance the efficiency of the device. Demonstrated efficiency of such devices is relatively high, e.g., reaching 50% at 1 GHz, and the typical gain is about 25 dB at 1 GHz.
The usual source of electrons for such microwave tube devices is a thermionic emission cathode, which is typically formed from tungsten that is either coated with barium or barium oxide, or mixed with thorium oxide. Thermionic emission cathodes must be heated to temperatures around 1000° C. to produce sufficient thermionic electron emission current, e.g., on the order of amperes per square centimeter. (As used herein, thermionic cathode indicates a cathode that must be heated to at least 800° C. to provide measurable emission.) The necessity of heating thermionic cathodes to such high temperatures creates several problems. For example, the heating limits the lifetime of the cathodes, introduces warm-up delays, requires bulky auxiliary equipment for cooling, and tends to interfere with modulation of emission in gridded tubes. The limited lifetime is due to the fact that the high operating temperatures cause constituents of the cathode, such as the barium or barium oxide, to evaporate from the hot surface. It is possible for the evaporated barium or barium oxide to then deposit onto the grid, which causes undesirable grid emission that essentially renders the device ineffective. Moreover, once the barium is depleted from the cathode, the cathode (and hence the tube) no longer functions. Many thermionic vacuum tubes therefore have operating lives of less than a year. The delay in emission is due to the time required for temperature ramp-up, and delays as long as four minutes are not uncommon. Such delays are unacceptable for many applications.
For gridded tubes, such as the klystrode
10
of
FIG. 1
, the high temperature environment near the grid electrode tends to introduce thermally induced geometrical and/or dimensional instability that changes the cathode-grid spacing, e.g., due to thermal expansion mismatch or structural sagging. These changes to the spacing tend to significantly interfere with the ability of the grid to modulate the cathode emission, and thus interfere with the overall operation of the gridded tube. Moreover, there is a certain minimum cathode-grid spacing that must be maintained, to ensure that such dimensional changes do not result in contact between the cathode and grid. Because of this minimum spacing requirement, it is not possible to move the cathode and grid closer together in order to decrease the cathode-grid transit time, which would in turn increase the maximum operating frequency of the device. For this reason, the frequency of gridded tubes with thermionic cathodes is limited.
Thus, there is a need for an improved electron source for microwave tube devices, particularly gridded tubes, which avoids problems of conventional devices and is able to reduce transit times.
SUMMARY OF THE INVENTION
The invention relates to an improved gridded-type microwave tube, in which a cold cathode containing carbon nanotube emitters is used. Use of the cold cathode avoids the problems encountered with thermionic cathodes, and allows the grid and cathode spacing to be substantially reduced, thereby reducing transit time of the electrons. In fact, a microwave tube of the invention generally exhibits a transit time at least 2× shorter than a similar tube having a thermionic cathode. And the operating frequency of a microwave tube of the invention is generally enhanced at least 2× compared to a similar tube having a thermionic cathode.
The gridded tube of the invention contains a cold cathode, an anode, and a grid located between the anode and cathode, such as shown by the conventional klystrode of FIG.
1
. In one embodiment, the cold cathode has a refractory metal substrate and carbon nanotube emitters. The nanotube emitters have a diameter of 1 to 300 nm and a length of 0.05 to 100 &mgr;m (length indicating the actual length of the nanotubes regardless of their geometrical configuration). Advantageously, the height of the nanotubes from the cathode substrate is relatively uniform, e.g., at least 90% of the nanotubes have a height within about 20% of the average height. The grid-cathode spacing is 1 to 100 &mgr;m, the grid contains apertures having a maximum dimension of 0.1 to 100 &mgr;m, and the grid thickness is 0.1 to 50 &mgr;m. Emission from the cathode directly onto the grid material itself, which undesirably heats the grid, is reduced by either (a) the presence of a shadow mask on the emitters or (b) selective formation of the emitters in locations that correspond to the grid apertures. The microwave tube operates at a frequency of greater than 0.5 GHz, advantageously greater than 2 GHz.


REFERENCES:
patent: 4453108 (1984-06-01), Freeman, Jr.
patent: 5680011 (1997-10-01), Makishima
patent: 5796211 (1998-08-01), Graebner et al.
patent: 5872422 (1999-02-01), Xu et al.
patent: 5935639 (1999-08-01), Sullivan et al.
patent: 5977693 (1999-11-01), Nakamoto et al.
patent: 6114808 (2000-09-01), Takahashi
A. S. Gilmour, Jr., Microwave Tubes, Artech House, pp. 191-313 (1986).
Ren et al.,Science, vol. 282, 1105 (1998).
Fan et al.,Science, vol. 283, 12 (1999).
Rinzler et al.,S

Affiliated with

Also associated with

Rating

Microwave vacuum tube device employing grid-modulated cold... does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Microwave vacuum tube device employing grid-modulated cold..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Microwave vacuum tube device employing grid-modulated cold... will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2603370