Patent Application
20060091831
The present invention relates generally to inductive output tubes. More particularly, the present invention relates to an inductive output tube adapted to operate in the L-band frequency range.
Since the late 1980s the Inductive Output Tube (also known as an “IOT” and a brand of which is marketed by Eimac under the trademark “Klystrode®”) has established itself as a useful device for broadcast, applied science and industrial applications in the UHF frequency range, typically operating in the 100 MHz-900 MHz range. Compared to a klystron, the IOT compensates for its lower gain with both superior efficiency and linearity, and it outperforms the tetrode, its next of kin in the electron device family, with regard to power capability and gain. However, it has long been thought that transit time effects limit the useful frequency range of IOTs to frequencies below 1000 MHz. It has been a commonly held belief in the industry that 1000 MHz is a hard threshold beyond which the performance of IOTs as fundamental frequency amplifiers would fall off rapidly.
The idea of employing higher-harmonic versions of IOTs at higher frequency bands was born early on. In a second-harmonic IOT, for example, the frequency-sensitive grid-cathode circuit (see, e.g., U.S. Pat. No. 5,767,625 entitled High Frequency Vacuum Tube with Closely Spaced Cathode and Non-Emissive Grid to Shrader et al.) could still be operated reliably in the well-experienced UHF regime, while the re-entrant output cavity could be tuned to a higher harmonic in an L-Band frequency. The main drawback to this approach is the relative length of the electron bunch that the low drive frequency forms. During its passage through the output gap the RF voltage in the output cavity changes its polarity twice: from the acceleration into the deceleration phase and back. Although the maximum of the current passes within the deceleration phase and thus ensures power conversion into the desired frequency, a considerable amount of electrons become accelerated, marginalizing efficiency and gain and causing problems with collector dissipation and X-ray radiation.
An investigation was conducted to see how far up in frequency the fundamental-frequency IOT could be tuned in computer simulation without jeopardizing its performance characteristics, particularly the operation of its critical grid-cathode configuration. An existing one-dimensional IOT computer code of proven reliability was modified to include the effects of grid-cathode transit time into the simulation.
As a first step an IOT electron gun with an established track record in UHF broadcast and science applications was analyzed to determine the change of electron bunch waveform and fundamental RF current versus frequency. The results of the simulation are shown in
Accordingly, it would be highly desirable to develop a fundamental mode L-band IOT with reasonable performance characteristics.
An inductive output tube (IOT) adapted to operate at frequencies above 1000 MHz includes a cathode for emitting a linear electron beam; a grid comprised of non-electron emissive material for density modulating the beam, wherein an input RF signal is applied between the cathode and the grid; an anode for forming an electric field in combination with the cathode for accelerating the beam; a collector for collecting the spent beam (which may be of the single-stage or multi-stage depressed collector (MSDC) type); and an output cavity resonant to a frequency of the input RF signal, which is positioned between the anode and the collector. Electrons passing through the interaction gap within the cavity induce an RF field in the cavity. A coupler responsive to the RF signal couples the RF power from the cavity to the load.
In an aspect of the invention an output window is provided to separate a vacuum portion of the IOT from an atmospheric pressure portion of the IOT, the output window being surrounded by a cooling air manifold, the manifold including an air input port and a plurality of apertures permitting cooling air to move from the port, through the manifold and across the window into the atmospheric pressure portion of the IOT.
In another aspect of the invention the output cavity includes a liquid coolant input port; a lower coolant channel coupled to receive liquid coolant from the liquid coolant input port; a vertical coolant channel coupled to receive liquid coolant from the lower coolant channel; an upper coolant channel coupled to receive liquid coolant from the vertical coolant channel; and a liquid coolant exhaust port coupled to receive liquid coolant from the upper coolant channel.
In yet another aspect of the invention the output cavity includes a vacuum tight diaphragm which can be moved into and out of the output cavity by manipulating a tuning control accessible on the exterior of the IOT. The tuning control may be bolt moving in threads or another mechanical component adapted to move the diaphragm in and out of the output cavity. Movement of the diaphragm causes a corresponding change in the resonant frequency of the output cavity.
Other aspects of the inventions are described and claimed below, and a further understanding of the nature and advantages of the inventions may be realized by reference to the remaining portions of the specification and the attached drawings.
The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more embodiments of the present invention and, together with the detailed description, serve to explain the principles and implementations of the invention.
In the drawings:
Embodiments of the present invention described in the following detailed description are directed at L-band IOTs. Those of ordinary skill in the art will realize that the detailed description is illustrative only and is not intended to restrict the scope of the claimed inventions in any way. Other embodiments of the present invention, beyond those embodiments described in the detailed description, will readily suggest themselves to those of ordinary skill in the art having the benefit of this disclosure. Reference will now be made in detail to implementations of the present invention as illustrated in the accompanying drawings. Where appropriate, the same reference indicators will be used throughout the drawings and the following detailed description to refer to the same or similar parts.
In the interest of clarity, not all of the routine features of the implementations described herein are shown and described. It will, of course, be appreciated that in the development of any such actual implementation, numerous implementation-specific decisions must be made in order to achieve the developer's specific goals, such as compliance with application- and business-related constraints, and that these specific goals will vary from one implementation to another and from one developer to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of engineering for those of ordinary skill in the art having the benefit of this disclosure.
Based on the findings discussed above, a complete 1300 MHz/15 kW continuous wave IOT was simulated, maintaining the above-described gun configuration. The simulated fundamental mode IOT in accordance with an embodiment of the present invention operating at 1300 MHz at a power output level of 16.4 kW results are in Table 1. Operational data for the simulated IOT is set forth in Table 1 set forth below.
Accordingly, a prototype unit was built in accordance with these principles by modifying an existing EIMAC K2 Series UHF IOT to operate at 1300 MHz. The external UHF output section was replaced with an internal 1300 MHz resonator. A 1⅝-inch diameter coaxial output feeder was used which contains an alumina window of the same type commonly used with L-Band klystron devices. The cavity is water-cooled as described in detail below in order to remove waste heat from the cavity as well as to provide stability against de-tuning which above 1000 MHz becomes much more critical than at lower frequencies.
The input circuit is more complex. The input impedance of an IOT is of the order of 10 ohms, thus the input circuit has to transform the impedance downward from that of the input feeder (typically 50 ohms), instead of upward as in the case of a klystron. The input signal has to be transferred safely and reliably from the ground level to the high-voltage DC potential of the electron gun assembly. High-voltage-safe dimensions and low impedance are not easily married. The input circuit utilized on the 1300 MHz IOT is a modified version of a conventional UHF IOT input circuit. The tuning paddle has been removed and a stub tuner has been added for the purpose of matching the drive signal to the tube. This is shown in
Turning now to
Operating the IOT 43 at L-Band frequencies results in a relatively large amount of waste heat being deposited in the structure of the output cavity 52. Absent an efficient mechanism for removing this waste heat, the waste heat would result in distortion of the structure of the output cavity 52 and consequent undesired distortions in the output signal. For example, any shift in the size or shape of the output cavity 52 would likely change the resonant frequency of the structure and thus its impedance at a given operating frequency. To reduce or eliminate these distortions, a cooling system is provided for the output cavity 52. A liquid coolant such as pressurized deionized water (or another suitable liquid coolant such as a cooling oil, air, polyethylene glycol, polyethylene glycol mixed with water, mixtures of deionized water and other materials or other well-known non-corrosive coolants) is provided to the cooling system through input port 70. From port 70 the liquid coolant passes into lower chamber 72 where it circulates about the lower chamber (which may be formed in a circular or other convenient shape) to remove heat from the structure, then passes through port 74 into vertical channel 76 (there is preferably a single vertical channel) and up through vertical channel 76, through port 78 and into upper chamber 80 (which may be formed in a circular or other convenient shape) where it circulates to remove heat from the structure, through port 82 and out water exhaust port 84. The structure of the output cavity 52 may be constructed, for example, of oxygen-free high-conductivity copper to provide good thermal conductivity and low corrosion so that the waste heat is efficiently removed by the output cavity cooling system.
The output cavity 52 can be tuned slightly in frequency. In order to accomplish this, a diaphragm 88 is mounted on a flexible flange 90 (
As with all linear beam types, the L-Band IOT design can be fabricated with a multi-stage depressed collector (MSDC), fed with a plurality of power supplies if desired.
The integral output cavity 52 used in the present invention includes its resonant structure as a part of the vacuum envelope, whereas the more common method for IOTs is to use an external tuning box to adjust the resonant frequency. This approach yields a tube of a relatively fixed frequency, but manufacturing variations may result in the tube having a resonant frequency that is slightly different than that desired. Accordingly, the diaphragm and flange tuning system described in detail above is used herein to adjust the volume of the integral output cavity 52 for the purpose of fine-tuning the resonant frequency of the IOT.
Table 2 lists typical test results for output power levels in the 20-30 kW range.
It is believed that these tests mark the first time that an IOT had been operated at a frequency beyond the UHF band (i.e., above 1000 MHz).
While embodiments and applications of this invention have been shown and described, it will now be apparent to those skilled in the art having the benefit of this disclosure that many more modifications than mentioned above are possible without departing from the inventive concepts disclosed herein. Therefore, the appended claims are intended to encompass within their scope all such modifications as are within the true spirit and scope of this invention.
