Electric lamp and discharge devices: systems – Cathode ray tube circuits – Combined cathode ray tube and circuit element structure
Reexamination Certificate
2000-02-11
2001-10-09
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Combined cathode ray tube and circuit element structure
C315S005320, C315S005440, C330S044000, C330S045000, C331S079000, C331S082000
Reexamination Certificate
active
06300715
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to radiofrequency generators of very high power. These generators, used notably for scientific applications, must operate at frequencies of the order of 20 to 200 MHz, or even slightly higher, and in a pulsed regime provides peak powers of several tens of megawatts. In the continuous regime the powers are significantly lower.
DESCRIPTION OF THE PRIOR ART
These generators make use of vacuum tubes. At these frequencies classical grid tubes such as tetrode-type tubes are limited to a few megawatts of power, the present limit being about 10 MW.
Klystrons can supply these powers but they work at microwave frequencies, in other words much higher frequency.
Inductive Output Tubes (IOT) are used in the frequency range of the grid tubes at much smaller powers; they are mainly used in UHF television transmitters. They are configured as shown in FIG.
1
. By adapting their input and output resonant circuits to the desired frequency they can also be used at shortwave and VHF frequencies. These tubes would appear to be the most suitable to obtain the required performance since they are derived from klystron technology.
Nonetheless, they still present certain problems. An IOT has an axial electron beam and applies the principle of amplitude modulation on its input, as in classical grid tubes; the output uses the axial structure of speed modulation tubes, as in klystrons.
More precisely, the tube contains successively an electron gun
1
built around an axis of revolution XX′ and along the axis an anode
5
forming the first drift tube
5
which opens into an interaction space
6
of a single output resonant cavity
7
, the interaction space
6
being delimited by a second drift tube
8
axially facing the first drift tube
5
, then a collector
15
. The two rims of the drift tubes lie axially opposite each other. The gun
1
includes a cathode
2
, a heating filament
3
and a grid
4
. The cathode
2
/grid
4
space forms the input circuit and the routing of the input signal E to the input circuit of the tube is generally via a coaxial resonant input cavity
9
coupled to the cathode/grid space. The input signal E to be amplified is introduced into the cavity
9
by looped inductive coupling means in the embodiment described. This input signal E is supplied by means external to the tube, generally including a preamplifier (not shown).
The grid
4
and cathode
2
are raised to continuous high negative voltages and the electrons emitted by the cathode emerge from the grid
4
in the form of a beam
10
in packets already modulated in density by the input signal E, contrary to what happens in a klystron. The beam
10
is longitudinal, following the axis XX′. The electrons of the beam
10
, attracted and focused by the anode
5
, penetrate the output cavity
7
and traverse the interaction space
6
where they couple to the electromagnetic field of the resonant cavity
7
. An output signal S of power significantly higher than that of the input signal E can be extracted from this output cavity
7
. The electrons having lost most of their energy then impinge on the wall of the collector
15
. The anode
5
is generally grounded; the collector
15
may also be grounded or at a slightly different potential from the ground.
When the IOT is intended to work with a modulated output power, as in television stations broadcasting transmitters, the input signal E carries the modulation. The coaxial input cavity
9
, formed of two coaxial conducting cylinders
90
,
91
is generally provided with a device
11
to regulate its resonant frequency, for example of the type including a piston whose position is adjustable. For safety reasons and to decouple the preamplifier from the high tension, this coaxial input is grounded. A decoupling capacitor C
1
provides electrical isolation, from a continuous point of view, between the internal cylinder
90
and the cathode
2
and another decoupling capacitor C
2
provides electrical isolation between the outer cylinder
91
and the modulation grid
4
. These capacitors C
1
, C
2
can be made of insulating sheets squeezed respectively between a cavity cylinder
90
,
91
and a cylindrical part
13
,
16
connected respectively to the electrode
2
,
4
.
In this application, as a UHF transmitter, the high tensions are of the order of a few tens of kilovolts, the cathode being less negative than the grid.
The output signal S, of amplified power compared with the input signal E, is extracted from the output cavity
7
by capacitive or inductive coupling. In
FIG. 1
, inductive coupling is shown in the form of a conductor
12
making a loop in the output cavity
7
. The output signal is passed to a user device such as an antenna (not shown).
The inside of the tube is classically under vacuum. Sealing of the output cavity
7
is assured by a dielectric sleeve
14
which allows the energy to be extracted to pass. Part of the output cavity
7
is external. It is delimited by walls in contact with the sleeve on the side where there is no vacuum.
Klystrons, operating either continuously or in pulses, can produce high powers because the gun is raised to high tension whereas the input signal to be amplified is injected into the first cavity of the tube. There is no interference between the high tension and the signal to be amplified.
In lOTs, on the other hand, the input signal to be amplified is injected in the cathode-grid space and the cathode and grid are simultaneously raised to high voltages. To obtain the required output powers (several tens or even hundreds of megawatts), the high tensions are no longer of a few tens of kilovolts but as much as a few hundred kilovolts. In these conditions, the capacitors between the two walls of the coaxial input cavity and respectively the cathode and the grid, of the same type as those in
FIG. 1
, will be ineffective. Decoupling becomes difficult to achieve because the risk of breakdown is very high owing to the very high voltages and small dimensions of the components.
SUMMARY OF THE INVENTION
The present invention resolves this problem and reduces the risks of breakdown. For this purpose, the invention includes an enclosure that is electrostatically screened and electrically isolated from the potential of the anode and that serves to confine the means producing the signal to be amplified (which is applied to the cathode/grid space), the means to passing it to the gun, and the gun of the IOT; this surrounding screened enclosure is connected to the means producing the high tension. With this structure the means of decoupling, between the input resonant circuit and either the grid or the cathode, that were the source of the problem described above, become unnecessary.
More precisely, the invention is a radiofrequency generator including an Inductive Output Tube (IOT) with an electron gun followed by an anode, the gun being raised to a high voltage in use, means producing an input radiofrequency signal, and means of transmitting it to said IOT such that it provides an output signal whose power is amplified compared to said input signal, wherein said means producing said input radiofrequency signal, said means transmitting it to said IOT and said gun are confined in an electrostatically screened enclosure that is electrically isolated from the potential of the anode and can be raised to a high voltage, said gun receiving its high voltage from said screened enclosure.
For safety reasons, the anode is generally grounded (i.e. at ground potential) and the screened enclosure can be placed on at least one dielectric support on the floor.
The means used to produce the input radiofrequency signal can include a radiofrequency source which feeds a preamplifier producing the input radiofrequency signal.
The means to deliver the input radiofrequency signal to the IOT can include an input resonant circuit connected between the grid and the cathode of the tube.
The resonant circuit may be with distributed or localized constants, this choice depending notably on the frequency chosen.
Th
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Thomson Tubes Electroniques
Vu Jimmy T.
Wong Don
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