Device for generating an on-off modulated electron beam

Abstract: A device comprising an electron gun and a high tension switch permits the achievement of high-power electron beams which are ON-OFF modulated by DC up to frequencies exceeding kHz. The electron gun comprises a cathode, at least two accelerating electrodes, a cathode heater, and a possible focussing system. The or each accelerating voltage, generated by any source, which may in the case of space applications also be a tether, is applied between accelerating electrodes and cathode, with the exception of an appropriate accelerating electrode to which there is applied, by means of the switch, alternately its accelerating voltage or the same voltage as that of the cathode, obtaining an ON-OFF modulation of the electron beam emitted by the gun.

Description: The invention refers to an electron gun device for generating an off-on modulated electron beam including a first accelerating electrode, a tension switch connecting said first accelerating electrode to an accelerating voltage, and controlling means for said switch. An electron gun of the above mentioned type is disclosed in IBM Technical Disclosure Bulletin, Vol. 29, No. 6, Nov. 1986, pages 2543-2544. This known device is provided with a cathode, a pulsed Wehnelt grid and a further anode. A first pulsed voltage is applied to the grid and a second pulsed voltage at higher frequency is applied to the further anode. The anode is responsible for modulating the electron beam, and it is never connected to the voltage of the cathode. The ON-OFF modulation of the power electron beam (in the order of 1 kW or tens of kW) emitted by an electron gun has hitherto been obtained in various ways. One of these is the use of a control electrode (grid) to which there is alternately applied a negative potential in the order of tens/hundreds of volts with respect to that of the cathode, in such a manner as to obtain beam prohibition (OFF), and a potential close to that of the cathode or even positive, to permit beam emission (ON). Such a solution involves the generation of a dedicated voltage, modulated between two levels, and its application to an appropriate electrode (grid) (Fig. 1). Another problem with regard to grids is their resistance to mechanical and thermal stresses and thus their reliability, which is important especially in the space and occupational field. Further solutions, which do not take into account the use of a dedicated electrode (grid), act on the accelerating voltage applied to the electron gun and to be modulated at the desired frequency or act on the cathode emission, the latter however being band-limited to a few Hz by the thermal time constant of the cathode. Systems already used in the space field are: the interposition of a solid state interruptor in series with the cathode bias (Fig. 2), which has permitted the achievement of the ON-OFF modulation of a beam using 1 keV, 100 mA [P. Banks et al., VCAP EXPERIMENTS ON STS-3], ie. a beam of limited power; this is because such a solution implies the interruption, at high tension, of the cathode emission current; the use of a high-tension power generator, itself having a modulated output, as generator of the accelerating voltage. In this case, it is necessary to provide: ON-OFF modulation of the full output power; optimization of the design of the generator having regard to the load (characteristics of the gun). According to the invention the electron gun device is provided with a double interruptor switch, which alternately connects the accelerating electrode to the accelerating voltage or to the voltage of the cathode. Further advantageous features of the electron gun of the present invention are set forth in the dependent claims. The invention and the relevant advantages thereof will be better understood by the following description in connection to the drawings, wherein: Figs 1 and 2 show known solutions, which have already been illustrated hereinabove; Figs 3 and 4 show a solution according to the invention, in the ON condition and in the OFF condition respectively. The object of the present invention is to obtain an ON-OFF modulated power electron beam, using an appropriate accelerating electrode of the electron gun, its own accelerating voltage and a high tension switch. The object of the present invention further comprises the possibility of regulating the beam current emitted in the ON condition by means of a closed-loop control, which regulates the heating power and thus the temperature of the cathode in such a manner as to bring into coincidence the emitted beam current intensity and the required intensity by means of a digital command. More specifically, by means of the switch M diagrammatically represented in Fig. 3 (with a double interruptor) operating as modulator, the accelerating voltage V E is applied to the accelerating electrode E only during the time intervals in which beam emission is desired (ON); in order to prohibit the beam (OFF, Fig.4), on the other hand, there is applied to the electrode E the same voltage as is present on the cathode K. In the OFF condition there may be a beam current, which is indeed a function of the design of the electron gun, this being negligible as compared with that in the ON condition, due to the presence of the other accelerating electrodes E1 which are continuously biased. The closed-loop regulation of the emitted beam current is possible in the ON condition (Fig. 3), and is undertaken by comparing the required value (by means of the user interface) with the value measured by the sensor S. Such comparison is undertaken by the electronic system C, which provides for the consequent regulation of the heating power applied to the cathode by means of a command to the feeder R, and finally bringing said cathode to the temperature which assures the required emission. The system described is advantageous inasmuch as: it uses one E of said electrodes E, EO etc., and a respective accelerating voltage generator GE, which are necessary for continuous operation; to achieve the ON-OFF modulation it is necessary to interrupt only the anode leakage current (which may be minimized when designing the gun by increasing the efficiency thereof, which is understood as being the ratio between the beam current downstream of the accelerating electrodes and the cathode emission) and not the entire cathode emission current, reducing the problems associated with the construction thereof; it is independent of the output of the gun employed and of the technology with which the gun is made, as a result of which it is possible for high currents to be interrupted; it is independent of the procedures of application of the accelerating voltage (anodes positive and cathode grounded, or anodes grounded and cathode negative) as the switch has equivalent branches for isolation and resistance to the high tension, and is connected in parallel with the gun. In particular, the object of the invention is also to obtain an ON-OFF modulated beam, using a switch of the solid state type obtained by conecting two solid state high tension interruptors as in Figs 3 and 4, each one of which can be realized as in Italian pat.appl.No. 9533 A/90,filed Nov.28,1990 laid open for public action Oct.10,1991. The accelerating voltage which the switch is able to manage is dependent upon the stages employed on each interruptor, and can therefore be adapted to the particular requirement. The switch forming the subject of the invention permits the modulation of beams of power in the order of tens of kW by DC up to frequencies exceeding kHz. In summary, Fig. 1 shows a known electron gun having a cathode K1, and an accelerator electrode E1 fed with accelerating voltage VE1 and equipped with an appropriate modulating electrode GM1 included between K1 and E1, with relative bias with respect to the cathode K, in the form of voltage VG1. The anode is indicated by A. Fig. 2 shows a known electron gun K2, E2; the modulation is obtained by interrupting the cathode emission current originating from the generator G2, with an interruptor I2. Figs 3 and 4 show an exemplification of the gun-switch device forming the subject of the present invention. The exemplification set forth concerns a gun with two accelerating electrodes EO and E, of which one (EO) is fed by the voltage VE, and the other (electrode E) is conditioned by the modulator M in such a manner as to obtain the ON-OFF modulation of the beam, with the switching of the two interruptors of said modulator M, as shown in Figs 3 and 4. The switch M is conected to the pertinent electronic control system and interface C. The switch M itself provides galvanic separation resistant to high voltages between the operational parts in association with the gun and the electronic system C. The value of the modulated current may be selected by means of a digital command to be passed to the electronic control system and interface C. This value is utilized by the electronic system C as set point for a closed-loop control of the beam current from the cathode.

Claims: An electron gun device for generating an off-on modulated electron beam including:a cathode (K), a first accelerating electrode (E); a tension switch (M) connecting said first accelerating electrode (E) to an accelerating voltage (VE); and controlling means (C) for said switch, characterized in that said switch (M) is provided with a double interruptor, to alternately switch said first accelerating electrode (E) to the accelerating voltage (V E ) or to the voltage of the cathode (K). The device as claimed in claim 1, wherein the high tension switch (M) is provided with a solid state double interruptor. The device as claimed in claim 1 or 2, including a second accelerating electrode (EO), said first accelerating electrode (E) being arranged between said cathode (K) and said second accelerating electrode (EO). The device as claimed in claim 3, wherein said second accelerating electrode (EO) is connected at the accelerating voltage and said double interruptor switch (M) alternately switches said first accelerating electrode (E) to the voltage of said second accelerating electrode (EO) or to the voltage of the cathode. The device as claimed in one or more of the preceding claims, including a loop control which regulates the heating power fed to the cathode in order to regulate the beam current emitted by the device.

Inventors: CIRRI GIANFRANCO, CIRRI, GIANFRANCO
Applicants: PROEL TECNOLOGIE SPA, PROEL TECNOLOGIE S.P.A.
Application year: 1991
Main classification: H01J3
IPC codes: H01J21, H01J37, H01J3