US2403745A - Apparatus and method for making tubes - Google Patents

Description

July 9, 1946.

VACUUM R, L. NORTON Filed Dec. 21, 1945 PUMP PULSE POWER /9 5UPPLVY 1 L 22 r 25 5 0' /.2/ r M I'P PULSER $6 INVENTOR Robe/7 Le/gh ATTORNEY Patented July 9, 1946 UNITED STAT'ESPATENT OFFICE] APPARATUS AND METHOD FOR MAKING TUBES Robert Leigh Norton, Burlingame, Calif., to Eitel-McCullough, Inc., San Bruno, corporation of California assignor Calif., a

Application December 21, 1943, Serial No. 515,189

tube to go soft. Thoroughly outgassing the plate electrode is particularly important because this is a relatively massive part.

A method commonly used for heating an electrode during exhaust is known as finternal bombardmentff involving bombarding the electrode with electrons from the cathode. This system has numerous advantages over heating tube parts by high frequency. induction. A problem in .the internal bombardment method however is that electron collisionwith gas atoms or mole-,

cules in the tube is likely to cause excessive ionization. This is bad for several reasons. One is that positive ion bombardment of the cathode is destructive.

It is among the objects of my invention to overcome the above diiiiculties by providing a method of electronically bombarding an electrode to heat the same without causing harmful ionization in the tube. I

The invention possesses other objects and features of advantage, some, of which, with the foregoing, will be set forth in the following description of my invention, It is to be understood that I do not limit myself to this disclosure of species of my invention as I may adopt variant embodiments thereof within the scope of the claims.

Referring to the drawing:

Figure 1 is a diagrammatic view showing a system for carrying out my method in conjunction with exhausting a tube; and I Figure 2 is a similar view showing a modified pumping system. v

In terms ofbroad inclusion, my invention involves periodically bombarding an electrode with electrons, and limiting each period of bombardment to a time intervalso small that the ionization of gas in the tube is insuificient to cause destructive bombardment of. the cathode. When used in a pumping system the tube is continuously evacuated during such bombardment.

In greater detail, and referring to the drawing, Figure 1 shows a plate pulsed type of pumping 9 Claims. (01. s s-a6) system embodying my method. Tube 2 to be exhausted is connected by tubulation 3 to a suitable vacuum pump- 4. The latter preferably includes an oil diffusion pump capable of producing a high degree of vacuum in the envelope of the tube. For purposes of simplicity I have shown a single tube, it being understood that a group of tubes may be pumped simultaneously by my' method.

While the tube is being evacuated, cathode 6 is heated by suitable means to provide a source of electrons. Thus the filamentary cathode illustrated is connected to secondary 1 of a transformer 3. Primary 9 of this transformer is connected through a voltage regulator H to a suitablesource of A. C. power supplied at terminals l2.. Regulator ll permits adjusting the filament voltage to obtain the desired emission from the cathode.

In the practice of the invention an electrode, such as plate l3, to beoutgassed is bombarded with electrons to heat the same for driving out the occluded gas.- Gas liberated from the plate is withdrawn from the tube .by pump 4 which operates continuously during the process. As hereinbefore pointed out, .the principal problem encountered in this system of electrode heating is that electron collision with gas atoms and molecules causes excessive ionization. This subjects the cathode to harmful positive ion bombardment. Furthermore, ionized gas is materially harder to remove from the tube than unionized gas, making-it more difficult if not impossible to achieve a high degree of vacuum.

In order to overcome this problem I bombard the plate periodically, and limit each period of bombardment to a time interval so small that the ionization of gas-in the tube is insumcient to cause destructive bombardment of the cathode. While. little is known of the exact time required for a gas to ionize, dependent as it is upon so many variables such as the nature of the gas, number, of, collisions between electrons and gas atoms orImolecules, etc.; nevertheless a measurable period is required. Fo practical purposes this period may be defined as lessthan about 25 microseconds. I have used p p Syste s i which electron bombardment of an electrode was limited to periods as short as /2 to 6 microseconds with good results. 7 I

In Figure 1 the periodic bombardment is accomplished by intermittently applying, short pulses of positive potential to plate l3 of the tube, the pulse lengths being of the order mentionedabova For this purpose a suitable pulser I4 is provided with its positive output terminal connected to plate l3 by conductor l and its negative terminal connected to the cathode circuit by conductor l6. In this set up grid I1 is not specially heated and is therefore merely tied to the cathode by conductor [8.

The voltage applied to the plate and the repetition rate of the pulses may be varied within wide limits, as long as suflicient power is applied to heat the plate to the desired temperature for driving out the occluded gas. I have used potentials up to 30,000 volts and repetition rates from 500 to 4,000 cycles per second with satisfactory results, it being understood that still higher voltages and other repetition rates may be employed.

It is also understood that other electrodes in a tube may be heated in a similar manner by applying pulses of positive potential thereto, and, if desired, two electrodes may be heated simultaneously by alternate application of pulses to them. In any case the advantages of my invention are obtained if the pulse length is so short that the ionization of gas in the tube is insufiicient to cause destructive bombardment of the cathode.

Figure 2 shows a modified pumping system in which bombardment of the plate is controlled by the grid. For this system a D. C. power supply 19 capable of delivering up to say 30,000 volts is provided with its positive terminal connected to plate [3 by a conductor 20 and its negative terminal connected to the cathode circuit by conductor 2|, a suitable condenser 22 being connected externally between the plate and cathode. In this case periodic bombardment of the plate is controlled by biasing the grid to cutoii (viz.,

applying sufiicient negative potential to the grid to normally cut off the electron stream from the cathode to the plate) and intermittently applying pulses of positive potential to grid I1. The positive terminal of a pulser 23 is connected to the grid by conductor 24 and the negative terminal is connected to the cathode circuit by conductor 26. Pulser 23 is desi ned to deliver a suitable potential, say up to 5,000 volts, the length of pulse being less than about 25 microseconds as hereinbefore discussed. Bias voltage is supplied by suitable means, as by a battery 21 interposed in lead 24 with its negative terminal connected to the grid. Sufiicient battery voltage is used to normally bias the grid to cut off the electron stream from the cathode to the plate. As a result, no electrons bombard the plate except during the periods when positive pulses are applied to the grid.

I claim:

1. The method of making a vacuum tube having an electrode, which comprises intermittently bombarding the electrode with electrons emitted by a continuously heated cathode to heat the electrode. and limiting each period of bombardment to a time interval so small that the ionization of gas in the tube is insufiicient to cause destructive bombardment of the cathode.

2. The method of making a vacuum tube hav-- ing an electrode, which comprises intermittently bombarding the electrode with electrons emitted by a continuously heated cathode to heat the electrode. and limiting each period of bombardment to a time interval less than about 25 microseconds. said interval being so small that the ionization of gas in the tube is insuflicient to cause destructive bombardment of the cathode.

3. The method of making a vacuum tube having an electrode, which comprises intermittently bombarding the electrode with electrons to heat the same for driving out occluded gas, limiting each period of bombardment to a time interval so small that the ionization of gas in the tube is insufiicient to cause destructive bombardment of the cathode, and evacuating the tube during said bombardment.

4. The method of making a vacuum tube having a cathode and another electrode, which comprises heating the cathode to provide a source of electrons, intermittently applying a positive potential to said other electrode to intermittently bombard it with said electrons to heat the electrode for driving out occluded gas, limiting each period of bombardment to a time interval so small that the ionization of gas in the tube is insufiicient to cause destructive bombardment of the cathode, and evacuating the tube during said bombardment.

5. The method of making a vacuum tube having a cathode and grid and another electrode, which comprises heating the cathode to provide a source of electrons, applying a positive potential to said other electrode, applying suflicient negative potential to the grid to cut oil the electron stream from the cathode to said other electrode, intermittently making the grid positive with respect to the cathode to intermittenly bombard the other electrode With said electrons to heat it for driving out occluded gas, limiting each period of bombardment to a time interval so small that the ionization of gas in the tube is insufi'icient to cause destructive bombardment of the cathode, and evacuating the tube during said bombardment.

6. The method of making a Vacuum tube having a cathode and another electrode, which comprises heating the cathode to provide a source of electrons, intermittently applying a positive potential to the other electrode to periodically attract electrons from the cathode limiting each period of said applied potential to a time interval less than about 25 microseconds, said interval being so small that the ionization of gas in the tube is insufficient to cause destructive bombardment of the cathode, and simultaneously evacuating the tube.

'7. In apparatus for making a vacuum tube having a cathode and another electrode, the combination with means for heating the cathode to provide a source of electrons, of a pulser for intermittently applying a positive potential to said other electrode to effect intermittent electron bombardment thereof, and thereby to heat the electrode for driving out occluded gas, said pulser limiting each period of bombardment to a time interval so small that the ionization of gas in the tube is insufiicient to cause destructive bombardment of the cathode, and means for evacuating the tube during said bombardment.

8. Apparatus for making a vacuum tube having a cathode, a grid, and another electrode, the combination with means for heating the cathode to provide a source of electrons, of means for applying a positive potential to said other electrode, means for applying suflicient negative potential to the grid to cut off the electron stream from the cathode to said other electrode, a pulser for intermittently making the grid positive with respect to the cathode to effect intermittent electron bombardment of the other electrode to heat the latter for driving out occluded gas. said pulser limiting each period of bombardment to a time interval so small that the ionization of gas the tube is insufiicient to cause destructive having a cathode and other electrodes, the combination with meansfor heating the cathode to provide a source of electrons, of a pulser for intermittently applying a positive potential to an ment of one-0f said electrodes, and thereby to 6 r l I 7 heat the bombarded electrode for driving out 00- cluded gas, said pulser limiting each period of bombardment to a time interval so small that n the ionization of gas in the tube is insuflicient to cause destructive bombardment of the cathode,

and means for evacuating the tube during said 7 I bombardment. electrode to effect intermittent electron bombardr ROBERT LEIGH NORTON.

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