US2044350A - Gaseous discharge tube and method of manufacture - Google Patents

Description

Patented June 16, 1936 ATENT oFFieE GASEOUS DISCHARGE TUBE AND METHOD OF MANUFACTURE ware No Drawing. Application April 13, 1934, Serial No. 720,433

7 Claims. (Cl. 25027.5)

This invention relates to gaseous discharge devices in general, but is particularly applicable to hot cathode discharge tubes.

The object is to prevent the failure of an occasional tube in early life.

The invention may be applied to any tube structure without change other than in the process of manufacture as described below.

The tube is given the usual treatment for pro- 10 ducing a high vacuum, i. e., the cathode is developed, the bulbbaked, and the metal parts treated with high frequency current in the usual manner. According to our invention, we then fill the tube with a gas different from that which 5 is to be used for the final filling and operate it for a few minutes. This gas is th n pumped out and the tube refilled with the same or similar gas and again operated. This treatment is repeated several times. With the cathode dis- 20 connected, we then run cold cathode discharges between as many electrodes as possible by applying suitable voltages and currents.

The gas used in this treatment must be inert to the tube parts and must have a higher ioniza- 25 tion potential than the gas which is to be used for the final filling. For argon filled tubes we use neon or helium as the treating gas, for xenon or krypton tubes we use argon for treatment. By this choice of gases all discharges occur with 30 higher voltage drops between electrodes, with greater losses in the tube and with consequently higher temperatures and greater ionization of imptu'ities than will occur with the final gas filling.

35 In some cases requiring extreme reliability of the tube we repeat the above treatments, using a gas or vapor of higher atomic weight than that of the final gas filling, such as: krypton or xenon for argon filled tubes, or mercury vapor 40 for xenon. By this treatment the tube may be operated on the pump at greater current rating, without melting the parts, than will be experienced in actual operation. Due to its greater atomic weight this treating gas neutralizes more 45 space charge in front of the cathode and therefore the cathode may be activated to a higher emissivity than will be required with the lighter final gas atoms. The heavier gas also permits operation on the pump at much higher voltages 50 than can be encountered with the final filling.

The discharges with the treating gas are not run at the same pressure as the final gas filling but at a pressure such that the mean free path for ionization is the same as that of the final gas 55 filling.

After the above treatment and any additional high frequency bombardment that may be necessary the tube is evacuated, filled with the final gas at the correct pressure and then sealed. off.

We believe that high frequency and heat treat- 5 ment removes the gas held in chemical combination by many compounds which are stable in vacuum at normal temperature but dissociate at high temperatures; and also absorbed, adsorbed and dissolved gas. This still leaves many chem- 1o ical compounds which break down when subjected to the ion bombardment occurring in normal operation. It seems probable that some of these chemical compounds break up due to electrolytic dissociation when ion bombardment is present. We have also found that there are several glass forming carbonates which break down in vacuum at high temperature only if in the crystalline state. If they are covered with an amorphous layer of carbonate the breakdown does not occur until bombardment initiates the conversion to the crystalline form. After penetrating the amorphous layer with bombardment these compounds will gas copiously.

With treating gas of greater atomic weight than the final filling the degassing effect may be due to the generation of some soft X-rays in the tube by high speed ions just before or during the instant of breakdown, which are active in decomposing chemical compounds containing gas or possibly the greater activating effect on the cathode is the cause of the more complete degassing.

By our treatment the parts of the tube are subjected to more intense gas forming influences while on the pump than will be encountered in operation, with the result that tubes may be tested thereafter with confidence, that gradual gassing will not change the characteristics thereafter and render the test meaningless. It eliminates the necessity for extended aging and the uncertainties as to when it is safe to terminate the aging and start the normal life of the tube.

We claim:-

1. A method of manufacture for a gaseous discharge device which includes operation during manufacture with a rare gas of greater ionization potential than that of the final gas filling and with a pressure of said rare gas which gives substantially the same mean free path for ionization as the said filling.

2. A method of manufacture for a gaseous discharge device which will have a final filling of rare gas at least as heavy as neon, which includes operation during manufacture with an inert gas of greater atomic weight than that of the final gas filling.

3. A method of manufacture for a gaseous discharge device which includes operation during manufacture with a filling of one of the gases neon and argon, exhausting the same and substituting a filling of one of the heavier gases krypton and xenon, the pressure of the treating gas for said operation being selected to give substantially the same mean free path for ionization as the said filling gas.

4. A method of manufacture for a gaseous discharge device which includes operation during manufacture with a filling of krypton, exhausting the krypton and substituting a filling of argon.

5. A method of manufacture for a gaseous discharge device which includes operation with a rare gas of greater ionization potential than that of the final gas filling, operation with a gas of greater atomic weight than that of said filling, said gases being exhausted after their respective operations, and filling the tube with a final rare gas.

6. A method of manufacture for a gaseous discharge device which comprises operating the device with a filling of argon, exhausting the argon,

operating the device with a filling of mercury vapor, exhausting the mercury vapor, and finally substituting a filling of one of the heavy rare gases, krypton and. xenon.

7. A method of manufacture for a gaseous discharge device which includes operation during manufacture with a gas of greater atomic weight than xenon, exhausting said gas and substituting a filling of one of the heavy rare gases xenon and krypton.

DONALD V. EDWARDS. EARL K. SMITH.