Electric lamp and discharge devices: systems – With cathode or cathode heater supply circuit – Pulsating or a.c. supply to the cathode or heater circuit
Patent
1994-07-22
1997-08-26
Lee, Benny T.
Electric lamp and discharge devices: systems
With cathode or cathode heater supply circuit
Pulsating or a.c. supply to the cathode or heater circuit
315244, 315 541, 250423R, 313337, 313569, 21912127, H01J 2702
Patent
active
056613695
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to apparatus for generating a stream of charged particles for use, for example, for heating in generating a stream of electrons in electron beam welding apparatus or streams of ions for use in other applications.
BACKGROUND OF THE INVENTION
In electron beam welding, an electron gun produces from a heated cathode a stream of electrons which is accelerated and focused onto a workpiece. Electron beam welding is extremely advantageous where deep penetration and a narrow heat-affected zone are required but a suitable electron beam generator requires a very high accelerating voltage, typically 150 kV is used. In U.S. Pat. No. 4,020,318 describe an electron beam generating system in which the auxiliary supply for as least one electrode of the electron gun (for example the filament or biassing electrode) is provided by a high frequency oscillator connected to a tansformer or other coupling means, the output of which is applied as the electron beam gun column to a component which is itself connected to the received output of the high voltage generator. The advantage of this arrangement is that it dispenses with the superimposition of the auxiliary supply on the HT (High Tension) voltage at a remote high voltage generator and with the corresponding auxiliary supply conductor in the conventional high voltage cable between the high voltage generator and the electron beam gun column. This system has worked well for filament and bias supplies for a simple triode gun but the control of the system for an indirectly heated or "back-bombardment" type of gun is complex.
In a back-bombardment system, the filament generates a stream of electrons which are accelerated onto a target (cathode) which is thereby heated and energised to generate its own stream of electrons which is then accelerated though an anode onto the workpiece. In order to control the system, it is first necessary to set up the back-bombardment system. This involves causing current to flow through the filament and then to monitor the electron flow to the target (which is held at a positive voltage of the order of 1 kV relative to the filament) until the correct operating filament emission current is reached. The procedure is known as "peaking" the filament. Thereafter, the main beam current from the target cathode to the workpiece must be optimized by adjusting the back bombardment accelerating voltage onto the target. Not only must this process be carried out at start-up, but also during use since the filament itself will gradually thin necessitating adjustment of the filament current to avoid filament overheating and failure of the primary filament.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, apparatus for generating a stream of charged particles comprises a charged particle source and a target connected respectively in series with and parallel to a resonant electrical circuit and relatively juxtaposed such that under working conditions, when the circuit is in resonance, electric current passing through the source causes emission of first charged particles and the potential difference between the source and the target accelerates the first charged particles towards the target.
This invention provides a novel arrangement in which the source and target are provided within the same resonant circuit and this has the effect of providing a self-limiting system. That is, when the current in the source circuit reaches a sufficient level to cause the source to emit the first charged particles, the associated emission current loads the target circuit leading to a limiting effect whereby with increasing power supplied to the whole circuit, the rapidly increasing charged particle emission current causes a loading of the resonant circuit whereby neither the filament current nor the accelerating voltage between the source and target increase as would otherwise be the case. As will be appreciated, the invention uses the inherent effect of a series resonant circuit to provide the high c
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patent: 3218508 (1965-11-01), Holce
patent: 3760279 (1973-09-01), Rudolph
patent: 3909663 (1975-09-01), Thomas et al.
patent: 4020318 (1977-04-01), Sanderson
patent: 4422013 (1983-12-01), Turchi et al.
patent: 4489254 (1984-12-01), Koinuma et al.
patent: 4772823 (1988-09-01), Matsuzaki et al.
patent: 5302881 (1994-04-01), O'Loughlin
Bettendorf Justin P.
Lee Benny T.
Novack Martin M.
The Welding Institute
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