Electric lamp and discharge devices – With positive or negative ion acceleration
Reexamination Certificate
2000-06-13
2002-06-11
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
With positive or negative ion acceleration
C313S310000, C313S341000
Reexamination Certificate
active
06404115
ABSTRACT:
The invention relates to charged particle emitting assemblies and emitters. The invention is concerned for example with the generation of high power electron beams (EB) and transmission into vacuum chambers operated at pressures in the range approximately 10
−1
mbar up to several hundred millibar. However, the invention is also applicable to other types of charged particle beams including those defined by negatively and positively charged ions. For convenience, only electron beams will be discussed.
Electron beams are readily produced by release of free electrons from an emitter and acceleration in an electric field. For electron beams which are merely used for applications such as vacuum melting of metals, beam quality in terms of energy density distribution, beam brightness and beam profile, is of little importance. Typically, “brightness” is defined as current density/steradian.
For other applications, beam quality is extremely important and moreover must be stable and reproducible. In the case of electron beam welding (EBW), for example, the ability to produce repeatedly deep narrow fusion zones of consistent depth and width is critically dependent on:
i) the beam energy density distribution
ii) the position of focus with respect to the workpiece surface, and
iii) the beam brightness which involves both spot size and convergence angle factors.
Ideally, for electron beam welding, it is important to achieve a clearly defined energy density distribution and usually this is Gaussian in form. Also, to perform deep narrow welding, the angle of convergence of the beam needs to be controlled within a relatively tight range. Certainly for welding of steels, for example in section thickness of 100 mm-150 mm, a beam semi-angle of greater than 1 degree leads to weld pool instabilities and internal defects. A beam which is near parallel, on the other hand, may be highly suited to welding such thick sections but is unsuitable for producing very narrow welds in steel sections of 1 mm-10 mm. In addition, in the case of the thinner section range, welding beam energy distribution is much more important. If for any reason the energy distribution includes a significant fringe, this is reflected in the weld fusion zone shape. Thus, instead of achieving a near parallel fusion zone as in the case of the Gaussian distribution, a much wider non-parallel fusion zone with a so-called “nail head” feature is produced. More beam power is required for the same weld depth, lateral shrinkage after welding is overall greater and because of the wider weld width at the top compared with the bottom, uneven shrinkage occurs resulting in distortion of the component as indicated. For precision components this is often unacceptable and may also lead to weld cracking.
Similarly, it is important, particularly for thin section welding, to achieve sufficient intensity in the focal spot. For systems which produce a near parallel beam, even without fringes, insufficient intensity leads to relatively wide tapered fusion zones accompanied by excessive distortion and again a risk of cracking. Near parallel beams are not necessarily focusable, space charge spreading can still occur in a vacuum environment even in spite of strong positive ion neutralisation effects. Thus, for a near parallel beam entering a focusing lens attempts to focus a beam over long distance result in little if any reduction in the beam diameter. Indeed, the beam profile and intensity characteristics can often at medium and high powers, be totally dominated by ion-electron interactions.
It is, therefore, very important to launch the beam from the electron gun with a well defined divergence (within a specified range), high brightness, low aberration and without fringes.
One possible means of achieving higher convergence angle to combat space charge spreading in the medium to high beam current range with a triode gun is to employ electrodes which produce a more strongly focusing field. This, however, leads to excessive convergence at low current when the grid field becomes an additional powerful focusing element. Large swings in convergence angle are generally undesirable even for high vacuum EBW and present greater difficulties in a system which employ a beam transfer system for reduced pressure (5×10
−1
to ~250 mbar) or non-vacuum (~1000 mbar) operation where fine bore nozzles are employed to restrict gas leakage into the gun region.
Yet another method of achieving greater convergence, to combat space charge spreading at high current levels, is to design a gun in which the cathode, grid electrode and anode are placed in close proximity. This leads to more rapid acceleration of the electrons over a shorter axial distance reducing the possibility of mutual electron repulsion. Unfortunately, such an arrangement increases the electrical surface stresses on the electrodes and can lead to increased high voltage breakdown tendency.
Avoidance of beam fringes and optimum focusing of electron beams is extremely important when the beam must be transmitted through narrow orifices in order to extract the electrons from the high vacuum (5.10
−5
−5.10
−6
mbar) region of a gun housing into working chambers operating over the approximate pressure range of 5.10
−2
to 1000 mbar. Here, the rate of leakage of gas from the working chamber into the gun housing is primarily determined by the diameter and length of the orifices apart from the number of orifices and the pumping capacity of the interstage pumps.
Beam fringes tend to contain large amounts of power and even for low total power operation (e.g. 5 kW), the ability to absorb this extraneous power on the orifice nozzles is limited even if substantial water cooling is applied; unlike electron microscope devices, where the beam power is extremely small, it is impractical to strip off the unwanted fringe on interception diaphragms. For similar reasons, it is important to avoid a low brightness, near parallel beam because of the large beam diameter.
Beam quality and whether or not a particular electron gun produces a pure un-aberrated beam with a well defined divergence is very dependent on gun design and particularly the cathode design and the detailed geometry of the electrodes in the immediate vicinity of the cathode.
Most electron guns used for EBW are triodes. The use of the grid electrode ensures that at low beam current, cathode emission is limited to a central portion of the cathode but the presence of the strong electric field created by the grid leads to considerable beam aberration.
The outer electron trajectories have a shorter focal length in the strong grid field because they are closer to the edge of the grid cup hole than the more central electrons. Also, as the grid voltage is reduced to increase beam current, the emission area expands and may even permit electrons to be released from the cathode edges where adverse geometrical features produce electron trajectory flight paths radically different to the main electron flow. In addition, the weakening grid field combined with increased space charge in the beam, as the beam current is increased, can result in gross spreading of the beam and loss of primary focus. Also, the primary focus waist and the virtual image position (apparent to the first focusing lens) can move considerable distances up and down the beam axis dependent on beam current level.
Beam fringes produced by such a gun, the drift of primary focus with beam current, the lack of beam convergence angle at high current and the relatively high convergence angle at low current, can adversely effect welding performance for even conventional systems projecting beams into relatively high vacuum chambers (5×10
−3
mbar). For beams which need to be transmitted through small orifices, operations can be difficult or even impossible, particularly for high power (greater than say 30 kW) operation.
In accordance with a first aspect of the present invention, a charged particle emitting assembly comprises an emitter member for emitting charged particle
Novack Martin
Patel Vip
The Welding Institute
LandOfFree
Particle beam emitting assembly does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Particle beam emitting assembly, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Particle beam emitting assembly will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2977552