Pulse forming networks
This is a continuation of application Ser. No. 07/004,046, filed Jan. 16, 1987, which was abandoned upon the filing hereof. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to pulse-forming networks and is particularly concerned with impedance-matched pulse-forming networks. 
     Pulse-forming networks (PFNs) are used to deliver a flat-top electrical pulse of either current or voltage to an impedance-matched load. In order to obtain maximum energy transfer from the PFN to the load within an allocated time interval, the PFN impedance has to be equal to the load impedance--otherwise the energy transfer is not maximised. Certain loads, for example some laser discharges, have continuously falling impedance and therefore the impedance-matched condition for maximum energy transfer is only satisfied briefly during the PFN discharge resulting in an energy transfer to the load of less than the maximum possible within the allocated interval. This can seriously degrade the performance of the laser. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided an electrical circuit including current supply means and a variable impedance load connected to the supply means to draw current therefrom, the supply means including saturable inductor means operable to cause the output impedance of the supply means to vary with the current drawn by the load and hence to vary with the impedance of the load, the presence of the saturable inductor means thereby causing the overall matching between the impedances of the supply means and load to be substantially better than would be the case if the impedance of the supply means were fixed. 
     The supply means can take the form of a single section or a multiple section inductance/capacitance network for delivery of voltage and current pulses to a load such as a gas discharge laser, with magnetisable material being positioned at least near the inductance of one or more of the sections to render the impedance of the network dependent upon the current drawn by the load. 
     Alternatively, the supply means can take the form of a pulse-forming high voltage conductor line which delivers high voltage pulses to a load, with a continuous strip of magnetisable material being positioned at least near the conductor line to render the impedance of the line dependent upon the current being drawn by the load. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawing in which: 
     FIG. 1 is a line-simulating, five-section pulse-forming network (PFN); and 
     FIG. 2 is a B-H curve for a magnetic material of the type used according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENT 
     In FIG. 1, a line-simulating, five-section PFN 1 is shown connected to a load 2. The network 1 comprises five inductors 3, each having an inductance L, and five capacitors 4, each having a capacitance C. When switch 5 is closed, the network 1 delivers a pulse of energy to the load 2. The amount of energy transferred to the load depends on the impedance of both the network Z N  and the load Z L . For maximum energy transfer within the allocated time interval, the impedances need to be matched, i.e. Z N  =Z L . As the total capacitance and in ductance for the PFN 1 are fixed because the components making up the network are fixed, the impedance of the network Z N  is given by: ##EQU1## where L&#39; and C&#39; are the total inductance and capacitance respectively for the network. Therefore, if the load impedance Z L  varies, the amount of energy transferred from the PFN to the load is reduced as the network impendance is no longer matched to that of the load. 
     The inductors normally used in a PFN are either wire wound coils with air cores or single turn air cored inductors, both of which have fixed inductance. If these inductors were replaced by inductors which contain a magnetic core with a non-linear B-H curve, the inductance of the inductors would be dependent on the permeability of the magnetic material and hence varible. 
     FIG. 2 shows a typical B-H curve for a magnetic material. The permeability of the material is given by the slope of the curve at any point, i.e.: ##EQU2## where μ m  is the permeability of the material, 
     μ o  is the permeability of free space, 
     B is the magnetic flux density, and 
     H is the magnetic intensity. 
     However, as ##EQU3## where n is the number of turns of the coil, 1 is magnetic path length of core, and I is the current flowing through the coil, the permeability of the magnetic material is current-dependent. This leads to the impedance of the inductors being also current-dependent over certain portions of the hysteresis curve. As the magnetic intensity H increases due to an increase in current, the permeability μ m  changes. The magnetic material may be set prior to the start of the current pulse so as to induce the required change in μ m . For example, if the material is set at the point X in FIG. 2 an increase in I, and hence H, will produce an increase in μ m  up to the point where H=O. The material may be set to any point on its B-H loop by positioning a subsidiary current loop around the material or by biasing the material with a permanent magnet. It may be arranged that μ m  is inversely variable or inversely variable with H. 
     Therefore, in a PFN having inductors with magnetic cores connected to a load as shown in FIG. 1, in which the impedance varies as shown by the B-H curve of FIG. 2, the network impedance will vary accordingly so that Z N  =Z L  as before and in general, the PFN impedance will tend to follow the load impedance even if the load impedance oscillates. 
     Naturally, the above applies to any line-simulating PFN, i.e. with any number of sections. 
     This technique can also be used in pulse-forming lines (PFLs) e.g. high voltage conductors which utilise distributed inductances and capacitances. In such an arrangement, a continuous length of magnetic material is placed near to the conductor to produce the current-dependency of the conductor impedance.