Abstract:
A method of operating an arc lamp. The method includes introducing a primary voltage pulse into the arc lamp, thereby inducing a primary flow of electrical current in the arc lamp. The method also includes introducing at least one secondary voltage pulse into the arc lamp, before the current flow has substantially decayed, thereby inducing a respective secondary flow of electrical current in the arc lamp.

Description:
FIELD AND BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to arc lamps and, more particularly, to a method of shaping the light pulses emitted by an arc lamp.  
         [0002]     Pulsed arc lamps have many applications in warfare, in medicine and in the fabrication of semiconductor devices.  
         [0003]      FIG. 1  shows a prior art circuit  10  for driving an arc lamp  12 . Circuit  10  includes a DC power supply  14 , a capacitor  16 , a gating switch  18 , a diode  20  and a coil  22  connected as shown. In the circuit as drawn in  FIG. 1 , electrical current from circuit  10  is fed to the anode  24  of arc lamp  12 , and the cathode  26  of arc lamp  12  is grounded. Alternatively, electrical current from circuit  10  is fed to cathode  26  and anode  24  is grounded.  
         [0004]     Power supply  14  supplies electrical current at a voltage of between 200V and 400V. Capacitor  16  is relatively large, to act as an energy reservoir. In the example shown, capacitor  16  has a capacitance of two millifarads. Gating switch  18  is shown as an insulated gate bipolar transistor (IGBT). Gating switch  18  is opened and closed by a driver  28  to provide pulses of electrical current from power supply  14  to arc lamp  12 . Diode  20  serves to discharge coil  22  when gating switch  18  is opened. Coil  22  has a ferrite core and is used to shape the voltage pulses from gating switch  18 . Coil  22  also is the secondary coil of a transformer  30  whose primary coil  32  is energized by a trigger pulse source (igniter)  34 .  
         [0005]     To turn on arc lamp  12 , igniter  34  is turned on to create an ignition pulse that provides a high (˜20 KV) voltage, low current trigger pulse between anode  24  and cathode  26  to create a conductive path from anode  24  to cathode  26  by ionizing the gas, that fills arc lamp  12 , between anode  24  and cathode  26 . Then an operating voltage pulse at a lower voltage of between 200V and 400V is introduced to arc lamp  12  by closing and then opening gating switch  18 .  FIG. 2  shows the shapes of the voltage V S  provided by gating switch  18  and the resulting electrical current I L  in arc lamp  12  as a function of time t. V S  is a square voltage pulse that lasts from time t 1 , when gating switch  18  is closed, to time t 2 , when gating switch  18  is opened. While gating switch  18  is closed, I L  is (V S /L)(t−t 1 ), where L is the inductance of coil  22 . Initially, the ferrite core of coil  22  gives coil  22  a high inductance L, so the slope of I L (t) is very low. When the ferrite core of coil  22  becomes saturated, at time t s , the inductance L of coil  22  falls to the inductance of an air coil, and the slope of I L (t) increases. I L (t) rises to a maximum value of I Lmax  at time t 2 . When gating switch  18  is opened at time t 2 , I L (t) starts to decay exponentially with a time constant of L/R where R is the effective resistance of diode  20 , arc lamp  12 , coil  22  and the wires that connect them. The overall shape of the current pulse I L  that actually flows through arc lamp  12  is approximately triangular. The intensity of the light emitted by arc lamp  12  is proportional to I L .  
         [0006]     Some applications of pulsed arc lamps require that the shape of the intensity profile of the light pulses be other than triangular, for example square. Perkin-Elmer of Wellesley Mass., USA, has developed a rather complicated circuit for driving an arc lamp in a way that provides light pulses with square intensity profiles. This circuit is described on the World Wide Web at http://optoelectronics.perkinelmer.com/content/RelatedLinks/pulsed_power_applications.pdf  
         [0000]     This circuit is considerably more complicated than prior art circuit  10 .  
       SUMMARY OF THE INVENTION  
       [0007]     It is an object of the present invention to provide a circuit, for driving a pulsed arc lamp, so as to produce light pulses with arbitrary intensity profiles, that is not significantly more complicated than prior art circuit  10 .  
         [0008]     According to the present invention there is provided a method of operating an arc lamp, including the steps of: (a) introducing a primary voltage pulse into the arc lamp, thereby inducing a primary flow of electrical current in the arc lamp; and (b) before the current flow has substantially decayed, introducing at least one secondary voltage pulse into the arc lamp, thereby inducing a respective secondary flow of electrical current in the arc lamp.  
         [0009]     According to the present invention there is provided a current source for operating an arc lamp including: (a) a power supply; (b) a switch for operationally connecting the power supply to the arc lamp; and (c) a timing mechanism for closing and opening the switch in a manner that provides a plurality of voltage pulses from the power supply to the arc lamp so as to induce, in the arc lamp, a flow of electrical current that has a desired shape.  
         [0010]     According to the method of the present invention, an arc lamp is energized by introducing a primary operating voltage pulse into the arc lamp, as in the prior art method described above, thereby inducing a primary flow of electrical current in the arc lamp. Then, unlike the prior art method, before the flow of electrical current in the arc lamp has substantially decayed, at least one secondary voltage pulse is introduced into the arc lamp, thereby inducing, for each secondary voltage pulse, a respective secondary electrical current flow in the arc lamp. Preferably, the secondary voltage pulse(s) is/are introduced into the arc lamp starting before the flow of electrical current in the arc lamp has decayed to half of its maximum value.  
         [0011]     Preferably, the primary voltage pulse and the secondary voltage pulses are square pulses.  
         [0012]     Preferably, a plurality of secondary voltage pulses are introduced into the arc lamp. Each secondary voltage pulse, subsequent to the first secondary voltage pulse, is introduced into the arc lamp before the total electrical current flow induced in the arc lamp by the voltage pulses up to and including the immediately preceding secondary voltage pulse has substantially decayed. Most preferably, each secondary voltage pulse subsequent to the first secondary voltage pulse is introduced into the arc lamp starting before the total current flow induced in the arc lamp has decayed to half of its most recent maximum value.  
         [0013]     Preferably, the duration(s) of the secondary voltage pulse(s), and the delay of the secondary voltage pulse(s) relative to the immediately preceding voltage pulse(s) (i.e., the delay of the first secondary voltage pulse relative to the primary voltage pulse, and the delay of each subsequent secondary voltage pulse, if any, relative to the immediately preceding secondary voltage pulse), are selected to give a desired shape to the sum of the primary electrical current flow and the secondary electrical current flow(s). Most preferably, the desired shape is substantially square. Alternatively, the desired shape is substantially Gaussian, or substantially sinusoidal.  
         [0014]     Preferably, the primary voltage pulse has a duration of between about 10 microseconds and about 40 microseconds. Preferably, each of the secondary voltage pulses has a duration of between about 10 microseconds and about 20 microseconds. Preferably, the total electrical current flow in the arc lamp has a rise time of between about 10 microseconds and about 20 microseconds during each secondary voltage pulse and/or a fall time, between successive secondary voltage pulses, of between about 50 microseconds and about 100 microseconds. Preferably, the total electrical current flow in the arc lamp has a duration, from when the primary electrical current flow starts until the total electric current flow substantially decays after the end of the last secondary voltage pulse, of between about 200 microseconds and about 10 milliseconds.  
         [0015]     The current source of the present invention includes a power supply, a switch for operationally connecting the power supply to an arc lamp, and a timing mechanism for opening and closing the switch in a manner that provides a plurality of voltage pulses from the power supply to the arc lamp so as to induce in the arc lamp a flow of electrical current that has a desired shape.  
         [0016]     Preferably, the switch includes an insulated gate bipolar transistor.  
         [0017]     Preferably, the switch and the timing mechanism are operative to give the primary voltage pulse a duration of between about 10 microseconds and about 40 microseconds. Preferably, the switch and the timing mechanism are operative to give the secondary voltage pulses respective durations of between about 10 microseconds and about 20 microseconds. Preferably, the current source is operative to give the total flow of electrical current in the arc lamp a rise time of between about 10 microseconds and about 20 microseconds during each secondary voltage pulse and/or a fall time between successive secondary voltage pulses of between about 50 microseconds and about 100 microseconds and/or a duration of between about 200 microseconds and about 10 milliseconds.  
         [0018]     The present invention also includes within its scope a source of light pulses that includes both the current source of the present invention and an arc lamp operationally connected to the current source.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:  
         [0020]      FIG. 1  is a circuit diagram of a prior art circuit for driving an arc lamp;  
         [0021]      FIG. 2  is a plot of a voltage pulse provided to the arc lamp by the circuit of  FIG. 1  and a plot of the consequent current flow in the arc lamp;  
         [0022]      FIG. 3  is a circuit diagram of a circuit of the present invention for driving an arc lamp;  
         [0023]      FIG. 4  is a plot of the voltage pulses provided to the arc lamp by the circuit of  FIG. 3  and a plot of the consequent current flow in the arc lamp. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]     The present invention is of a method for pulsing an arc lamp, and a circuit for implementing the method. Specifically, the present invention can be used to produce light pulses whose intensity profiles have any desired shapes.  
         [0025]     The principles and operation of an arc lamp driving circuit according to the present invention may be better understood with reference to the drawings and the accompanying description.  
         [0026]     Referring now to the drawings,  FIG. 3  is a circuit diagram of a circuit  40  of the present invention for driving arc lamp  12 . Circuit  40  is identical to circuit  10 , as indicated by the use of identical reference numerals in  FIGS. 1 and 3 , except for the substitution of a programmable driver  42  for driver  28 . Driver  42  is programmable to introduce multiple pulses of operating voltage into arc lamp  12 .  
         [0027]      FIG. 4  shows an example of multiple pulses of operating voltage V S  provided by using driver  42  to open and close gating switch  18  so as to induce a flow of current I L  in arc lamp  12  that has a substantially square pulse shape. After power supply  34  has been turned on briefly to provide the trigger pulse, driver  42  closes and opens gating switch  18  to provide a primary operating voltage pulse  44  that is substantially identical to the voltage pulse illustrated in  FIG. 2 . Then, after the current flow I L  in arc lamp  12  has decayed from I Lmax  to a preselected intermediate value I Lint , driver  42  again closes gating switch  18 , and I L  rises with a slope V S /L. When I L  again reaches I Lmax , driver  42  opens gating switch  18 . In other words, after I L  has decayed from I Lmax  to I Lint , driver  42  closes and opens gating switch  18  to provide a secondary operating voltage pulse  46 A that brings I L  back up to I Lmax .  
         [0028]     Four more times, as illustrated in  FIG. 4 , after the current flow I L  in arc lamp  12  has again decayed from I Lmax  to I Lint , driver  42  again closes and opens gating switch  18  to provide four more secondary operating voltage pulses  46 B through  46 E that bring I L  back up to I Lmax . After the last pulse, gating switch  18  is kept open and I L  is allowed to decay exponentially with the time constant L/R. The resulting current flow I L  in arc lamp  12  has a pulse shape that, rather than being substantially triangular as in the prior art, is substantially square, or at least closer to square than the prior art pulse shape. Correspondingly, the intensity profile of the light emitted by arc lamp  12  is substantially square as a function of time.  
         [0029]     It will be clear to those skilled in the art that, in principle, any desired pulse shape can be achieved by the correct selection of the delays between successive operating voltage pulses, and by the correct selection of the duration of the operating voltage pulses. Preferably, the delay between successive operating voltage pulses is such that I L  decays to not less than half of its immediately preceding maximum value between successive operating voltage pulses. This guarantees that the conductive path between anode  24  and cathode  26  is maintained between operating voltage pulses so that additional trigger pulses are not needed.  
         [0030]     Preferably, the duration of the primary operating voltage pulse is between about 10 microseconds and about 40 microseconds. Preferably, the duration of each secondary operating voltage pulse is between about 10 microsecond and about 20 microseconds. Preferably, the various components of circuit  40  are such that I L  has a rise time between about 10 microseconds and about 20 microseconds during each secondary operating voltage pulse and a fall time, between successive secondary operating voltage pulses, of between about 50 microseconds and about 100 microseconds. Corresponding parameters for the components of circuit  40  include: for the inductance of coil  22  when its core is saturated: 5 to 10 microhenries; for the inductance of the connecting wires: 5 to 10 microhenries; and for the total resistance of all components to the right of capacitor  16  in  FIG. 3 : 100 to 200 milliohms.  
         [0031]     Preferably, the duration of I L , from t 1  until a time to the right of  FIG. 4  at which I L  has substantially decayed, is between about 200 microseconds and about 10 milliseconds.  
         [0032]     While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.