Abstract:
The power supply device ( 14 ) for an ion-bombardment-induced secondary-emission electron source in a low-pressure chamber includes a control input, two high-voltage outputs, an element for generating a plurality of positive pulses on a high-voltage output, and an element for generating a negative pulse on the other high-voltage output after at least some of the positive pulses.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to the field of pulsed electron sources and devices that make use of such sources, notably gas lasers with electronic excitation or X-ray pulsed pre-ionisation. A pulsed electron source emits an electron beam under the effect of ion bombardment. 
     2. Description of the Related Art 
     Reference may be made to the publications FR 2 204 882 or FR 2 591 035. The device comprises an ionisation chamber and an acceleration chamber communicating with the ionisation chamber through a grid. A preliminary discharge takes place in the ionisation chamber. Some of the positive ions thus created are accelerated towards a cathode located in the acceleration chamber. The accelerated ions bombard the cathode and cause the secondary emission of electrons. The accelerated secondary electrons, being repelled by the negative voltage applied to the cathode, then form an electron beam extracted through the grid between the two chambers. 
     In fact, it tends to become more and more difficult to trigger the discharge in the ionisation chamber as the use of the device continues. The discharge is thus initiated progressively later and there is a danger that it will occur at the same time as the negative voltage impulse applied to the cathode. The simultaneous application of the positive voltage in the ionisation chamber and of the negative voltage in the acceleration chamber risks causing a breakdown or even destruction of the device and the systems for which the device is used. The delayed triggering of the discharge will in any case cause a deterioration in the characteristics of the electron beam obtained as it leaves the source. Natural and hence uncontrolled delaying of the triggering of the discharge in the ionisation chamber is unsatisfactory. 
     SUMMARY OF THE INVENTION 
     The present invention sets out to remedy the drawbacks outlined above. 
     The aim of the invention is in particular to obtain a stable triggering of the electron source which is relatively independent of the operating conditions, such as the ageing of the source. 
     The electricity supply device for an ion-bombardment-induced secondary-emission electron source in a low pressure chamber comprises a control input, two high voltage outputs, a means for generating a plurality of positive pulses at one high voltage output and a means for generating a negative pulse at the other high voltage output after at least some of the positive pulses. Generating a plurality of positive pulses that can be applied to an electrode of an ionisation chamber makes it easier to trigger the discharge. 
     In one embodiment, the device comprises means for generating a delay between the end of the operation of the means for generating a plurality of positive pulses and the start of the operation of the means for generating a negative pulse. The delay may be constant or adjustable in order to adapt to the operating parameters, notably the pressure, the molecular mass of the gas, etc. 
     In one embodiment, the means for generating a plurality of positive pulses is configured so that the first pulse is at a voltage which is higher than that of the following pulses. Even if the first discharge in the ionisation chamber is delayed, the initiation delay stabilises rapidly. The negative pulse can then be controlled after a length of time D 1  has elapsed since the command to initiate the last positive pulse, while the length of time D 2  between the actuation of the last positive pulse and the triggering of the last discharge in the ionisation chamber may be known precisely. The length of time D 3  between the triggering of the last discharge in the ionisation chamber and the actuation of the negative pulse may be determined using the formula D 3 =D 1 −D 2 . Thanks to the invention, the uncertainty as to the length of time D 2  is substantially reduced. 
     The method for supplying electricity to an ion-bombardment-induced secondary-emission electron source in a low pressure chamber comprises a step of generating a plurality of positive pulses at one high voltage output, and a step of generating a negative pulse at another high voltage output after at least some of the positive pulses. 
     In one embodiment, a delay other than zero separates the end of the last positive pulse of the series of positive pulses from the start of the negative pulse. This ensures the safety of the device. 
     In one embodiment, the peak voltage of the first positive pulse is greater than the peak voltage of the following positive pulses. The first discharge is made easier by a first high voltage pulse. The discharge can easily be obtained during the following pulses with a lower voltage. The energy consumption is reduced and the ageing of the electricity supply is less. 
     In one embodiment, the peak voltage of the following positive pulses is substantially equal. 
     In one embodiment, the duration of the following positive pulses is substantially constant. The reduction in uncertainty as to the length of time D 2  makes it possible to increase the precision of the length of time D 3 . 
     The voltage of at least one pulse may be increased in the course of ageing. 
     The electron source comprises a low pressure chamber, an acceleration chamber, a cathode located in the acceleration chamber, an anode located in the low pressure chamber, and an electricity supply device provided with two high voltage outputs, one connected to the anode and the other to the cathode. The electricity supply device comprises means for generating a plurality of positive pulses and means for generating a negative pulse after the positive pulses. 
     In one embodiment, the source comprises a command module for the means for generating a plurality of positive pulses and for the means for generating a negative pulse. The command module may be configured so as to calculate the delay that will prevent a positive pulse and a negative pulse from occurring simultaneously. 
     In this way the risks of malfunction, or even failure, of the electron source are considerably reduced. The service life of the electron source is also increased by the reduction in the ageing of the electricity supply and of the ionisation chamber. The cost of using the electron source is thus optimised. 
     It is also possible to progressively increase the voltage generating the discharge in the course of ageing. 
     It would also be possible to use an auxiliary source at the cathode, optionally coupled with a system for magnetic confinement of the electrons. However, the service life of the source is then limited because of the vaporisation of the hot anode and the deposit of vaporised materials that forms on the walls of the ionisation chamber, causing a deterioration in the functioning of the source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The present invention will be better understood from a study of the detailed description of a number of embodiments taken as non-restrictive examples and illustrated by the attached drawings, wherein: 
         FIG. 1  is a schematic view of an electron source; 
         FIG. 2  is a curve showing the evolution of the outputs of the command module; 
         FIG. 3  is a curve showing the evolution over time of the supply voltage and current; 
         FIG. 4  is a curve showing the evolution over time of the voltage at the terminals of the electrode of the ionisation chamber; and 
         FIG. 5  is a schematic view of the electricity supply. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As can be seen in  FIG. 1 , the electron source  1  comprises an acceleration chamber  2  and an ionisation chamber  3  defined by an enclosure  4 . The ionisation chamber  3  may be elongated in a main direction. 
     The enclosure  4  comprises an outer casing  5  and an inner wall  6  separating the chambers  2  and  3 . The enclosure  4  may be made of metal, for example based on brass or stainless steel. The inner walls defining the acceleration chamber  2  on the one hand and the ionisation chamber  3  on the other hand may be covered with a metal or a metal alloy suitable for the intended use, notably in terms of the electric voltage applied and the gas in the enclosure  4 , particularly the nature and pressure of the gas. For example, a coating based on aluminium or nickel may be used to cover the walls of the acceleration chamber  2 , and/or the walls of the ionisation chamber  3 . 
     The acceleration chamber  2  and the ionisation chamber  3  are connected via a passage  7  in the form of a through-hole formed in the inner wall  6 . The passage  7  may be provided with a grid  8 , generally made of metal. An exit  9  is provided in an outer wall of the ionisation chamber  3  opposite the inner wall  6 . The exit  9  may be open or fitted with a grid, especially if a gas of a similar nature and at a similar pressure is present in the enclosure  4  and around the enclosure  4 . If the conditions of pressure and/or the nature of the gas are different, the exit  9  is generally provided with a seal, not shown, for example in the form of a part made of synthetic material which is impermeable to gas and at least partly permeable to electrons so as to allow the electron flux generated in the source  1  to escape. The seal may also be covered with a layer of metal, notably based on metal with a high atomic mass of, for example, more than 50, with a view to generating X-rays under the effect of the electron bombardment. 
     The electron source  1  comprises a cathode  10  mounted in the acceleration chamber  2 . The cathode  10  may be fixed or rotary. The cathode  10  may be made of a material based on stainless steel or an aluminium alloy. The cathode  10  may take the form of a disc presenting a flat surface  10   a  facing the passage  7  or a cylinder. The passages  7  and  9  and the flat surface  10   a  of the cathode  10  are aligned. The cathode  10  is supported by a gas-tight insulator  11 , fixed in a hole formed in an outer wall of the casing  5 . The insulator  11  may also be aligned with the openings  7  and  9 . The insulator  11  forms an electrical pathway allowing the cathode  10  to be supplied with electricity from outside the casing  5 . 
     The electron source  1  comprises an anode  12  arranged in the ionisation chamber  3 . The anode  12  may take the form of one or more wires elongated in the main direction of the chamber  3 . The wire may be supplied with power at both ends with a view to increasing the homogeneity of the electrical field. 
     The anode  12  is supported by a leaktight insulator  13  fixed to a side wall of the outer casing  5 , forming a gastight seal and providing the electrical pathway. The anode  12  is offset relative to the alignment of the openings  7  and  9 . 
     The electron source  1  comprises an electricity supply  14  comprising a supply module  15  for the cathode  10 , a supply module  16  for the anode  12  and a command module  17 . The supply module  15  and the supply module  16  may be of the type shown in  FIG. 5 . The command module  17  is configured so as to generate pulse control signals which are offset in time between the signal sent to the supply module  16  and the signal sent to the supply module  15 . This time offset may be adjusted as a function of the gas pressure in the acceleration chamber  2  and ionisation chamber  3  and the nature of the gas or the gaseous mixture, notably the atomic mass. 
     In operation, the command module  17  sends a signal  18 , see  FIG. 2 , to the supply module  16 . The signal  18  is in the form of a plurality of rectangular signals, notably five such signals. The number of pulses may be increased over time to compensate for the ageing of the source  1 . Then, the command module  17  sends a signal  19  to the supply module  15  to apply a high negative voltage to the cathode  10 . The signal  19  may be synchronised with the end of the signal  18 , optionally with a delay (not shown), or be sent before the end of the signal  18  but after the beginning. 
     In  FIG. 3  the bold lines indicate the waveforms of the voltage while the fine lines show the current supplied by the supply module  16  to the anode  12 . The number N denotes the rank of the voltage pulse applied. At the first voltage pulse, the current discharge does not take place until after a high voltage has been applied for a relatively long period. Then this period of a high voltage preceding the discharge decreases from the first to the fourth pulse and remains substantially constant at the fifth pulse. It will be understood that in  FIG. 3  the time scales relating to each pulse have been aligned vertically for the purposes of the drawing. Naturally, the pulse of rank N occurs after the pulse of rank N−1. After the last, in this case the fifth, pulse, the command module  17  sends the signal  19  to the supply module  15 , causing a high negative voltage to be applied in the form of the curve  20  to the cathode  10 . The negative voltage pulse  20  applied to the cathode  10  starts after a length of time D 4  has elapsed after the end of the maximum value of the positive voltage pulse on the anode  12 , or in other words substantially after the end of the last command pulse of the signal  18  received by the supply module  16 . Insofar as the duration of the positive voltage pulse on the anode  12  is substantially constant at the n th  pulse, with N=5 in this case, the said duration can be determined by the operating conditions such as the voltage value, the gas pressure, the nature of the gas, the distance between the anode  12  and the walls of the ionisation chamber  3 , etc. The duration of the n th  positive voltage pulse can be estimated or measured experimentally. The command module  17  can be configured simply and economically to generate the command pulse  19  after a period of time equal to the sum of the length of time D 4  and the duration of the positive voltage pulse has elapsed after the end of the command pulse  18 . 
     In one embodiment, shown in  FIG. 4 , the command module  17  generates a positive voltage command signal comprising a first pulse of a duration greater than the duration of the other pulses of the signal  18 , resulting in a longer charge time of the supply module  16  and a higher voltage for the first positive voltage pulse applied to the electrode  12  than that of ranks 2 or more. The Applicant has in fact noticed that the first discharge is particularly difficult to achieve and can be obtained faster and more easily with a higher voltage. The positive voltage pulses of ranks 2 or more can be obtained with a lower voltage, resulting in less stress on the supply module  16  which is subjected to less wear in this case. The optimum voltage can be selected for the first pulse for triggering the first discharge and the optimum voltage for the following pulses can be selected for the stability of the discharges. The voltage of the following pulses may be between 80 and 100% of the voltage of the first pulse. For this purpose a supply module  16  of the pulsed type may be chosen wherein the charging time T-supply is greater than the periodicity of the pulses T. The first discharge is triggered by a higher voltage than the other discharges. 
     Thanks to the invention, the electron source with multi-pulse triggering supplies a stable electron beam with reduced ageing, while being largely unaffected by the factors of duration and conditions of use. To compensate for the ageing it is also possible to increase over time the voltage of the first pulse, the voltage of the following pulses and/or the number of the following pulses. A regulating knob or automatic regulator may be provided for this purpose. Maintenance is very easy. 
     During operation, the acceleration chamber  2  and ionisation chamber  3  are filled with a gas, for example helium at a low pressure of between 1 and 20 Pascal, for example. The application of a positive voltage to the anode  12 , while the enclosure  4  is connected to earth, causes a voltage pulse discharge. The electrical discharge in the ionisation chamber  3  containing gas causes positive ions to be emitted. Then the voltage pulse at the anode  12  ceases and the negative voltage pulse at the cathode  10  is produced. The positive ions are then attracted by the cathode  10  and travel through the passage  7  to bombard the flat surface  10   a  of the electrode  10  along the trajectory indicated by the arrow  21 . The ion bombardment of the cathode  10  causes electrons to be emitted, which are subjected to a repelling effect of the cathode  10  as a result of the high negative voltage applied by the supply module  15 . The electrons are accelerated along the trajectory indicated by the arrow  22 , travel through the passage  7  then through the exit  9  and thus provide an electron beam. 
     As shown in  FIG. 5 , the electricity supply  15  comprises a pulse transformer  28  provided with a primary  29  and a secondary  30 . The primary  29  of the pulse transformer  28  is connected to earth on the one hand and to a capacitor  31  on the other hand. On the opposite side from the primary  29 , the capacitor  31  is connected to a voltage source U 0  and to a switch  32 . The switch  32  is also connected to earth so as to be able to short-circuit the capacitor  31  and the primary  29 . The secondary  30  is connected to the earth of the power supply on the one hand and to the cathode  10  of the electron source  1  on the other hand. 
     The electricity supply  15  may also comprise, mounted parallel to the secondary  30 , an auxiliary voltage source supplying the bias voltage and connected to the earth of the power supply on the one hand and to the common point between the secondary  30  and the electrode  3 , on the other hand. A protective device may be arranged in series with the auxiliary source so as to limit the current circulation. The protective device may comprise at least one diode, a capacitor and/or an inductor. Moreover, a current sensor may be provided at the output from the power supply  15  for measuring the current consumed in the ionisation chamber  2 . 
     During the first phase, the switch  32  forms an open circuit. The capacitor  31  is charged to the voltage U 0 . 
     The auxiliary voltage source may maintain the cathode  10  at the positive bias voltage. To limit the losses in the secondary  30 , a diode, not shown, may be arranged between the secondary  30  and the point that is common to the protective device and to the cathode  10 . After the switch  32  has been closed, short-circuiting the capacitor  31  and the primary  29  of the transformer  28 , a high negative voltage pulse −U gun  is supplied by the secondary  30  of the transformer  28  and applied to the cathode  10 . 
     The electron source  1  may be modelled electrically by a parasitic capacitance C gun . The parasitic capacitance C gun  may be reduced considerably on account of the absence or, failing that, the very small amount, of plasma in the acceleration chamber  2  during the first ionisation step. When plasma is present in the acceleration chamber  2 , the polarisation of the plasma generates a strong parasitic capacitance. Thanks to the application of the positive bias voltage which prevents positive ions from the plasma from entering the acceleration chamber  2  during the first step, the acceleration chamber  2  is substantially free from plasma at the moment when the high negative voltage −U gun  is applied to the cathode  10 . The parasitic capacitance C gun  therefore remains low. The charging voltage U 0  of the power supply  15  may be reduced. Alternatively, the transformation ratio of the transformer  28  may be reduced.