The present invention relates to time gated image intensifier tubes and particularly to an improved time gated image intensifier tube for use in ultrafast imaging applications, such as time resolved optical tomography.

BACKGROUND OF THE INVENTION

Time resolved optical imaging devices for clinical applications, such as brain functional imaging, generally comprise a light source, an ultrafast detection device, such as a photomultiplier or single photon avalanche diode and electronic control units. Optical coupling is generally carried out by the use of optical fiber and consequently the number of measurement point is restricted to a few units. An emerging and promising new approach is to use ultrafast time gated camera. Such devices aim at providing two dimensional images of a lightened zone with a good temporal resolution. To that purpose, the intensified camera must then be able to provide a temporal resolution of 200 ps Full Width at Half Maximum (FWHM) and a repetition rate of 70MHz or above.

An ultrafast time-gated intensified camera generally comprises a time gated image intensifier tube and a CCD or CMOS camera. A time gated image intensifier tube generally comprises three active components, which are a photocathode, a micro channel plate and a phosphor screen. The photocathode receives the incident photons coming from the exterior environment to convert them into photoelectrons. The micro channel plate (MCP) multiplies the photoelectrons, which are then transformed by the phosphor screen into an intensified light signal. The temporal resolution and the repetition rate of the ultrafast time gated camera depend on the gating speed and on the gating repetition rate of the time gated image intensifier tube. For clinical applications, such as brain functional imaging, the time gated image intensifier tube must be able to reach a temporal gate of 200 ps or less and a repetition rate of 70MHz or above.

To get a fast shutter of the time gated image intensifier tube, rapid electrical pulses whose duration is typically of the order of 100 ps to a few nanoseconds are used. The shutter of the tube may be obtained through two methods: the MCP gating and the photocathode gating. The MCP gating is very effective, and allows, through the non-linear relationship between the gain and the voltage applied to the MCP very fast shutter times. But two major drawbacks limit the usefulness of this method:

- The shutter electromagnetic wave propagates roughly half as fast as in the MCP in the vacuum, which generates spatiotemporal dispersion in the tube;

- the voltage to be applied to the MCP wafer is of the order of 1 kV , which requires a lot of energy and which therefore limits the repetition rate to some kHz.

The photocathode gating requires a lower voltage, typically ten to a few hundred volts maximum. We can see on Figure 1 c that the opening of the tube only requires a voltage of about 2.5V. It is therefore more interesting in the case of a high repetition rate operation. However, it should maintain a sufficiently high voltage to maintain sufficient spatial resolution. Otherwise, as the electric field potentially moves in the gap between the photocathode and the MCP, the propagation velocity can reach the speed of light, which ensures minimal spatiotemporal distortion. The invention concerns this type of shutter.

As a matter of fact, by applying a fast voltage pulse on a conventional time gated image intensifier tube, two major phenomena are observed: an abnormally slow displacement of the electric field in the tube and multiple reflections of the current pulse through the tube.

The article entitled "200 ps FWHM and 1 00 MHz Repetition Rate Ultrafast Gated Camera for Optical Medical Functional Imaging" by Wilfried Uhring et al., discloses a time gated image intensifier tube enabling to achieve an improved gating speed and repetition rate. To that purpose, the article discloses that the propagation of the electromagnetic wave in the photocathode has to be decoupled from the propagation in the rings. Thus the publication proposes a new design of photocathode shape that lead to a main central waveguide between the photocathode and the MCP with a vacuum gap.

The new shape of the photocathode of the time gated image intensifier tube allows reaching a time gate of 200 ps or less. However, the electrical losses in this time gated image intensifier tube are important and some harmonics are generated. Such harmonics should be minimized.

SUMMARY OF THE INVENTION The invention aims at providing a time gated image intensifier tube presenting a high gating speed and a good repetition rate while minimizing electrical losses and undesired harmonics when an electrical wave is guided across the time gated image intensifier tube.

To that purpose, a first aspect of the invention proposes a time gated image intensifier tube comprising:

- a photocathode,

- a micro channel plate,

- a screen,

- an housing holding the photocathode, the micro channel plate and the screen, the housing comprising:

o a first metal ring connected to the photocathode;

o a second metal ring connected to the micro channel plate;

o a ceramic ring disposed between the first and the second metal ring; the time gated image intensifier tube being characterized in that the first metal ring is connected to the second metal ring via at least one resistive link, named "filtering resistive link", said filtering resistive link comprising at least one resistive element arranged between the first and the second metal ring and covering an angular section of between 30° and 150° with reference to a point of application of an electrical pulse on the first metal ring.

The filtering resistive link forming an angle of 30° to 150° with the point of application of an electrical pulse enables to avoid parasitic waves to propagate along to the housing of the time gated image intensifier tube, such that the filtering resistive link enables to minimize undesired harmonics when an electrical wave is guided across the time gated image intensifier tube.

The time gated image intensifier tube according to the first aspect of the invention may also comprise one or several of the following technical features, taken individually or according to all possible combinations:

- the photocathode has a shape wherein :

o two sides disposed along a first axis passing through the point of the first metal ring and wherein said two sides are connected to the first metal ring; and, o two longitudinal sides disposed along a second axis, perpendicular to the first axis, and wherein said two longitudinal sides are disconnected from the first metal ring with a gap between said two longitudinal sides and the first metal ring.

- the time gated image intensifier tube further comprises a filtering resistive link comprising at least one resistive element arranged between the first and the second metal ring at an angle of 90° with reference to the point of application of an electrical pulse on the first metal ring;

- at least one filtering resistive link comprises a resistor;

- at least one filtering resistive link comprises a resistive ribbon;

- at least one filtering resistive link comprises a capacitor;

- at least one filtering resistive link comprises a capacitive ribbon;

- at least one filtering resistive link comprises a printed circuit board on which at least one resistor is mounted;

- at least one capacitor is further mounted on the printed circuit board;

- the total conductance per length of all filtering resistive links linking the first and the second metal ring is included between 0.2 S.m ^"1 and 5 S.m ^"1 , preferably between 1 S.m ^"1 and 2 S.m ^"1 ;

- the first metal ring and the second metal ring are further connected by an adaptation resistive link, the adaptation resistive link being located on the first metal ring in order to be diametrically opposed to the point of application of the electrical pulse on the first metal ring.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of methods in accordance with embodiments of the present invention are now described, by way of example only, and with reference to the accompanying drawings, in which :

- Figure 1 a schematically represents a camera according to one embodiment of the invention ;

- Figure 1 b represents a schematic representation of a time gated image intensifier tube according to one embodiment of the invention ;

- Figure 1 c represents the sensitivity of a time gated image intensifier tube of the prior art as a function of the photocathode voltage;

- Figure 1 d represents the electromagnetic wave propagation of a 45 ps rise time electrical pulse applied to an image intensifier of the prior art; - Figure 2a schematically represents a front view of a time gated image intensifier tube according to one embodiment of the invention ;

- Figure 2b represents the electromagnetic wave propagation in the time gated image intensifier tube of figure 2a;

- Figure 3 schematically represents a sectional view of a time gated image intensifier tube according to one embodiment of the invention ;

- Figure 4 schematically represents a front view of a time gated image intensifier tube according to one embodiment of the invention;

- Figure 5 schematically represents a front view of a time gated image intensifier tube according to one embodiment of the invention;

- Figure 6 schematically represents a front view of a time gated image intensifier tube according to one embodiment of the invention;

- Figure 7 schematically represents a sectional view of a time gated image intensifier tube according to another embodiment of the invention;

- Figure 8 schematically represents a front view of a time gated image intensifier tube according to one embodiment of the invention;

- Figure 9 schematically represents a sectional view of an time gated image intensifier tube according to another embodiment of the invention ;

- Figure 1 0 represents the evolution of the attenuation constant in a housing of an time gated image intensifier tube together with the evolution of the attenuation at the end of the housing, both as a function of the conductance of such housing;

- Figures 1 1 and 12 represent the evolution of the voltage at a point B of the housing of different embodiments of a time gated image intensifier tube as a function of the time;

- Figure 1 3 represents an electric circuit enabling to explain the electric behavior of an housing of an time gated image intensifier tube according to one embodiment of the invention ;

- Figure 14 schematically represents a front view of a time gated image intensifier tube according to another embodiment of the invention;

- Figure 15 represents the photocathode voltage as a function of time in the time gated image intensifier tube of figure 14

- Figure 1 6 represents the temporal gate width and sensitivity of an intensified camera according to one embodiment of the invention according to the photocathode DC bias;

- Figure 17 represents a comb generator topology, - Figure 18 represents a transient simulation of the comb generator behavior of figure 1 7;

- Figure 19 represents another generator that may be used;

- Figure 20 represents another generator that may be used;

- Figure 21 represents another generator that may be used.

DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT

Figure 1 a represents a camera according to one embodiment of the invention, for use in medical imaging applications. This camera comprises a lens 1 , a filter wheel 2, a time gated image intensifier tube 3, a CCD or CMOS camera 4. The camera also comprises a pulse generator 5 enabling to control the time gated image intensifier tube 3 and a controller 6 enabling to control the pulse generator 5. The camera also consists of a light source 8 configured in such a way that it illuminates, through an optical fiber, part of the image 7. The laser 8 is also linked to the controller 6. The intensified camera also comprises a processor 9 enabling to acquire the images of the CCD or CMOS camera.

The very core of the camera is the time gated image intensifier tube 3 and its associated electrical pulse generator 5. This time gated image intensifier tube 2 is represented more precisely on figure 2a and 3. It comprises an input window 1 3, a photocathode 14, a micro channel plate 1 5 and a screen 1 6.

The light emitted by the light source 8 is injected in the optical fiber 7 to obtain a uniform illumination spot on the patient's skin. Backscattered photons 1 1 are collected by the lens 1 and transmit to the time gated image intensifier tube 3 through the filter wheel 2. The photocathode 14 of the time gated image intensifier tube receives these photons and converts them into photoelectrons 12. The micro channel plate (MCP) 1 5 multiplies these photoelectrons. The screen 1 6 converts these photoelectrons into an intensified light signal which is then recorded by the CCD or CMOS camera 4. The photocathode 14 of the time gated image intensifier tube is powered with electrical pulses generated by the pulse generator 5. The structure of the image intensifier may be the one disclosed in EP-B- 2218089.

As a matter of fact, the time gated image intensifier tube is open by applying a short electrical pulse on the time gated image intensifier tube. With reference to figures 2a and 3, the time gated image intensifier tube comprises a housing 1 9 holding the photocathode 13, the MCP 15 and the screen 1 6.

The housing 1 9 comprises:

- a first metal ring 20 connected to the photocathode 1 3;

- a second metal ring 22 connected to a first side of the MCP 15;

- a third metal ring 24 connected to a second side of the MCP 15.

The housing 1 9 further comprises:

- a first ceramic ring 21 inserted between the first and the second metal ring 20 and 22;

- a second ceramic ring 23 inserted between the second metal ring 22 and the third metal ring 24.

In the tubes of the prior art, when an electrical pulse is applied to a point 25 of the first metal ring, instead of directly crossing the time gated image intensifier tube, the electrical wave is partially guided across the first metal ring 20 and across the second metal ring 22. The first ceramic ring 21 has a high dielectric constant ε, i.e. superior to 9.5, which tends to delay the electrical wave. As a matter of fact, in a waveguide, the electrical field is most concentrated in the high dielectric material and has a velocity given by:

ν _ε = (0)

Thus, the electrical pulse propagates along the waveguide 30 formed by the first and the second metal rings 20, 22 that surround the high dielectric constant material. The electrical pulse is split into two parts that propagate on each side of the time gated image intensifier tube and joined themselves at the opposite side of the pulse application point.

The time i _f for the two waves to travel along this path is given by equation (2)

Where d is the metal ring diameter. To overcome the limitation presented above, the propagation of the electromagnetic wave in the photocathode has to be decoupled from the propagation in the rings. The principle is to physically separate the waveguides.

To that purpose, the photocathode 13 presents the shape represented on figure 2a. This shape is such that: - the photocathode has two sides 26, 27 disposed along an axis X passing through the point 25 of the first electrical ring where the electrical pulses are applied. These two sides 26, 27 are connected to the first metal ring 20.

- The photocathode also comprises two longitudinal sides 28, 29 disposed along a second axis Y perpendicular to the first axis X. These two longitudinal sides 28, 29 are disconnected from the first metal ring 20. In other words, there is a gap between each longitudinal side 28, 29 of the photocathode 13 and the first metal ring 20.

This configuration leads to a main central waveguide between the photocathode and the MCP with a vacuum gap.

By comparing the electromagnetic wave propagation in a time gated image intensifier tube of the prior art, represented on figure 1 d, and the electromagnetic wave propagation in the time gated image intensifier tube of figure 2a represented on figure 2b, it is clearly apparent that the photocathode shape enables to improve the gating speed of the time gated image intensifier tube.

Curve A of figure 1 1 shows the voltage measured in a point diametrically opposed to point 25 where the electrical pulse is applied, in a time gated image intensifier tube of the prior art not having the photocathode shape described with reference to figure 3. Curve B of figure 12 shows the same voltage measured in the time gated image intensifier tube of figure 3.

Consequently, the complete aperture of the time gated image intensifier tube is quicker and shorter in the time gated image intensifier tube of figure 2a than in the time gated image intensifier tube of the prior art.

However, curve B of figure 1 2 also shows that propagation through the parasitic wave guide 30 occur during the aperture of the time gated image intensifier tube of figure 2a.

To solve this problem, an improved time gated image intensifier tube is proposed. This improved time gated image intensifier tube is represented more precisely on figures 3 to 9.

This time gated image intensifier tube comprises at least a resistive link 31 , 41 connecting the first metal ring 20 and the second metal ring 22. The resistive link 31 ,41 comprises a resistive component 32 and it may also comprise a capacitive component 33. The attenuation of the parasitic reflections depends on the position and the value of the resistive link(s) 31 , 41 .

Thus, the time gated image intensifier tube may comprise a first resistive link, hereinafter referred to as "adaptation resistive link"41 . This adaptation resistive link 41 is located on the first metal ring 20 in order to be diametrically opposed to the point 25 where the electrical pulse is applied. The adaptation resistive link 41 comprises a resistive component 42 and preferably a capacitive component 43. The adaptation resistive link 41 enables to dampen the parasitic harmonics generated during the reflection of the electromagnetic wave as it clearly appears on curve A of figure 1 1 and as a result the obtained voltage is curve B of figure 1 2.

However, as it is also apparent from curve B of figure 12, the adaptation resistive Iink41 , disposed on the opposite side of the first metal ring with respect to the point 25 where the electrical pulse is applied, does not enable to remove all parasitic harmonics.

To further reduce these parasitic harmonics, the time gated image intensifier tube also comprises at least one resistive link 31 , hereinafter referred to as "filtering resistive link" in the present document. Each filtering resistive link 31 connects the first metal ring 20 to the second metal ring 22. Each filtering resistive link 31 comprises at least a resistive portion forming an angle of between 30° and 150° with the point 25 where the electrical pulse is applied.

As represented on figure 4, the filtering resistive link(s) 31 comprises a resistive component 32 that may be a discreet resistor 32a or a resistive ribbon 32b. The filtering resistive link(s) may also comprise a capacitive component 33. According to different embodiments, the capacitive component 33 may be located between the resistive component 32 and the first metal ring 20 or conversely the resistive component 32 may be located between the capacitive component 33 and the first metal ring 20. Besides, when the time gated image intensifier tube comprises several filtering resistive links, the filtering resistive links may be all identical or they may be all different as represented on figure 4.

Other embodiments of the invention are represented on figures 5 to 9. According to a first embodiment represented on figure 5, the time gated image intensifier tube comprises two filtering resistive link 31 , each filtering resistive link 31 being located on the first metal ring 20 in order to form an angle of 90° with the point 25 where the electrical pulse is applied. Each of these filtering resistive links 31 comprises a discrete resistor, and it preferably also comprises a discrete capacitor. These two filtering resistive links preferably present a total conductance per length unit Gp of between 0.2 S/m and 5 S/m and preferably of between 1 S/m to 2 S/m. These two filtering resistive links 31 enable to reduce the amplitude of the parasitic electromagnetic wave circulating in the parasitic wave guide 30 of the time gated image intensifier tube and to reduce the parasitic harmonics as it is clearly apparent on curve C of figure 12.

According to a second embodiment represented on figure 6, the time gated image intensifier tube comprises n filtering resistive links 31 circumferentially distributed over the periphery of the first metal ring 20. These n filtering resistive links preferably present a total conductance Gp of between 0.2 S/m and 5 S/m and preferably of between 1 S/m to 2 S/m. This second embodiment enables to further reduce the undesired harmonics in the electromagnetic wave as it is clearly apparent from curve D of figure 12.

According to a third embodiment represented on figure 7 and 8, the time gated image intensifier tube comprises a filtering resistive link 31 comprising a resistive ribbon 32. The resistive ribbon may completely surround the first metal ring 20. This resistive ribbon preferably presents a conductance Gp of between 0.2 S/m and 5 S/m and preferably of between 1 S/m and 2 S/m. This third embodiment allows an improvement of the attenuation of the amplitude of the parasitic electromagnetic wave circulating in the parasitic wave guide 30 together with a decrease of the parasitic harmonics as represented on Curve E of figure 12. Alternatively, instead of covering the whole circumference of the first metal ring, the resistive ribbon may only cover a portion of the circumference of the first metal ring as represented on figure 8. In this last case, at least one portion of the resistive ribbon is a circular portion extending from an angle of 30° to an angle of 135° with respect to an axis of symmetry passing through point 25 where the electrical pulse is applied. These portions of resistive ribbon are employed to absorb the parasitic wave propagating along to the first metal ring of the time gated image intensifier tube. Besides, as represented on figure 8, the time gated image intensifier tube may comprise several filtering resistive link comprising each a resistive ribbon covering a portion of the first metal ring 20. The tube may further comprise an adaptation resistive link 41 located on a side of the first metal ring opposed to the point 25 where the electrical pulse is applied. This adaptation resistive link 41 enables to absorb the main wave propagating in the photocathode. Each resistive link may further comprise a capacitive ribbon 33, 43 or a capacitor connected to the resistive ribbon.

According to a fourth embodiment represented on figure 9, the tube may comprise a filtering resistive link 31 comprising a printed circuit board 34 on which are mounted a resistive component 32 and preferably also a capacitive component 33. The printed circuit board is preferably inserted between the first metal ring 20 and the second metal ring 22 such that a first side 39 of the printed circuit board is in electric contact with the first metal ring 20 and a second side 40 of the printed circuit board is in electric contact with the second metal ring 22. An electric connection 44 goes through the printed circuit board in order to electrically connect the first side 39 of the printed circuit board with the second side 40 of the printed circuit board. The resistive component 32 may be on the first side 39 or on the second side 40 of the printed circuit board. The capacitive component 33 may be on the first side 39 or on the second side 40 of the printed circuit board.

To understand why the presence of the filtering resistive link(s) comprising at least a resistive portion extending from an angle of 30° to an angle of 1 35° with respect to an axis of symmetry passing through the point where the electrical pulse is applied, we can approximate the time gated image intensifier tube by the distributed circuit represented on figure 1 presents a characteristic impedance Z _c given by:

Where L, C, R, G are distributed components which described the waveguide parameters given respectively in H/m, F/m, Ohm/m and S/m.

The propagation constant k in this waveguide is given by:

Where a is the attenuation constant (m ^~1 ) of the parasitic wave guide and is given by - afLC + RG - afLC) + ( o ^L {RC + LG)

The higher alpha is, the higher the line attenuation is. The distributed parameters C and L are given by waveguide geometry and ceramic rings. We assume that these parameters cannot be modified once the time gated image intensifier tube is designed. They are estimated thank to the housing geometry. These parameters are set by the geometry of the image intensifier on the basis of the geometric dimensions of the ring and the used ceramic. The higher R and G are, the more important the attenuation. The distributed resistance is mainly defined by the characteristics of the metal of the rings. The properties of metals (covar, copper), the shape of the ring and the skin effect define the resistance. The distributed conductance G is generally given by the ceramic used in the ceramic rings, the so called dielectric loss G _r , but it can be modified by adding filtering resistive link, i.e. additional distributed conductance G _p , along the rings to increase the attenuation constant oc.The total distributed conductance G is the sum of the dielectric loss G _r and of the additional distributed conductance G _p . If we assume that there is no loss arising from the resistive part {R=0) and the dielectric part of the tube housing ( Gr= ), the attenuation factor becomes:

Thanks to the attenuation constant, one can easily calculate the attenuation factor Att of the signal propagating through the parasitic waveguide as :

Att = l - e (0)

Where x is the length of the parasitic waveguide, i.e. the half the metal rings.

Equation (0) and (0) can be used to determine the required distributed conductance G _p in order to have a sufficient attenuation and consequently, they can be used to determine the required distributed conductance of the filtering resistive link(s) of the time gated image intensifier tube.

Figure 10 shows the attenuation constant a and the attenuation factor Att according to the conductance G _p in the range between 0 and 5 S/m for a parasitic waveguide of L=317nH/m, C=1 69pF/m, a length x of 44 mm. A pulsation of 200π.10 ⁶ rad/s, i.e. a frequency of 100 MHz is assumed. Note that, the higher is the frequency, the higher are the losses. For these typical values, in order to achieve an attenuation of more than 95%, that ensures a negligible parasitic signal, a conductance per unit length G _p superior to 1 is preferably used.

If only one conductance is used, in order to have G>1 S/m, the resistor value is preferably less than R _eq <22 Ohms, i.e

G = l/(R _eq x) ^ R _e¾ = l/(G - x) (0)

If only one discrete resistor is used, the use of a too small value of R _eq , i.e. lower than 5 Ω, should be avoided as reflections could appear in the parasitic waveguide.

If several resistors R, on several section x, of the first metal ring are used, as for example represented on figure 6, the parallel connections of the electric resistors along the parasitic waveguide should preferably lead to a conductance G as described below:

∑ = G (0)

R , ^■ x

Where x, is the distance between two consecutive resistors.

If we use n equal resistors R _p equally spaced among the parasitic waveguide, we can reduce the equation (0) to:

_2_ _{= σ} « _{Λ =} _5_ (0)

R _p x ^p G - x

Application on two resistors gives F? _p < 44 Ohms for a G > 1 S/m, thus an attenuation above 95%, in the typical value of the above mentioned example.

Application on three resistors gives R _p < 66 Ohms, etc.

In case of filtering resistive link with capacitor in series, the value of the capacitor should be chosen according to the frequency present in the photocathode voltage signal, typically from 10MHz up to 1 GHz. The impedance of the capacitor at this frequency should be as low as possible. Practical value can be found by using:

C=1 /(0.1 ^* 2 ^* pi ^* f)

Where f is the considered operating frequency. Typical value are in the range from 1 nF up to 10nF. As previously mentioned, the resistive links may comprise a capacitor or they may not comprise a capacitor. The resistive links comprising a capacitor are named "AC", while the resistive links without a capacitor are named "DC". The DC configuration is optimal in term of short signal path and thus in high frequency behavior. The AC configuration allows adding an interesting feature to the tube gating: the adjustment of the temporal gate width as it makes it possible to add a DC offset to the photocathode voltage.

Figures 14 to 1 6 represent embodiment in which the terminals of the resistive links are specifically AC connected to the ground.

The photocathode voltage has to be lower than a threshold voltage in order to the tube to be gate on, i.e. the shutter is open, as represented on figure 1 b. Thus, by assuming a given shape of photocathode voltage e.g. Gaussian or triangular, of the electrical pulse generated for the gating operation, adding a DC offset on the AC coupled electrical pulse allows to reduce the temporal gate width at the cost of a lower sensitivity.

In the example of figure 16, the temporal gate width can be modified from 1000 ps down to 100 ps FWHM thank to the DC offset of the photocathode biasing feature. Meanwhile, the sensitivity keeps relatively constant in the temporal gate width range of 1000 to 400 ps FWHM and decrease to about 50% at a temporal gate width of 200 ps FWHM.

To enable this feature, the first metal ring, i.e. the photocathode, is DC coupled to a static voltage through a choke inductor 45 and/or resistor 46 and the impedance matching resistor/ribbon is AC coupled to the ground.

While the present invention has been particularly described with reference to the preferred embodiments, it should be readily apparent to those of ordinary skill in the art that changes and modifications in form and details may be made without departing from the scope of the invention. For example the number and composition of the resistive link may vary. Besides the resistive links may be directly fixed to the metal rings or they may be fixed on an intermediary piece which is itself fixed on the metal rings.

Generator

In order to ensure a good spatial resolution, the photocathode to MCP voltage during the aperture of the temporal gate has to be as high as possible, typically more than 1 0 volts are necessary to ensure a spatial resolution better than 10 line pair per millimeter. Nevertheless, the total characteristic impedance of the tube is relatively low, typically around 5 Ohms. To drive the photocathode voltage above 10 Volts, the pulse current should be of more than 2 amperes.

A fast pulse generator that can operate at a high repetition rate is the step recovery diode (SRD) based comb generator as represented on figure 1 7. On figure 18, curve (a) represents the generator E _g voltage, curve (b) represents the SRD current, curve (c) represents the load voltage, T is the period of the repetition rate, t ₀ is the full width at half maximum of the voltage pulse, V _p is the voltage pulse amplitude.

This kind of pulse generator generates an electrical pulse of less than 1 nanosecond at a repetition rate of more than 1 0 MHz and up to several GHz. The maximal voltage peak value Vp is limited by the absolute maximal reverse voltage that can be applied to the SRD but also by the maximal current intensity, thus the maximal input power. For low load impedance typically less than 1 0 Ohms, the limiting factor is the maximal current intensity, in the range of 1 ampere for high power SRD, than can be applied to this device. Consequently, the pulse peak V _p is limited below 1 0 Volts with a classical comb generator. To overcome this limitation, another technology or circuit topology must be used. This circuit topology combines several SRD to increase the pulse amplitude. This circuit topology is represented on figure 19. It comprises a periodic generator, a RF amplifier, a splitter, at least two phase tuning, two coils, two E _B i , and two diodes.

The generator can be extended to a multipath topology as during the commutation, the SRD behave as an opened circuit and the inductor behave as a current source that is injected into the load, i.e. the tube. Consequently, the voltage is approximately increase by N, where N is the number of SRD channel. Nevertheless, the mismatch between the SRD produce a mismatch in the pulse generation timing. Thus a coarse phase tuning is mandatory to ensure that each SRD commutes at the same times. When the phase adjustment is sufficient, the parallel SRD are self-synchronizing. It is also possible to use several RF amplifier as shown on figure 20. The phase tuning can be carried out by an adjustable LC or any other technique.

Another way to generate the phase adjusted periodic signals is to use a multi output PLL (phase locked Loop) generator followed by several RF amplifier as represented on figure 21 . While the present invention has been particularly described with reference to the preferred embodiments, it should be readily apparent to those of ordinary skill in the art that changes and modifications in form and details may be made without departing from the scope of the invention.

For example, instead of using a resistive link, we could use a capacitive-resistive link. In that case, the capacitive-resistive link preferably comprises a capacitor and a resistor.