This application claims priority of German Application Serial No. 20 2006 017 713.2, filed Nov. 16, 2006, which is hereby incorporated by reference herein.
1. Field of the Invention
The present invention relates to a beam analyzing system and a method for analyzing pulsed particle or laser beams.
2. Discussion of Prior Art
In accelerator machines in which a pulsed beam (particle or laser beam) is used the exact determination of the position of a beam pulse in time and space as well as the determination of the energy of the beam pulses is of major importance. For example, in a free electron laser a pulsed electron beam is used to generate pulses of coherent light in a so-called undulator installed at the end of the course of beam. For this purpose the electron beam is steered on a sinusoidal path in the undulator by magnetic fields, such that coherent light is emitted into the forward direction of the electron beam. By virtue of the pulsed electron beam this coherent light is also pulsed.
In experiments with free electron lasers these pulsed characteristics are for example used to conduct measurements of ex-cited states of atoms or molecules, wherein the excited states are initially generated by a pulse of an excitation laser. In experiments of this kind the time between the excitation pulse on the one hand and the pulse of the free electron laser on the other hand is selectively varied to thereby determine properties of excited states, such as decay times.
It results from this that it is of major importance for experiments of this kind to know the exact timing of the beam pulses relative to a pulsed reference signal, wherein a pulsed laser beam may be used as reference signal.
In the following, this timing is referred to as “phasing” of the beam pulses.
Moreover, the spatial position of the particle beam is also of interest as in certain sections of the accelerator a variation of the transverse position corresponds to a variation in energy. An exact determination of the energy of the particle beam can therefore in those sections be accomplished by a precise measurement of the transverse position of the particle beam.
From the prior art it is known to use an antenna in a beam pipe section for the determination of the timing of the particle beam. By means of the beam pulse a voltage pulse is induced in the antenna and e.g. conducted through a band pass filter and amplified afterwards. The amplified signal is mixed with a reference signal in an HF-mixer to a lower frequency. The phase information is then extracted from the low-frequency signal.
It is disadvantageous about a system of this kind that the mixed output signal is inter alia affected by variations of the frequency or the phase of the reference signal, thus a drift appearing in the reference signal leads to the output signal varying independently from the timing of the beam pulse. Due to the fact that just a small frequency band of the electrode signal is analyzed the signal levels are in addition low, which limits the resolution of this method.
For the measurement of the arrival time of laser beam pulses a method for determining the cross-relations between a reference laser beam and a laser beam to be analyzed by means of a frequency-doubling crystal is known from the prior art. However, it is a major disadvantage that there is not a suitable crystal for any combination of a reference laser with a laser beam to be analyzed available, e.g. in the UV-domain or at too low power outputs.
Starting form the prior art, it is therefore the object of the present invention to provide a system and a method for analyzing a beam, such that the timing and the spatial position of a particle or laser beam pulse as well as its energy can be determined with a high precision and without the disadvantages described above.
According to a first aspect of the present invention, this object is solved by a beam analyzing system including a detector unit, a unit for generating a pulsed reference laser beam, a first electro-optical modulator and a first readout photo detector, wherein the optical input of the first electro-optical modulator is connected with the unit for generating a pulsed reference laser beam, wherein the optical output of the first electro-optical modulator is connected with the first readout photo detector and wherein the signal input of the first electro-optical modulator is connected to the detector unit.
Using the beam analyzing system according to the invention it is exploited in case of a particle beam that a signal pulse generated during the passage of a particle pulse by an electrode arrangement serving as the detector unit arranged at a beam pipe section is used to modulate the intensity of one or more pulses of the pulsed reference laser beam in the first electro-optical modulator.
Provided that the voltage pulse of the detector unit has a reference point such as a zero-point that lies in the range of the pulse where the voltage changes heavily, the timing of the reference point relative to the reference laser pulse can be deduced from the modulated intensity of the reference laser pulse. In case of a laser beam a photo detector serves as the detector unit arranged such that a voltage pulse is generated if a laser pulse hits it. Apart from that the functionality is the same.
In general, the following method for measuring the timing of a pulsed beam relative to a reference signal can be conducted. A pulsed reference laser beam is used as the reference signal and fed into a modulation system. Furthermore, a voltage pulse of a detector unit interacting with the particle or laser beam is conducted as a control signal to the modulation system, wherein the voltage pulse is induced when a particle or laser beam pulse passes or hits, respectively, the detector unit. The modulation system modulates the intensity of the pulses of the reference laser beam dependent on the phasing of the volt-age pulses relative to the reference laser pulses, such that the intensity of the reference laser pulses is a measure for the relative phasing.
By this the advantage arises that, if the pulsed reference laser beam is used to synchronize the whole accelerator machine or the laser system, respectively, this optical signal used for the synchronization can directly be used to determine the timing of the particle or laser beam pulses.
Furthermore, the beam analyzing system according to the present invention enables for instance to measure the arrival time of a particle or laser beam pulse relative to the reference signal with significantly higher resolution compared to the prior art. A resolution of up to 10 fs is reached by this.
Within the scope of the present invention a “beam pipe section” relates to a portion generally under vacuum condition through which a beam pulse of a particle or laser beam is directed. This can be a conventional beam pipe section or a resonator cavity of the accelerator section. In case of a laser beam a beam pipe is not necessary in this sense, nevertheless a laser beam can be directed through a beam pipe. In addition, a photo detector in terms of the present invention relates to a unit that, upon a hit of a light pulse, generates an electric signal of which the intensity corresponds to that of the light pulse. For instance, this can be a photo diode, a photo transistor or a photo multiplier. Finally, “connection” between two or more components of the system or two or more components of the system “connected” means within the scope of the present invention that these components comprise an electrical, optical or other connection suitable for a signal transport.
According to an embodiment of the invention there is a first delay unit interposed between the unit for generating a pulsed reference laser beam and the optical input of the first electro-optical modulator for adjusting a delay time for the pulsed reference laser beam. This has the advantage that the system can be adjusted to the voltage pulse generated by the detector unit, such that, when the particle or laser beam pulse has the desired phasing relative to the reference laser pulse, a zero-point of the voltage pulse coincides in the electro-optical modulator with a reference laser pulse.
Preferably, the electro-optical modulator is adjusted such that the intensity of the reference laser pulse output by the modulator is lowered to a defined pre-adjusted level when then voltage at the signal input is zero and raised or lowered with respect to this pre-adjusted level dependent on the impressed voltage.
Thereby, a deviation from the desired phasing causes a deviation in the intensity at the optical output of the electro-optical modulator with respect to the pre-adjusted level, wherein this deviation is a direct measure for the shift in the phasing.
If the system is, according to a preferred embodiment of the invention, to detect the arrival time of a particle beam pulse, the detector unit in form of an electrode arrangement arranged in a beam pipe section preferably comprises an annular bracket and electrode members, wherein the electrode members are electrically insulated with respect to the bracket. Furthermore, radial bores are provided in the annular bracket, wherein the electrode members are formed as bolts extending inside the bores and wherein the portion of the bolts facing the particle beam comprises an essentially constant diameter. Finally, the bolts are retained by insulating bushes in the bores.
Such an arrangement makes sure that the voltage pulse output by the electrode system has a range about the zero-point that is monotonously rising or falling and comprises a high gradient. The latter has the advantage that an intensity deviation of the laser pulse output at the output of the electro-optical modulator with respect to the pre-adjusted level arises al-ready at a small phase shift. So, with this preferred embodiment of the invention a high precision of the determination of the phase shift can be achieved.
As an alternative to the above-mentioned detection of the arrival time of particle beam pulses another preferred embodiment of the invention can also be used to determine the spatial position of the particle beam pulse perpendicular to the beam direction in a beam pipe section.
With this the electrode system comprises an electrode member extending transversely with respect to the direction of the particle beam pulses and comprising first and a second end portion. Furthermore, there is a second electro-optical modulator and a second readout photo detector provided, wherein the optical input of the second electro-optical modulator is connected with the unit for generating a pulsed reference laser beam and the optical input of the second electro-optical modulator is connected with the second readout photo detector. Finally, the first end portion of the electrode element is connected with the signal input of the first electro-optical modulator and the second end portion of the electrode element is connected with the signal input of the second electro-optical modulator.
Moreover and preferably, there is a second delay unit arranged between the unit for generating a pulsed reference laser beam and the optical input of the second electro-optical modulator for adjusting a delay time for the pulsed reference laser beam. This enables, as already for the determination of the arrival time, to adjust the position of the laser pulses relative to the voltage pulses output by the electrode system.
If the system is, according to a preferred embodiment of the invention, to detect the arrival time of a laser beam pulse, a photo detector serves as the detector unit, which is arranged such that a laser beam pulse hits it completely or partially and a voltage pulse is thereby generated. The faster the photo detector is the steeper is the voltage pulse. As described above, by means of a delay unit put in place upstream the relative position of the reference laser pulse can be adjusted such that a steep shoulder of the voltage pulse is sampled. The arrival time of the laser beam to analyze can then be deduced from the deviation of the amplitude of the reference laser beam pulse at the output of the electro-optical modulator.
With this, fluctuations in the amplitude of the laser beam to be analyzed are problematical. These would be misinterpreted as a shift in the arrival time using the system described above. This can be corrected by splitting up the reference laser beam and sampling the early shoulder as well as the late shoulder of the voltage pulse by means of two electro-optical modulators and two delay units that are put in place upstream with respect to the two electro-optical modulators, respectively.
Therefore, there are a second electro-optical modulator and a second readout photo detector provided in another embodiment of the inventive beam analyzing system, wherein the optical input of the second electro-optical modulator is connected with the unit for generating a pulsed reference laser beam and the optical output of the second electro-optical modulator is connected with the second readout photo detector. In addition, the signal output of the photo detector is connected with the signal inputs of the first electro-optical modulator and the second electro-optical modulator, respectively.
With this embodiment of the invention amplitude fluctuations of the laser beam to be analyzed lead to symmetrical amplitude fluctuations of the two reference laser pulses which can be well separated from shifts in the arrival time of the laser beam to be analyzed that lead to asymmetrical amplitude shifts of the two reference laser pulses.
According to a second aspect of the invention the above-mentioned object is achieved by a method for analyzing a pulsed particle or laser beam,
In the inventive method the pulsed reference laser that is used as the reference signal beam is modulated by the voltage pulse that may serve as a control signal from the detector unit. In particular, when a particle or laser beam passes or hits, respectively, the detector unit the intensity of the pulses of the reference laser beam are modulated dependent on the phasing of the voltage pulse relative to the reference laser pulses. Thereby, the measured intensity of the reference laser pulses is a measure for the relative phasing between the voltage pulses and the reference laser pulses.
The inventive method has the advantage, that in case the pulsed reference laser beam is, as already mentioned, used to synchronize the complete accelerator system or the laser system, respectively, this optical signal used for the synchronization can directly be used for the determination of the timing or, where applicable, the spatial position of the particle or laser beam, such that a very precise measurement is made possible. With this, resolutions of up to 10 fs are yielded. Furthermore, using a preferred embodiment of the inventive method, in case of a laser beam to be analyzed, the phasing of an accelerator field for the particles as well as the amplitude thereof can be determined with a high precision.
Preferably, a nominal value of the phasing of the pulsed reference laser beam relative to the pulsed particle or laser beam is adjusted. This can for example be accomplished by means of a delay unit downstream with respect to a unit for generating a pulsed reference laser beam, which makes it possible that the system can be adjusted to the voltage pulse generated by the detector unit, such that if the particle or laser beam has the desired phasing with respect to the reference laser pulse corresponding to the nominal value a zero-point of the voltage pulse coincides with a reference laser pulse during the modulation, such that the reference laser pulse is in this case modulated in intensity as it is given for a voltage of 0 Volts.
In case of analyzing a particle beam an electrode system may serve as a detector unit. The voltage pulse generated by the electrode system may either be induced by the pulsed particle beam itself if the electrode system is arranged in a conventional beam pipe section or else by an accelerator field if the electrode system is arranged at a resonator. In the latter case the pulses have sinusoidal shape.
In another preferred embodiment of the method for analyzing a particle beam the transversal position of the particle beam in the beam pipe section can be determined. The transversal position can be a measure for the energy of the particle pulses.
There, the electrode system arranged in a beam pipe section comprises an electrode element extending transversely with respect to the particle beam direction and having a first and a second end, and at both ends of the electrode element a voltage pulse is output when a particle beam passes the electrode system. The pulsed reference laser beam is split up into a first pulsed reference laser beam and a second pulsed reference laser beam and the intensity of the first pulsed reference laser beam is modulated by the first voltage pulses as well as the intensity of the second pulsed reference laser beam is modulated by the second voltage pulses. The intensities of the modulated reference laser pulses are acquired, wherein the phasing of the first voltage pulses relative to the first reference laser pulses are determined from the intensity of the modulated by the first reference laser pulses and the phasing of the second voltage pulses relative to the second reference laser pulses are determined from the intensity of the modulated by the second reference laser pulses.
Also in this electrode system the voltage pulses comprise a zero-point and a monotonously rising or falling range about the zero-point. The system can be adjusted such that in both electro-optical modulators the zero-point coincides with a reference laser pulse when the beam pulse pass through the beam pipe section in longitudinal direction at a transversal reference position. In this case the intensity of the laser pulses in both modulators is not modulated with regard to the pre-adjusted level.
Provided though that the beam pulse passes through the beam pipe section in longitudinal direction at a transversal position different from the reference position the zero-points do not coincide with the reference laser pulse as the voltage pulses output at the ends are shifted with respect to those which are induced when the beam pulse pass through the beam pipe section at the transversal reference position. This time shift of the voltage pulse results in turn in that the laser pulses in the electro-optical modulator are then modulated with regard to the pre-adjusted level, wherein the variation of the intensity is a measure for the displacement of the transversal position of the beam with respect to the reference position.
There, first and/or second delay units may serve on the one hand to define the transversal reference position by adjusting a first and/or a second nominal value of the phasing of the first and/or reference laser pulses relative to the particle beam, respectively. On the other hand, the delay units serve to account for the phasing of the beam pulses relative to the reference laser beam in such a way that the zero-points of the voltage signals actually coincide with the laser pulses when the beam passes through the beam pipe section at the transversal reference position. Therefore, using the delay units a possible shift in the phasing can also be accounted for.
In case of an electrode system serving as a detector unit being arranged at a resonator that provides an accelerator field for a particle beam, also the phasing and the amplitude of the accelerator field can be determined. If the electrode system is arranged at a resonator it detects the accelerator field itself and a voltage signal can be tapped thereof which is proportional to the accelerator field and in particular to the timing shape thereof and can be fed in to the electro-optical modulators.
Given that the frequency of the pulsed reference signal is not an integer multiple of the frequency of the accelerator field or a fraction thereof a laser pulse coincides with the voltage signal having a different phasing with respect to the accelerator field, respectively. Thereby, each laser pulse samples a different point in the range of one wave length of the accelerator field, such that those modulated intensities output by the modulator contain an information about the phase and the amplitude of the accelerator field. In case the frequency of the reference signal is below that of the accelerator field, this can be referred to as “undersampling”.
For analyzing a laser beam a photo detector as detector unit is arranged such that laser beam pulses hit partially or fully the active surface of the photo detector and corresponding voltage pulses are generated.
In a preferred embodiment of the method for analyzing a laser beam a photo detector serves as a detector unit that is arranged such that a laser beam pulse hits partially or fully the active surface of it and a voltage pulse is generated thereby. As described above, using the delay units put in place upstream the relative position of the reference pulses can be adjusted such that a steep shoulder of a voltage pulse is sampled. From the variations of the amplitude of the reference laser pulses at the output of the electro-optical modulator the arrival time of the laser beam to be analyzed can then be deduced.
In order to prevent misinterpretations of amplitude fluctuations of the laser beam to be analyzed as shifts in the arrival time, the pulsed reference laser beam is, in another preferred embodiment of the method, wherein the voltage pulses generated by the laser beam in the photo detector comprise a first and a second shoulder, split up into a first pulsed reference laser beam and a second pulsed reference laser beam. The phasing of the first pulsed reference laser beam is adjusted such that the first shoulder of the voltage pulse is sampled and the phasing of the second pulsed reference laser beam is adjusted such that the second shoulder of the voltage pulse is sampled. The intensity of the first pulsed reference laser beam and the second pulsed reference laser beam is modulated by the first and the second voltage pulses, respectively, wherein the intensities of the modulated reference laser pulses are measured. The phasing of the first voltage pulses relative to the first reference laser pulses is determined from the intensity of the modulated first reference laser pulses and the phasing of the second voltage pulses relative to the second reference laser pulses is determined from the intensity of the modulated second reference laser pulses.
Thus, the reference laser beam is split up and the voltage pulses sampled at the early shoulder as well as at the late shoulder by means of two electro-optical modulators and delay units put in place upstream with respect to those. Amplitude fluctuations of the laser beam to be analyzed result in symmetrical amplitude fluctuation of both reference laser pulses and can be well separated from shifts in the arrival time of the laser beam to be analyzed which result in asymmetric amplitude variations in both reference laser pulses.
Other aspects and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments and the accompanying drawing figures.
Preferred embodiments of the invention are described in detail below with reference to the attached drawing figures, wherein:
The drawing figures do not limit the present invention to the specific embodiments disclosed and described herein. The drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the preferred embodiment.
The beam analyzing system 1 comprises an electrode system 5 that is arranged in the beam pipe section 3. There, the electrode system 5 is set up such that a voltage pulse is induced when the electrode system 5 is passed by a beam pulse that runs through the beam pipe section 3 along the beam axis 7.
In this embodiment the electrode system 5 has the setup shown in
By this, the electrode elements in form of the bolts 13 are arranged on the inner surface of the annular bracket 9. Additionally, this setup of the electrode system 5 results in that a voltage pulse is induced when the bolts 13 of the electrode system 5 are passed by a beam pulse that runs along the beam axis 7.
In particular, the electrode system shown in
As furthermore follows from
An electro-optical modulator 27 is arranged downstream with respect to the delay unit 25 wherein the delay unit 25 is connected with the optical input 29 of the modulator 27. The optical output 31 of the modulator 27 is connected to a readout photo detector 33 for measuring the intensity of the reference laser pulses, wherein a photo detector in this regard refers in terms of the present invention to an element that generates an electrical signal upon a hit by a light pulse where the strength of the electrical signal corresponds to the intensity of the light pulse. Preferably, InGaAs photo diodes are deployed as photo detectors.
Finally, the bolts 13 of the electrode system 5 are connected with the signal input 35 of the electro-optical modulator 27 such that the voltage pulse induced by a beam pulse may result in a modulation of the intensity of the laser pulses generated by the unit 23.
Moreover, a signal processing unit 37, for example in form of an analogue-digital converter, is arranged downstream with respect to the readout photo detector 33.
The beam analyzing system 1 described above can be used in the following way to determine the arrival time of a beam pulse at the electrode system 5, wherein
The unit 23 generates reference laser beam pulses 39 with a frequency that is significantly higher than that the accelerator system produces beam pulses with. But this is not necessarily the case. There, the delay unit 25 is initially adjusted such that a reference point such as a zero-point 17 of a voltage pulse induced by a beam pulse coincides at the electro-optical modulator 27 with a laser pulse 39 when the beam pulse has the desired phasing relative to the reference laser signal. By this, a nominal value of the phasing of the reference laser beam relative to the pulsed particle beam is adjusted.
Preferably, the modulator 27 is adjusted such that the intensity of the laser pulses output at the optical output 31 has a pre-adjusted level, for example 50% of the maximum level, when a voltage of 0 Volts is present at the signal input 35. However, if a positive or negative voltage is present the intensity is raised or lowered with respect to the preadjusted level.
Therewith, such an adjustment results in that the laser pulse going into the optical input 29 of the modulator 27 is not modulated in its intensity with respect to the pre-adjusted level when a beam pulse has the exactly the desired phasing corresponding to the nominal value. Thus, the laser pulses 39 are detected at the readout photo detector 33 with non-modulated intensity.
In case the beam pulse has not the desired phasing the zero-point 17 of the voltage pulse does not coincide with a laser pulse 39. The laser pulse 39, as seen in
In both cases the intensity of the laser pulses 39 is modulated with respect to the pre-adjusted level because of the voltage of the voltage signal differing from zero, wherein this modulation is measured by the readout photo detector 33 and processed further by the signal processing unit 37. There, the modulation is a direct measure for the shifting of the phasing with respect to the desired value because of the monotonously rising shape of the voltage signal between the points 19 and 21.
Here, the chosen electrode system 5 shown in
According to a preferred embodiment of the beam analyzing system the following method may therefore be performed for determining the timing of the pulsed particle beam relative to a reference signal. A pulsed reference laser beam that is used as the reference signal is fed into a modulation system. As a control signal a voltage pulse of an electrode system arranged in a beam pipe section is conveyed to the modulation system, wherein the voltage pulse is induced when a beam pulse passes the electrode system. By means of the modulation system the intensity of the pulses of the reference laser beam are modulated depending on the phasing of the voltage pulses relative to the laser pulses. Therewith, the intensity of the laser pulses is a measure for the relative phasing.
In
There is in this second embodiment an electrode system 5′ arranged in a beam pipe section 3′, wherein the electrode system 5′ comprises in this insofar preferred embodiment two electrode elements 41 transversely extending with respect to the direction 7′ of the beam pulses and having a first end 43 and a second end 45.
Besides the unit 23 for generating a pulsed reference laser beam the beam analyzing system 1′ comprises furthermore a first measuring leg comprising a first delay unit 25, a first electro-optical modulator 27, a first readout photo detector 33 and a first signal processing unit 37 which are connected with each other as described in connection with the first embodiment. Moreover, there is analogously provided a second measuring leg comprising a second delay unit 25′, a second electro-optical modulator 27′, a second readout photo detector 33′ and a second signal processing unit 37′. With this setup the pulsed reference laser pulse is split up into a first and a second pulsed reference laser beam.
The signal input 35 of the first electro-optical modulator 27 is connected with the first end 43 of the electrode element 41 and the second end 45 is connected with the second electro-optical modulator 27′. But is possible, though, that both first ends 43 are connected with the first modulator and both second ends 45 are connected with the second modulator 27′.
For determining the position of the beam pulses perpendicular to the beam direction 7′ following the method for analyzing a beam with the beam analyzing system 1′ according to the second embodiment one proceeds as follows.
At the first and the second ends 43, 45 of the electrode elements 41 first and second, respectively, voltage pulses are output when a beam pulse passes the electrode system 5′, wherein the voltage pulses show a shape similar to that of
The beam analyzing system 1′ is adjusted by means of the delay units 25, 25′ such that in both electro-optical modulators 27, 27′ the zero-point of the voltage pulse exactly coincides with a reference laser pulse when a beam pulse passes the beam pipe section at a transversal reference position. So, corresponding nominal values of the phasings are adjusted. In these cases the intensity of the laser pulses are not modulated by the modulators 27, 27′ with respect to the pre-adjusted level.
However, if the beam pulse passes the beam pipe section 5′ at another transversal position the zero-points do not coincide any more with the reference laser pulse as the voltage pulses output at the ends 43, 45 are shifted in time with respect to those that are induced when the particle beam pulse passes the beam pipe section 5′ at the transversal reference position. This time shift of the voltage pulses and the associated shift of the phasings results in that the laser pulses are then modulated in the intensity by the electro-optical modulators 27, 27′ with respect to the pre-adjusted level, wherein the modulation of the intensity is a measure for the shift of the transversal position of the particle beam pulse from the reference position.
Therein, the first and second delay units 25, 25′ may initially serve for defining the transversal reference position. Furthermore, the delay units 25, 25′ serve for taking into account the phasing of the particle beam pulses relative to the pulsed reference laser beam in such a way that the zero-points of the voltage pulses actually coincide with the laser pulses 39 when a particle beam passes the beam pipe section 3′ at the transversal reference position. That is to say if the phasing of the particle beam pulses fluctuates relative to the reference signal the effect that the zero-points shift relative to the reference signal occurs as described in connection with the determination of the arrival time. Thereby, a variation of the phasing may be accounted for by using the delay units 25, 25′.
In the
In particular,
The preferred forms of the invention described above are to be used as illustration only, and should not be utilized in a limiting sense in interpreting the scope of the present invention. Obvious modifications to the exemplary embodiments, as hereinabove set forth, could be readily made by those skilled in the art without departing from the spirit of the present invention.
The inventors hereby state their intent to rely on the Doctrine of Equivalents to determine and assess the reasonably fair scope of the present invention as pertains to any apparatus not materially departing from but outside the literal scope of the invention as set forth in the following claims.
Number | Date | Country | Kind |
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202006017713.2 | Nov 2006 | DE | national |