X-ray pulse source and method for generating X-ray pulses

Abstract

X-ray pulse source (100) for generating X-ray pulses (1) includes electron pulse source device (10) including photo-emitter device (11) being configured for photo-induced creation of free electron pulses (2) and driver device (12) being configured for creating electromagnetic driver pulses (3) accelerating electron pulses (2) along acceleration path (7), and electromagnetic interaction device (50) comprising electromagnetic pulse source device (51) being configured for creating electromagnetic pulses (4) in interaction section (5) of electromagnetic interaction device (50), wherein electron pulse source device (10) and electromagnetic interaction device (50) are operable for generating X-ray pulses (1) by an interaction of electron pulses (2) and electromagnetic pulses (4), and driver device (12) includes THz driver pulse source (13), which is configured for creating single cycle or multi cycle THz driver pulses (3). Furthermore, a method of creating X-ray pulses (1) is described.

Claims

1. An X-ray pulse source, being configured for generating X-ray pulses, comprising: an electron pulse source device comprising a photo-emitter device being configured for photo-induced creation of free electron pulses and a driver device being configured for creating electromagnetic driver pulses accelerating the electron pulses along an acceleration path, and an electromagnetic interaction device comprising an electromagnetic pulse source device being configured for creating electromagnetic pulses in an interaction section of the electromagnetic interaction device, wherein the electron pulse source device and the electromagnetic interaction device are operable for generating the X-ray pulses by an interaction of the electron pulses and the electromagnetic pulses, and the driver device includes a THz driver pulse source, which is configured for creating THz driver pulses.

2. The X-ray pulse source according to claim 1, wherein the photo-emitter device comprises a field emitter array and a laser source arranged for irradiating the field emitter array with excitation pulses.

3. The X-ray pulse source according to claim 2, wherein the laser source and the field emitter array are configured for creating temporarily modulated electron pulses.

4. The X-ray pulse source according to claim 1, wherein the THz driver pulse source comprises at least one of the features the THz driver pulse source is coupled via a THz driver waveguide with the acceleration path, and the THz driver pulse source includes multiple source stages each being configured for creating the THz driver pulses.

5. The X-ray pulse source according to claim 1, further comprising an accelerator device being arranged between the electron pulse source device and the electromagnetic interaction device and comprising at least one THz accelerator pulse source, wherein the accelerator device is arranged for additionally accelerating the electron pulses along the acceleration path.

6. The X-ray pulse source according to claim 1, comprising at least one of a compressor device being arranged between the electron pulse source device and the electromagnetic interaction device for temporarily compressing the electron pulses traveling along the acceleration path, and a focusing device being arranged between the electron pulse source device and the electromagnetic interaction device for focusing the electron pulses to the interaction section of the optical interaction device.

7. The X-ray pulse source according to claim 6, wherein the electron pulse source device and the electromagnetic pulse source device are operable such that the interaction of the electron pulses and the electromagnetic pulses is a coherent interaction, wherein the electromagnetic pulses provide an undulator field in the interaction section.

8. The X-ray pulse source according to claim 1, wherein the electron pulse source device and the electromagnetic pulse source device are operable such that the interaction of the electron pulses and the electromagnetic pulses is an incoherent interaction comprising an inverse Compton scattering.

9. A method of creating X-ray pulses, comprising the steps of photo-induced generating of electron pulses and accelerating the electron pulses along an acceleration path by the effect of electromagnetic driver pulses, and creating the X-ray pulses by an interaction of the electron pulses and electromagnetic pulses, wherein the driver pulses comprise THz driver pulses.

10. The method according to claim 9, wherein the electron pulses are generated by optical field emission.

11. The method according to claim 9, further comprising at least one of the steps of the THz driver pulses are coupled via a THz driver waveguide with the acceleration path, and the THz driver pulses are generated with multiple source stages each being configured for creating the THz driver pulses.

12. The method according to claim 9, further comprising additionally accelerating the electron pulses along the acceleration path.

13. The method according to claim 9, further comprising at least one of temporarily compressing the electron pulses traveling along the acceleration path, and focusing the electron pulses to an interaction section.

14. The method according to claim 9, wherein the interaction of the electron pulses and the electromagnetic pulses is a coherent interaction, wherein the electromagnetic pulses provide an undulator field.

15. The method according to claim 9, wherein the interaction of the electron pulses and the electromagnetic pulses is an incoherent interaction comprising inverse Compton scattering.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further details and advantages of the invention are described in the following with reference to the attached drawings, which show in:

(2) FIG. 1: a schematic illustration of an X-ray pulse source according to a preferred embodiment of the invention, and

(3) FIG. 2: further details of the X-ray pulse source according to FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(4) Features of preferred embodiments of the inventive X-ray source and the inventive method for creating X-ray pulses are described in the following with reference to FIGS. 1 and 2 showing the combination of an electron pulse source device including the THz driver pulse source for creating single cycle and multi-cycle THz driver pulses with the electromagnetic undulator. It is emphasized that details of the pulse sources for creating excitation pulses for the photoemission of the electrons, THz pulses for driving, accelerating and compressing purposes, and electromagnetic undulator pulses, as well as details of the inverse Compton scattering or FEL processes are not described as they are known as such from prior art. Furthermore, it is emphasized that the figures present schematic illustrations only. Details of the X-ray source, like sensor and monitoring equipment, vacuum equipment or enclosing vessels are not shown. The components of the X-ray source are illustrated as being controlled with a control device which may comprise a single control circuit or a plurality of control circuits separately coupled with the components of the X-ray source.

(5) The X-ray source is illustrated with reference to a preferred embodiment for coherent X-ray generation. To this end, driver, accelerator, compressor and focusing devices are provided as outlined below. It is emphasized that the accelerator, compressor and focusing devices represent optional features of the invention. In particular, the accelerator device could be omitted with both of the coherent or incoherent X-ray generation if the driver device provides sufficient acceleration, and the compressor and focusing devices can eventually be omitted especially with the incoherent X-ray generation.

(6) According to the embodiment of FIG. 1, the X-ray source 100 comprises an electron pulse source device 10, an accelerator device 20, a compressor device 30, a focusing device 40, an electro-magnetic interaction device 50 and a control device 60 coupled with each of the components 10 to 50. A straight acceleration path 7 extends in an evacuated waveguide channel (see FIG. 2) from the electron pulse source device 10, in particular from the photo-emitter device 11 thereof, to the electromagnetic interaction device 50 along a z-direction (axial direction). The plane perpendicular to the acceleration path 7 extends in an x-y-plane (radial directions).

(7) The electron pulse source device 10 comprises the photo-emitter device 11 and the driver device 12. The photo-emitter device 11 includes a laser source 16 and a field emitter array 14. The laser source 16 is a pulse laser, like e.g. an amplified femtosecond laser, which preferably is provided with a control of carrier-envelope phase (CEP). The field emitter array 14 (enlarged schematic illustration in FIG. 2) is a nanostructured photocathode having a characteristic extension in radial direction of about 5 m to 100 m. As an example, the field emitter array 14 comprises a planar arrangement of Si-tips with a mutual distance of e.g. 0.5 m to 1 m and a height in axial direction in the range of 2 m to 50 m. As an example, 10.sup.6 emitter tips provide an emitter portion of the field emitter array 14. With alternative examples, the single emitters of the field emitter array 14 comprise metallic islands with sharp tips on an atomic scale, having a height of 200 nm and a mutual distance of 400 nm, arranged on an emitter portion with an extension of 30 m 30 m. According to yet another example, the single field emitters can be provided by quantum well structures created in a substrate of a semiconductor material. The use of quantum well structures may have particular advantages in terms of creating low energy spread electrons (cold electrons).

(8) Excitation pulses 6 are created with the laser source 16 with a repetition rate f.sub.rep, a pulse duration .sub.exc and a wavelength .sub.exc selected with the control device 60, in particular in dependency on the material and geometry of the field emitter array 14. As examples, f.sub.rep is above 100 Hz, e.g. in the range from 1 kHz to 1 MHz, while .sub.exc is in a range from 5 fs to 200 fs, e. g. 10 fs, .sub.exc in a range from 200 nm to 10.000 nm, e. g. 800 nm, and an excitation pulse energy is in a range of 100 nJ to 10 J. The excitation pulses 6 may comprise rectangular shaped multi-cycle laser pulses that are polarized along the in-plane field emitter elements (y-axis). The incoming field strength of the excitation pulses 6 is tuned such that plasmonic and tip field enhancement lead to a resulting field at the tips, which is typically 10 to 100 times stronger than the incoming field alone and sufficient for field emission of preferably one electron per emitter and cycle.

(9) The field emitter array 14 is arranged at an input side of the waveguide channel 18. Photo-emitted electrons released by the field emitter array 14 are directly subjected to driving single- or multi cycle THz driver pulses 3 created by the driver device 12. Alternatively, the photo-emitted electrons are pre-accelerated as follows (see FIG. 2). The electrons freed at the field maximum in each cycle of the excitation pulses 6 can be accelerated by radially polarized THz pulses, which is created with a separate THz pulse source (not shown) or split from THz driver pulses 3 created by the driver device 12. The THz pulses are directed to the field emitter array 14 via a THz waveguide 19 (see FIG. 2), or they may propagate through the substrate of the field emitter array 14. This process results in the formation of the electron pulse 2, which is emitted from the field emitter array 14 and further accelerated by the strong THz driver pulses 3. Advantageously, the impact of the THz field on the emission is weak and very slow over the single or multi-cycle IR-field of the excitation light 6 by the physical arrangement of the field emitter array 14 as the longitudinal driving THz field is orthogonal to the field emitter array elements, or feature sizes are used, which are smaller than the THz skin depth, so that a plasmonic enhancement of the THz field is prevented.

(10) The driver device 12 comprises a THz driver pulse source 13 coupled via a waveguide 15 (see FIG. 2) with the evacuated waveguide channel 18. The THz pulse source 13 may be designed as described e.g. in reference [34]. The THz driver pulse source 13 creates single-cycle THz driver pulses 3 with a duration .sub.driv, a repetition rate f.sub.rep, a frequency of the carrier wave f.sub.driv, a phase .sub.driv and an amplitude A.sub.driv adjusted with the control device 60. The pulse duration .sub.driv is selected in a range of e.g. 1 ps to 100 ps. The repetition rate f.sub.rep of the THz driver pulses 13 is matched to the repetition rate of the excitation pulses 6. The THz driver pulses 3 are reflected via a mirror 17 onto the acceleration path 7. The mirror 17 includes a central hole through which electron pulses 2 can travel along the acceleration path. The polarization of the THz driver pulses 3 is selected such that the axial component of the electron pulses 2 is accelerated.

(11) With a practical example, the emitted electron pulses 2 have the following features. The radial size of the electron pulses 2 is in a range of 20 m to 100 m, and the axial length is in a range of 0.03 m to 10 m, e. g. 3 m. The irradiation spot size of the excitation pulses 6 on the field emitter array 14 is in a range of 20 m to 100 m. Each electron pulse 2 emitted at the field emitter array 14 has a duration of 0.01-10 fs and carries a charge of about 1 pC to 10 pC. By the application of the THz driver pulses 3, the electron pulses 2 are accelerated to relativistic energy of about 1 MeV to 2 MeV.

(12) The electron pulses 2 have a modulation structure as shown in the enlarged illustration in FIG. 2. The modulation structure is obtained by the excitation pulses 6 and the simultaneous excitation of all emitter tips of the field emitter array 14.

(13) The illustrated driver device 12 includes a THz driver pulse source 13 with one single source stage only. In a practical embodiment, the single THz driver pulse source 13 can have multiple THz driver stages, each being operated with an appropriately selected frequency f.sub.driv and/or phase .sub.driv, so that a net acceleration of the electron pulse 2 is obtained at the output of the electron pulse source 10.

(14) The accelerator device 20 is coupled with the waveguide channel 18 (see FIG. 2). The accelerator device 20 includes a THz accelerator pulse source with a consecutive arrangement of multiple accelerator source stages 21, 22, which are arranged for additionally accelerating the electron pulses 2 along the acceleration path 7. THz acceleration pulses are coupled via THz waveguides 23, 24 (see FIG. 2) and the mirrors 25 to the acceleration path 7 in the waveguide channel 18. The mirrors 25 have a double function in terms of superimposing a current THz pulse with an electron pulse and reflecting a previous THz pulse from a forgoing acceleration stage out of the waveguide channel 18 (see FIG. 2).

(15) The frequency of the carrier wave, phase and amplitude of the THz accelerator pulses are adjusted by the control device 60 for a constructive acceleration of the electron pulses 2. As a result, the electron pulses 2 have relativistic speeds of about 10 MeV to 30 MeV at the downstream side of the accelerator device 20. This acceleration can be obtained by single-cycle or multi-cycle THz accelerator pulses, which typically have an energy of 5 mJ to 20 mJ.

(16) The electron pulses 2 are accelerated in the waveguide channel 18 by the single cycle THz driver pulses 3 and the single-cycle or multi-cycle THz accelerator pulses. To achieve efficient acceleration, the waveguide channel 18 preferably provides phase matching between the phase velocity of the THz wave and the electron pulse 2 propagating at a speed less than c is necessary. This matching task is solved by using a dielectrically loaded metal waveguide excited with a TM01 mode. As an example, 20 mJ, 10-cycle pulses at 0.6 THz frequency can accelerate a 1 MeV initial electron bunch up to 10 MeV within only 4 cm propagation including the losses in copper and the group velocity dispersion of the waveguide.

(17) Subsequently, the electron pulses 2 are transferred via the waveguide channel 18 to the compressor device 30. The compressor device 30 includes a THz compressor source 31, which is arranged for temporarily compressing the electron pulses 2 traveling along the acceleration path 7. The THz compressor source 31 creates THz compression pulses, which are coupled via the THz waveguide 32 (see FIG. 2) and the mirrors 33 to the acceleration path 7 in the waveguide channel 18. By the effect of THz compression pulses, an initial pulse length in a range of 10 fs to 100 fs is compressed to a final pulse length in a range of 100 as to 1 fs. With a practical example, for a 1 pC bunch, a bunch length of 500 as is equivalent to a 2 kA electrical current.

(18) The compressor device 30 compresses the relativistic electron pulses 2 to small bunch lengths. For this purpose, the concept of rectilinear compression [10] is applied. The same structure as the accelerators will be used to achieve a THz traveling wave with phase velocity equal to the speed of light. However, the electron pulses 2 will reside in the zero crossing of the THz field, leading to zero total acceleration and effective compression of the bunch. As an example, the electron bunch compression using a 100 cycle 20 mJ THz beam traveling through the waveguide channel 18 provides a 60 fold compression of the electron pulses, meaning that the initial electron bunch length of about 30 m is compressed to 0.5 m at the waveguide output. This large amount of compression is feasible only with large field gradients available from THz beams.

(19) The THz accelerator source stages 21, 22 and the THz compressor source 31 comprise sources creating THz pulses by optical rectification as described e.g. in reference [34]. As illustrated in FIG. 1, the THz driver pulses 3, the THz accelerator pulses and the THz compressor pulses are created by separate THz sources. Alternatively, the THz accelerator and compressor sources 21, 22, 31 can be provided by a single common THz source coupled via waveguides and phase shifting components to the waveguide channel 18. Advantageously, both acceleration and compression tasks can be fulfilled by a single THz pulse source.

(20) At the downstream side of the compressor device 30, the focusing device 40 is coupled via the waveguide channel 18. The focusing device 40 comprises an arrangement of quadrupole magnets 41 (see FIG. 2), which are controlled such that the electron pulses 2 are focused into the interaction section 5 of the electromagnetic interaction device 50. With a practical example, an initial radial size in a range of 5 m to 10 m is focused to a final radial size in a range of 1 m to 3 m. The electron pulses 2 traveling to the interaction section 5 of the electromagnetic interaction device 50 have a charge of e. g. a 3 pC per pulse, a pulse length of with 100 as to fs, and about 15 MeV energy. The energy spread is around 1%.

(21) Finally, the electron pulses 2 enter the electromagnetic interaction device 50 comprising an interaction section 5, where the electron pulses 2 collide for example with high power electromagnetic pulses 4, e. g. laser pulses to produce coherent X-ray pulses 1 via the FEL process. The electromagnetic pulses 4 are created with an electromagnetic pulse source device 51, comprising e. g. a laser source with 1 J and 1 ps long pulses, which is controlled by the control device, in particular for setting the undulator wavelength .sub.0 in a range of 1 m to 200 m, the pulse length .sub.0 in a range of 500 fs to 20 ps, a laser spot size in a range of 5 m to 20 m and a laser pulse energy in a range of 0.1 J to 2 J. The repetition frequency f.sub.rep is adapted to the repetition frequency of the electron pulses 2. The electromagnetic pulses 4 comprise e. g. 500 cycle 1 J pulses at .sub.0=1 m wavelength focused to a w.sub.0=20 m spot size in the interaction section 5.

(22) Based on a one-dimensional FEL model, the creation of the X-ray pulses 1 can be described as follows. The equivalent laser strength parameter or normalized vector potential a.sub.0 for the above laser electromagnetic pulses 4 is:

(23) $\frac{E_0^2}{2Z_0}{w}_0^{2}500\frac{{}_0}{c\sqrt{4\ln 2}}=1J.Math.E_0=\sqrt{\frac{1\left[J\right]240c\sqrt{4\ln 2}}{500w_0^2{}_0}}=1.54{10}^{12}V\text{/}m.Math.a_0=\frac{{\mathrm{eE}}_0}{mc}=0.48$
(E.sub.0: electric field strength, Z.sub.0: free space impedance, e. g. 377 Ohm, : the laser angular frequency, c: the speed of light, e: the electron charge, and m: the electron rest mass) The radiated X-ray wavelength .sub.X will then be

(24) ${}_X=\frac{{}_0}{4{}^2}\left(1+a_0^2\right)=0.34\mathrm{nm}$

(25) It can be shown that in an inverse Compton scattering process the trajectory of the electrons are the same as the trajectory in a FEL process with .sub.u=.sub.0/2, with .sub.u being the undulator wavelength. The gain length for the resulting free electron laser is obtained as follows

(26) $\mathrm{Density}\mathrm{of}\mathrm{electrons}=n_e=\frac{3{10}^{-12}}{e{\left(5{10}^{-6}\right)}^{2}3{10}^{-8}}=2.5{10}^{25}\frac{l}{m^3}$$\frac{L_{g0}}{{}_l/2}=\frac{1}{\sqrt{3}}{\left(\frac{{}^3{m}_e}{{}_0{a}_0^2{e}^2{\left({}_l/2\right)}^2{n}_e}\right)}^{\frac{1}{3}}32.$

(27) The 500 cycle optical undulator will then be equivalent to about 16 gain lengths which is sufficient for achieving saturated FEL operation. At saturation, the amount of X-ray energy produced is determined by the parameter of the FEL, which reads as

(28) ${}_{\mathrm{FEL}}=\frac{1}{4\sqrt{3}}\frac{1}{32}=0.0015=0.15\%.E_{\mathrm{sat}}={}_{\mathrm{FEL}}{E}_{\mathrm{beam}}=1.5{10}^{-3}3{10}^{-12}15{10}^6=6.75{10}^{-8}J$

(29) The number of photons in each pulse is then obtained as follows:

(30) $E_{ph}=\mathrm{hv}=\frac{\mathrm{hc}}{{}_X}=5.8{10}^{-16}=3.65\mathrm{keV}$

(31) Total number of photons: 1.1610.sup.8

(32) Advantageously, the electron beam energy and the wavelength of the electromagnetic interaction device 50 can be independently increased or decreased to achieve different X-ray beam parameters selected in dependency on the particular application of the X-ray source 100.

(33) In the above calculations, the one-dimensional FEL model can be applied due to the large beam width compared to the laser pulses 4 wavelength. The criterion for validity of the 1D FEL theory is r.sub.b>>{square root over (L.sub.g0.sub.X)}, which yields 5 m>>1.36 nm. At a repetition rate of 1 kHz, the inventive X-ray source 100 would provide an average flux of 10.sup.11 photons per second, which is an order of magnitude above that necessary for phase contrast imaging, for example.

(34) In the following table, a comparison between the estimated flux from the inventive THz-driven X-ray source 100 with an optical undulator and LCLS (Linac Coherent Light Source) at SLAC in the USA is presented.

(35) TABLE-US-00001 Parameter THz-driven Source LCLS X-ray photon energy 4 keV 12.4 keV 9.6 keV Pulse charge [pC] 3 3 150 Electron beam energy [MeV] 15 25 10,000 Photon wavelength [nm] 0.3 0.1 0.13 Relativistic factor 30 50 20,000 Photon number 1.6 10.sup.8 1.6 10.sup.8 2 10.sup.12 Average photon flux [10.sup.12 ph/s] 0.6 0.6 2 10.sup.2 Peak flux [10.sup.25 ph/s] 1.2 1.2 2 Average power [mW] 0.2 0.6 500 Bandwidth (FWHM) [%] 1 1 0.2-0.5 Average brightness [10.sup.20 ph/ 1.4 10 160 (s 2% BW mm.sup.2 mrad.sup.2] Peak brightness [10.sup.33 ph/ 6 180 2 (s 2% BW mm.sup.2 mrad.sup.2] Pulse length [as] 100 7.5 100,000 Repetition rate 1 kHz 1 kHz 120 Hz

(36) The comparison between the two sources shows that the invented compact X-ray source 100 can produce X-ray radiation with similar peak flux compared to large scale FEL facilities. This application is a very advanced application for coherent X-ray sources targeting protein crystallography and high density and warm matter physics.

(37) Further applications would benefit from the quasi-monochromatic laboratory scale hard X-ray source of the invention, having orders of magnitude larger average flux and higher spatial coherence than current laboratory sources. Current laboratory scale sources are still based on rotating anode tubes working on the physical basis of the original tube invented by Roentgen more than 100 years ago. Such applications are phase contrast imaging to increase the spatial resolution in medical imaging and sensitivity to soft tissues, which only provide index contrast rather than an absorption change. Phase contrast imaging allows for micron level screening of the body, which is important to address artefacts in plague detection in vessels during cardio-vascular diagnosis. The quasi-monochromatic output enables a much lower dose to patients. The size of this source makes it fit to hospital installations. Other applications are various X-ray scattering techniques for material analysis. The high brightness and small spot size of the source, when compared to rotating anode tube sources, enable nm-scale resolution.

(38) The features of the invention disclosed in the above description, the drawing and the claims can be of significance both individually as well as in combination or sub-combination for the realization of the invention in its various embodiments.