Patent Grant
11770890
The present invention relates generally to x-ray sources. More specifically, it relates to devices and methods for generating tunable, intense, narrowband, fully coherent, soft X-rays.
X-ray methods are the most powerful non-destructive tools for analyzing matter. Electromagnetic radiation in the extreme ultraviolet (EUV) or soft x-ray spectral range (1-100 nm wavelengths or 0.01-1 keV photon energies) is rapidly gaining importance in both fundamental research and industrial applications.
However, successful application depends critically on the brilliance of the available sources. Currently, the degree of coherence and the average photon flux required by advanced applications is only available at large-scale synchrotron facilities and EUV Free Electron Lasers (FELs), severely limiting the range of applications.
In one aspect, the invention provides a compact, lab-sized and affordable soft X-ray source generating tunable, narrowband, fully coherent and intense soft X-ray photons, with a brilliance previously only provided by SLS and/or XFEL facilities.
The device combines an Ultra-Cold Electron Source (UCES) with an electron accelerator and a high-power laser in an Inverse-Compton-Scattering setup. The intense laser beam collides head-on with a counter propagating beam of electrons extracted from the ultra-cold electron source, travelling at a velocity close to the speed of light. Due to the relativistic Doppler effect the laser photons that bounce off the electrons are converted into (soft) X-ray photons, constituting a narrow (soft) X-ray beam travelling in the same direction as the electrons.
The electron pulses are created by a two-step photo-ionization process of an ultracold atomic gas, which enable precise tailoring of the initial electron density distribution in three dimensions. The initial longitudinal density distribution can be modulated by exciting the atoms using a standing wave of light. The excited atoms are then ionized to create a modulated electron distribution (micro-bunches), with a modulation period that is determined by the standing wave of light. The picosecond electron pulse is RF accelerated to a few MeV and simultaneously RF compressed by two orders of magnitude. This means that the modulation period is shrunk by the same two orders of magnitude. The modulation period is now equal to the wavelength of the soft x-ray pulse that is going to be generated. As a result the generated soft X-ray beam will be fully temporally coherent. In addition, the radiation generated by the individual micro bunches will add up coherently so that the intensity will be boosted by an amount proportional to the number of the electrons in the bunch. This boosts the intensity to intensities comparable to SLS and XFELs.
Simultaneously, the picosecond electron pulses extracted from the UCES source which are accelerated to a few MeV have an ultra-low electron temperature which means that the electron beam divergence is smaller than that of a diffraction limited soft X-ray beam; this guarantees the production of a fully spatially coherent soft X-ray beam.
Significantly, the device can generate tunable, narrowband (soft) X-ray beams which are fully coherent and have super-radiant intensity. This provides the realization of a table-top Compton soft X-ray free Electron Laser. This new type of table-top soft X-ray source has a performance in terms of brilliance, intensity and coherence vastly superior to all other compact sources, has many applications, in particular for wafer inspection in the semiconductor industry and high contrast imaging of biological samples in the 2-4 nm water window spectral regime.
At the present there is no alternative method to realize a fully coherent table-top soft X-ray free electron laser. The technique for extracting electrons from the ultra-fast ultra-cold electron source provides pre-bunching to reach longitudinal coherence and super-radiance, ultralow electron temperature (emittance) for transverse coherence. By combining spatial modulation of the photoionization process with radiofrequency bunch compression techniques, micro-bunching at EUV wavelengths and thus coherent amplification is realized.
The device may be used as an injector for an Inverse Compton Scattering (ICS) source. The high degree of coherence provided by the UCES allows the use of new, coherent regimes of ICS at EUV wavelengths. As a result, it has many important applications:
In one aspect, the invention provides a device for generating soft x-rays, the device comprising: an electron source configured to generate an electron beam comprising electron micro-bunches; an electron accelerator configured to accelerate the electron micro-bunches from the electron source; and a laser configured to generate a laser beam colliding with the accelerated electron micro-bunches in a counter-propagating direction to generate the soft x-rays; wherein the electron source comprises: a magneto-optical trap configured to produce an ultracold atomic gas; two counterpropagating excitation laser beams configured to produce a standing wave for inducing a periodic spatial modulation of the ultracold atomic gas along a beam propagation direction; an ionization laser configured to induce photo-ionization of the ultracold atomic gas.
Preferably, the electron accelerator comprises an RF compression cavity and X-band accelerator to simultaneously compress and accelerate the electron micro-bunches. Preferably, the electron accelerator comprises steering coils and a focusing magnetic coil. In some embodiments, wherein the electron accelerator comprises an RF compression cavity configured to operate in TM010 mode. In some embodiments, wherein the electron source comprises a DC plate configured to produce a DC acceleration field to extract the electron micro-bunches from the electron source.
In another aspect, the invention provides a method for generating soft x-rays, the method comprising: generating by an electron source an electron beam comprising electron micro- bunches; accelerating by an electron accelerator the electron micro-bunches from the electron source; and colliding a laser beam with the accelerated electron micro-bunches in a counter-propagating direction to generate the soft x-rays; wherein generating the electron beam comprising electron micro-bunches comprises: producing an ultracold atomic gas by a magneto-optical trap; producing a standing optical wave to induce a periodic spatial modulation of the ultracold atomic gas along a beam propagation direction; inducing photo-ionization of the ultracold atomic gas.
Preferably, wherein accelerating the electron micro-bunches comprises compressing the electron micro-bunches with an RF compression cavity and simultaneously accelerating the electron micro-bunches with an X-band accelerator. In some embodiments, wherein accelerating the electron micro-bunches comprises compressing the electron micro-bunches with an RF compression cavity operating in TM010 mode. Preferably, wherein generating the electron beam comprises extracting the electron micro-bunches from the electron source using a DC acceleration field. Preferably, wherein producing a standing optical wave to induce a periodic spatial modulation of the ultracold atomic gas along a beam propagation direction comprises inducing double-modulation.
An embodiment of the invention comprises an apparatus that entails the combination of an Ultra-Cold Electron Source (UCES) with an electron accelerator and a high-power laser in an Inverse-Compton-Scattering (ICS) setup. The intense laser beam collides head-on with a counter propagating beam of electrons extracted from the ultra-cold electron source, travelling at a velocity close to the speed of light. Due to the relativistic Doppler effect the laser photons that bounce off the electrons are converted into (soft) X-ray photons, constituting a narrow (soft) X-ray beam travelling in the same direction as the electrons. The implementation of the UCES as a source for ICS will lead to unprecedented soft x-ray coherence and brilliance. The electron pulses are created by a two-step photo-ionization process of an ultracold atomic gas, which enables precise tailoring of the initial electron density distribution in three dimensions. The initial longitudinal density distribution can be modulated by exciting the atoms using a standing wave of light.
Inverse Compton Scattering X-Ray Source
In the Inverse Compton Scattering (ICS) process light from an intense laser beam is bounced off a relativistic electron beam, turning it into a bright X-ray beam through the relativistic Doppler effect, as is schematically illustrated in
If high power laser light 100 with wavelength λ0, coming in at an angle θ0 with respect to an electron beam electron 102, is scattered into an angle θx, then the wavelength of the scattered light 104 is given by:
λX=λ0(1−β cos θX)/(1+β cos θ0) (1)
where β=v/c is the velocity of the electrons normalized to the speed of light. For a head-on collision, i.e., θ0=0, with electrons moving at velocities close to the speed of light, i.e., β≈1, Eq. (1) can be approximated by
λX≈λ0(1+(γθX)2)/4γ2 (2)
with γ=(1−β2)−1/2, the Lorentz factor of the relativistic electron beam. For example, for a laser wavelength λ0=500 nm and a moderately relativistic electron beam with kinetic energy Ukin=2 MeV, i.e., β=0.98 and γ=5, soft X-rays will be generated at wavelengths as short as λx=5 nm. The X-rays will be emitted in a cone with a half angle of about γ−1 centered around the direction of the electron beam, with the shortest wavelengths being generated in the forward direction (θX=0) and progressively longer wavelengths for increasing O. The intrinsic narrowband nature of an ICS based source, combined with its high degree of directionality and the straightforward way in which the X-ray wavelength can be tuned continuously by simply changing the electron beam energy, make it a very attractive method for generating X-rays. Arguably it is the cleanest, purest and most controlled way of generating X-rays.
Unfortunately, however, the efficiency of the ICS process is very low. Assuming the electron beam waist is much smaller than the laser beam waist, the number of X-ray photons Nx produced when a bunch of Ne electrons collides with a laser pulse of N0 photons is given by
NX=NeN0σT/2πw02, (3)
where στ=6.65×10−29 m2 is the Thomson scattering cross section and w0 is the waist of the laser beam. For example, if 500 nm, 100 mJ laser pulses are collided with 100 pC electron bunches at a repetition rate of 1 kHz in a laser beam waist w0=10 μm, then an X-ray flux ΦX≈2×1010 photons/s will be generated. This is an optimistic estimate, assuming state-of-the-art pulsed electron and laser beam technology, but it is still 2-3 orders of magnitude below the desired flux for advanced imaging applications. Moreover, the bandwidth will be large, as photons scattered at all angles are used in the estimate, and the spatial coherence of the generated soft X-ray beam will be very small, <10−2 partial coherence, due to the inevitably large angular spread of the electron beam, associated with the finite emittance of a 100 pC bunch.
Spatially Coherent Compton Scattering
In order to generate a soft X-ray beam by ICS with full spatial coherence, first and foremost an electron beam with very high transverse quality is required. Transverse beam quality is usually expressed in terms of the geometrical emittance ϵ, or focusability of the beam, expressed in units [m rad], which is equal to the product of beam size and uncorrelated angular spread. An electron beam can only generate a diffraction-limited, i.e. fully spatially coherent, X-ray beam if its emittance ϵ<λX/4π. Since geometrical emittance depends on beam energy, it is convenient to define the normalized emittance ϵn=γβϵ, which is a Lorentz invariant measure for beam quality. In terms of the normalized emittance the coherence condition becomes:
ϵn<γβλX/4π. (4)
By combining Eq. (1) with θ0=θX=0 and Eq. (4) with an equality sign, we can calculate the minimum conditions necessary for spatially coherent ICS, resulting in the plot shown in
The Ultracold Electron Source
The UCES is based on ultracold atomic gas, usually rubidium vapor, which is cooled and trapped in a Magneto Optica Trap (MOT), and subsequently photoionized, using a two-step photoionization scheme, as is illustrated in
The UCES is characterized by electron temperatures as low as 10 K, 2-3 orders of magnitude lower than conventional photoemission sources, as was demonstrated first by nanosecond photoionization [2,3] and later by femtosecond photoionization as well [4,5]. As the normalized emittance of a source can be written as
ϵn=σs(kTe/mc2)1/2, (5)
where σs is the root-mean-squared (RMS) transverse source size and Te is the source electron temperature, it is clear that the UCES allows much smaller normalized emittances than are possible with conventional photoemission sources. For example, for an RMS transverse size σs=25 μm and electron temperature Te=10 K, the normalized emittance ϵn=1 nm rad, a value that is routinely achieved with the UCES [4,5,6]. In a Rb MOT the size of the trapped gas cloud and thus the longitudinal size of the ionization volume is typically 1 mm and the densities can be as high as a few 1018 m−3, implying that Ne≈106-107 electrons can be created with ϵn=1 nm rad. This combination of bunch charge and beam quality should enable, e.g., single-shot protein crystallography [3,6,7], which is one of the main driving forces behind the development of the UCES. Note that to achieve a similar normalized emittance from a conventional photocathode would require a source size σs≤1 μm. To extract bunches with 106 electrons from such a small spot would require unrealistic GV/m electric field strengths. The UCES however, allows even smaller emittances: by reducing the size of the overlap between the excitation and the ionization laser (
Microbunching and Superradiance
The resonant two-step photoionization process, employing the combination of an excitation laser, tuned to an intermediate atomic level, and an ionization laser, exciting atoms from the intermediate state to the continuum, allows very precise control of the initial density distribution of the ionized gas: since atoms are only ionized in the region where the two laser beams overlap, the initial electron bunch distribution can be accurately tailored in 3D by modulating the beam profiles of the two lasers. This was beautifully demonstrated by the Scholten group at the University of Melbourne, who used a Spatial Light Modulator (SLM) to shape the excitation laser beam and thus create electron bunches with intricate, almost arbitrary charge distributions, with the smallest sized structures only limited by the diffraction of the laser light [8]. The low temperature of the source turns out to be essential to maintain these intricate structures, which immediately get blurred due to random thermal motion of the electrons at higher source temperatures.
Embodiments of the invention use an effective way to shape the initial charge distribution in a way extremely beneficial for boosting the ICS yield. As illustrated in
By using a 780 nm standing wave to excite the 52P3/2 state, the excited Rb atoms 402 in the MOT will be spatially modulated with a period of λmod=390 nm. The atoms 404 outside the standing wave 400 remain in their laser-cooled ground state. The periodic spatial modulation of excited atoms 402 are subsequently ionized by a femtosecond ionization laser (480 nm, in the case of Rb), aligned perpendicular to the excitation laser beam, thus almost instantly creating an electron bunch spatially modulated with a period equal to half the excitation laser wavelength, i.e., λmod=390 nm.
To generate EUV radiation by ICS, the electron bunch is accelerated to 0.5-2 MeV. This uses radiofrequency (RF) accelerator structures. A very compact accelerator structure operating at 12 GHz, in the so-called ‘X-band’, is used instead of the more conventional 3 GHz ‘S-band’ accelerating structures. Because of the high accelerating fields in the X-band accelerators, typically >50 MV/m, only 3 X-band cells, and thus less than 10 cm of accelerator structure is sufficient to cover the entire EUV spectral regime. Only acceleration, however, is not sufficient. In order to boost the ICS yield substantially coherent amplification is required. This can be accomplished by compressing the bunch in such a way that at the point where the accelerated bunch collides with the laser pulse, the period of the spatial modulation is decreased to the wavelength of the EUV radiation generated. For example, by accelerating a bunch with a normalized emittance ϵn=0.4 nm rad to an energy of 1 MeV and colliding it with a 500 nm laser pulse, spatially coherent EUV radiation is generated at a wavelength of 15 nm (see
Nx=(1+FNe)NeN0σT/2π02. (6)
Here 0≤F≤1 is the form factor associated with the electron bunch distribution: in absence of any density modulation F=0, while F=1 for a bunch with a perfect periodic longitudinal density distribution. Here perfect means that the Fourier transform of the longitudinal density distribution only contains spatial frequency components associated with the EUV wavelength to be generated. For bunch charges of 0.1 pC, i.e., Ne=6.2×105 electrons, colliding with 100 mJ, 512 nm laser pulses in a w0=10 μm waist at a repetition rate of 1 kHz, the incoherent ICS photon flux (Eg. (2)) is ΦX=1.7×107 ph/s. Assuming a perfect density modulation, the coherent photon flux is ΦX=1.0×1013 ph/s, more than sufficient for recording a full image. To obtain the same photon flux by incoherent ICS would require focusing a sub-ps, few MeV, 60 nC electron bunch to a spot smaller than 10 μm, which is not possible.
The coherent amplification of pulsed-electron-beam based radiation sources by this so-called superradiance mechanism is well known and has been applied times before. The challenge is always how to realize the required longitudinal density modulation, in the case of EUV radiation at the nanometer scale. Already in 1996 Carlsten et al. proposed to apply the density modulation in the transverse direction first, which can be done quite straightforwardly with a mask, and subsequently use a magnetic chicane to transfer it to the longitudinal direction [9]. The Graves group at MIT/ASU has recently devised a particularly smart variation of this method to actually realize nano-modulated electron beams and thus use superradiance to coherently amplify the soft X-ray photon yield in an ICS setup [10]. The UCES based method used here, has two major advantages: first, the two-step photoionization method allows extremely accurate shaping of the initial longitudinal bunch density distribution (see
EUV Compton FEL
The combination of superradiant amplification of the emission by microbunching of the electron bunch and fully spatially coherent emission, constitutes the realization of a Free Electron Laser operating at EUV wavelengths, an EUV Compton FEL. The UCES-based EUV Compton FEL would have a footprint of only a few square meters, in stark contrast with present-day FEL facilities. Clearly, this would be an enormously important development allowing wide-spread dissemination of EUV FELs in academic and industrial labs and potentially even in semiconductor fabs.
Although in principle the UCES provides the ingredients necessary to realize full spatial coherence and superradiant emission, there are still major obstacles facing actual realization of a EUV Compton FEL. These obstacles can be summarized in a single, major challenge: the control of space charge forces. To achieve a large photon flux, as many electrons as possible should radiate in perfect unison, while confined in a very small volume, both focused transversely to a few μm, and compressed longitudinally to a few 10 μm (temporal compression to ˜100 fs). The space charge forces associated with these high charge densities could cause deformation of the phase space distribution of the bunch, which could lead to irreversible emittance growth and thus loss of spatial coherence. Moreover, space charge forces could hamper bunch compression, leading to an imperfect bunch density modulation at the interaction point and thus reduced superradiance.
RF Compression by Velocity Bunching
In
Further Exploitation of the Initial Longitudinal Density Modulation
In other embodiments, the periodic spatial modulation in the MOT may be accomplished in the ground state gas by using the dipole force in the standing wave of two counter propagating laser beams at a wavelength far-detuned to the blue with respect to the transition to the intermediate state. In fact, this could be a superior method, since it would entail compressing the atoms prior to excitation, thus leading to higher initial bunch densities.
Interestingly, by combining the standing waves of the excitation laser and a ‘dipole force’ laser, a multi-periodic modulation would result, which would include structures at the scale of the difference of the two wavelengths, possibly much smaller than the diffraction limit at optical wavelengths. This could be useful when considering the possibility of using the UCES for realizing coherent amplification of hard X-rays. In addition, this would open the possibility of coherent amplification at two wavelengths simultaneously, and thus two-color operation at EUV wavelengths. To realize this, a doubly modulated bunch would collide with two laser pulses at different wavelengths. The normalized emittance, beam energy and laser wavelengths can be read off from
The Setup
In
Grating-MOT-Based UCES
In one embodiment, a laser-cooled and trapped cloud of rubidium atoms is created using a so-called ‘grating Magneto Optical Trap’, a technique[15] that allows a very compact design and turn-key operation, with minimal alignment of trapping and cooling lasers and maximal access for the excitation and ionization laser beams.
For the ICS setup, a dedicated grating-MOT-based UCES is used, specifically designed for achieving high atom densities in the MOT. An Optical Parametric Amplifier (OPA), fed by an amplified Ti:sapphire laser provides the tunable femtosecond 480 nm ionization laser pulses. Ever lower electron source temperatures may be obtained by appropriate selection of the bandwidth and the temporal profile of the ionization laser pulse.
RF Compression Cavity
The electron bunches are compressed by velocity bunching, employing a 3 GHz resonant RF cavity in TM010 mode, similar to those used for single-shot, 100 fs Ultrafast Electron Diffraction [11-13].
The RF compression cavity is very robust and reliable and has been sold by AccTec BV to many groups worldwide over the past few years. Synchronization of the compressed electron bunch with the ICS interaction laser pulse is accomplished by synchronization of the RF phase with the laser pulse [17].
X-Band Accelerating Section
Preferred embodiments use a very compact X-band accelerator structure operating at 12 GHz. Because of the high accelerating fields in the X-band accelerators, typically >50 MV/m, only a few X-band cells, and ˜10 cm of accelerator structure is sufficient to reach 1-2 MeV electron beam energies for generating EUV radiation by ICS. By injecting the bunches at the proper RF phase, acceleration could be combined with compression by velocity bunching in the X-band structure. However, we choose to separate compression and acceleration, as the RF bunch compression method is proven technology, allowing bunch compression to be controlled and optimized independently.
Generation of EUV Radiation
To maximize the EUV photon flux, a powerful, industrial pulsed sub-ps laser is preferably used to generate the laser beam that collides with the electron bunches in the interaction point. At present the most powerful turn-key systems are glass lasers providing 200 mJ, 1024 nm, sub-ps pulses at 1 kHz rep rate [18]. These expensive lasers are ideal for achieving a reliable high EUV photon yield. The 2nd harmonic (512 nm) is preferred, which can be generated with at least 50% efficiency. As can be seen in
The generated EUV beam may be characterized and optimized in terms of EUV wavelength, bandwidth, angular spread, photon flux, coherence and brilliance.
Following is an overview of the method of operating the device.
Step 1
Modulate the excited rubidium gas in the z direction (in the 52P3/2 state) using two counterpropagating 780 nm laser beams that produce a standing wave. The excited gas will be spatially modulated with a period of λmod=390 nm.
Step 2
The excited rubidium beamlets are ionized using an ultrafast ionization laser (<1 picosecond) with an optical wavelength tuned close to the ionization threshold, for example, a blue ultrafast ionization laser. In this way we create a micro-bunched electron beam with a modulation period determined by the standing wave λmod=390 nm. Additionally, due to the near-threshold photoionization the electrons have an ultra-low momentum spread which results in a beam emittance that is smaller than 1 nm rad. This creates a fully transverse coherent X-ray pulse.
In order to generate fully transverse coherent x-ray radiation by ICS, high quality electron beams are used with normalized emittances preferably below 1 nm rad. The ultra-cold electron source is used to deliver high-charge electron bunches of such a quality.
Step 3
The rubidium atoms are ionized in an electrostatic acceleration field which accelerates the electrons created inside the UCES to an energy of a few tens of keV. Since the electrons that are ionized at a position further away from the aperture in the anode are accelerated to higher kinetic energy than the ones initially closer to the anode, the electron pulse acquires a negative velocity chirp after exiting the DC acceleration field. As a result, after extraction the electron pulse will self-compress. After the self-compression point, the pulse will automatically acquire a positive velocity chirp and therefore stretch again. Subsequently, using an RF cavity operated in TM010 mode, the front of the electron pulse is decelerated while the back is accelerated, resulting in an electron pulse with again a negative velocity chirp.
Step 4
The negatively chirped picosecond electron pulse is RF accelerated to a few MeV and is simultaneously compressed by two orders of magnitude at the interaction point. This is due to the negative chirp acquired. It does not matter in which order the compression and the acceleration take place. The compression and the acceleration can also be realized simultaneously in a single RF accelerator.
As a result, the initial modulation period λmod=390 nm is shrunk by the same two orders of magnitude. The modulation period λmod=390 nm of the electron beam is now equal to the wavelength λ of the soft x-ray pulse that is generated in the interaction point.
As a result, the generated soft X-ray beam will be fully longitudinal coherent. In addition, the radiation generated by the individual micro bunches will add up coherently so that the intensity will be boosted by an amount proportional to the number of the electrons in the bunch. This boosts the intensity to intensities comparable to that of SLSs and XFELs.
Simultaneously, the ultra-low electron emittance makes sure that the electron beam divergence in the interaction point is smaller than that of a diffraction limited soft X-ray beam; this guarantees the production of a fully spatially coherent soft X-ray beam.
Embodiments of the invention provide a state-of-the-art method that can generate narrowband (soft) X-ray beams which are fully coherent and have super-radiant intensity, realizing a table-top Compton soft X-ray free Electron Laser. The entire setup can be constructed with a footprint smaller than 3 meters.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2019/079968 | 11/1/2019 | WO |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2020/089454 | 5/7/2020 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 20180220518 | Polyakov | Aug 2018 | A1 |
| Entry |
|---|
| Engelen et al. High-Coherence electron bunches produced by femtosecond photionization. Nature Communications 2013, pp. 1-5. |
| Shayeganrad. High resolution nanofocus X-ray source based on ultracold electrons from laser cooled-atoms. Cornell University Library 2018. |
| Franssen et al. From ultracold electrons to coherent soft x-rays. arxiv.org/pdf/1905.04031.pdf 2019. |
| Number | Date | Country | |
|---|---|---|---|
| 20210400796 A1 | Dec 2021 | US |
| Number | Date | Country | |
|---|---|---|---|
| 62755340 | Nov 2018 | US |
