he ultrashort all-optical source
Introduction: The ultrashort all-optical source is all-optical compact and femtosecond X-ray instruments that exploit the laser-plasma acceleration beam using electric fields of plasma to investigate the ultrafast structural features of objects (Miura, Kuroda, and Toyokawa, 2016 paragraph 1 line 1). They mainly use time-resolved X-ray diffraction and spectroscopy. Their pulses have temporal duration as few as tens of femtosecond. The laser-plasma accelerations that use electric fields of plasma wave to hasten electrons have been exploited to investigate the structural dynamics of materials (R. Rakowski et al., 2017 paragraph 3, line 12). The electron pulse duration of these types of equipment are very short (they are in the order of 10 fs) since the accelerating field’s wavelength is on the 10 µm order. Similarly, the ultrashort all-optical source can use high accelerating fields higher than 100 GV/m to realize a compact electron accelerator.
Problem Statement: There has been a lack of X-ray instruments that could generate pulses of short duration wavelength (femtosecond) to facilitate the study of ultrafast structural dynamics efficiently (R. Rakowski et al., 2017 paragraph 2 line 8). Various High-energy X-rays instruments that use photon energy on the order of 100 keV have been attractive due to their excellent features. However, these sources their pulses are not of short duration enough to permit the study of ultrafast structures, which has limited these studies. The High-Energy X-ray sources produce a pulse duration that is limited to about 30-100ps. This is not effectively short enough to permit studying the ultrafast features of structures on the atomic temporal scale.
Ultrashort all-optical X-ray sources were developed to solve this problem and allow the effective study of all ultrafast structural features since it produces electron beams and the subsequent X-ray pulses are of a few femtoseconds in wavelength and have excellent temporal synchronization to the laser-plasma (Miura, Kuroda, and Toyokawa, 2016 paragraph 1 line 6). Ultrashort all-optical X-ray sources effectively investigate the ultrafast structural dynamics of dentine, bone, enamel, and other structures. They can energy-filter the diffraction signal by employing an energy-resolving High-Energy X-rays camera and laser-plasma electron accelerator, which permits the extraction of nearly all background structures.
Instrument details: As state earlier, the ultrashort all-optical X-ray sources use plasma-laser electron acceleration to produce a pulse of short duration wavelength on the order of 10 fs, which is adequate due to a 10 µm on order accelerating field (Miura, Kuroda, and Toyokawa, 2016 paragraph 1 line 9). The plasma-laser acceleration is done using the electric field of plasma, permitting time-resolved X-ray diffraction and spectroscopy. They are generally made from the laser-accelerated electron beams. Laser Compton scattering X-ray source, abbreviated as an LCS X-ray source, is an example. They are made by scattering of femtosecond laser pulses off accelerated electron beams under high energy using a laser-Wakefield accelerator, which then will excite plasma wave (R. Rakowski et al., 2017 paragraph 3 line 3). The excited plasma wave will accelerate electrons to produce relativistic energy within the shortest distance, possibly a few millimeters which allow driving of X-rays sources that have a significantly compact footprint. The laser warrants both perfect electron beam and the synchronization of optical thus, they emit well-collimated and quasi-monochromatic (QM) X-ray beams. The desirable characteristics of this ultrashort all-optical X-ray source include the generation of high energy X-rays, which also have a large photon number. Secondly, its electron beam spread is large, so do its energy spread.
The third is a QM X-ray source, making its most pleasing for numerous purposes. Production of a quasi-monochromatic X-ray source using an LCS x-ray source necessitates a narrow energy spread (Miura, Kuroda, and Toyokawa, 2016 paragraph 2, line 6). Additionally, ultrashort all-optical X-ray sources exploit the full X-ray bandwidth for structural analysis, for instance, in the case of brain molecular structure, which allows captions of signals above the background despite contamination in the background (Rakowski et al., 2017 fourth paragraph, line 14). Besides, these sources allow enhancement of the signals through its source developments and reducing its X-ray propagation distance to the structure under study due to its vast pilling up of photon. This is Ultrashort X-rays sources (LCS) that accommodate ultrafast HEX-rays sources, which are slow in their photon per shot generation and with low repetitions rates.
Test procedure: In the experiment setup, the laser pulses that are used to accelerate electron beams are known as the main pulse and that for accelerating the LCS beam are termed as colliding pulse (Miura, Kuroda, and Toyokawa, 2016 paragraph 4 line 1). They were polarized horizontally. The main pulse was directed to the edge of the short HE gas jet (2-mm-long) by the f/14 off-axis parabolic mirror. It was set at an intensity of 4.7 × 1018 W/cm 2. On the other hand, the colliding pulse, laser pulse to accelerate LCS, was directed to the outward side of the main pulse away from the gas jet using a similar f/6 off-axis parabolic mirror. Here the intensity was set at 8.8 × 1017 W/cm 2. The colliding pulse’s incident angle was 20◦ on relation to the main pulse’ propagation axis in a vacuum called the main laser axis, and the plasma electron density was recorded as .5 × 1019 cm−3.
The scattering of laser Compton scattering X-rays took place in a progressive direction of accelerated electron beam emission (Miura, Kuroda, and Toyokawa, 2016, paragraph 5, line 1). The accelerated electron beams were then forced to bend using a dipole magnet at a 0.27 T magnetic field and was as well partially segregated from the X-rays. The X-rays and accelerated electron beams were strokes on a phosphor screen via an Al filter of 115-µm-thick. The researchers simultaneously captured X-rays and energy-resolved electrons’ images formed on the phosphor screen using a single short utilizing CCD camera. A keen observation of electrons with higher energies above 30 MeV was noted. Also, the electron beam charges were estimated detection system’s calibrated sensitivity, which was sensitive to those X-rays that had energies ranging from 15 to 150 keV.
Test results: A phosphor image was noted upon the production of LCS X-rays. Also, an energy-resolved electron image of a quasi-monochromatic electron beam was formed on the phosphor image as well as the LCS X-ray image (Miura, Kuroda, and Toyokawa, 2016 paragraph 5 line 1). The position of the main axis line was also noted. The peak energy was estimated as 60 MeV and quasi-monochromatic electron beam charge as 70 pC. The X-ray beam’s divergence angles were 6.2, and 4.2 mrad approximated horizontally and vertically from e −2 intensity radius of the image. ∼ 1/γ was used to determine the LCS’s divergence angle, where γ represents the Lorentz factor. The estimated divergence angle is 8.3 mrad electron energy of 60 MeV (γ ∼ 120), almost close to noted divergence angles. This confirms that the spot noted next to the main laser axis is LCS X-ray’s image (R. Rakowski et al., paragraph 6 line 9). The estimated range of photon energy of X-rays with a five mrad scattering angle was 30 to 120 keV when 40 and 70 MeV QME beam’s energy spread is accounted for.
The energy directed onto the phosphor screen is converted to fluorescence for electrons with an equal efficiency as that of X-rays when a similar phosphor screen is used (R. Rakowski et al., 2017 paragraph 6 line 13). This proves that energies in the phosphor screen dictate the phosphor screen’s sensitivity. Hence, sensitivity to X-rays can be determined from the analysis of the energies deposited in the phosphor layer grounded on sensitivity to electrons. Simulation code EGSS was employed to calculate accumulated energies. X-rays with 30 to 120 keV energies had a sensitivity of about an eighth to that of electrons with 60 MeV energies. X-ray photon number of about 2 × 107 per pulse was noted when the counts of the image showing the position of phosphor image. Importantly, X-ray achieved a photon number per pulse alike to LCS X-rays sources photon number per pulse utilizing the rf laser-plasma accelerators (Miura, Kuroda and Toyokawa, paragraph 6 line 5). Plasma-trapped betatron oscillation of electrons can produce X-ray radiation. The estimated critical energy of betatron radiation X-ray was seven keV for 60 MeV electrons. The detection system’s sensitivity to those X-rays that had energies below seven keV was very low.
The results were numerically simulated where the X-ray beam pattern and the observed beam pattern were vertically and horizontally elliptical, respectively (Miura, Kuroda, and Toyokawa, 2016 paragraph 8, line 3). The horizontally polarized main pulse interactions with accelerated electrons leading to a horizontal elliptical pattern of the quasi-monochromatic electron beam could be attributed to this. The horizontal and vertical divergence angles were 9.3 and 14.1 mrad, respectively. Divergence angles for the experiment were smaller compared to that of the simulation result, which can be attributed to possible uncertainty electron energy estimation in the test. Possible errors in determining electron energy were attributed to probable electron beam’s incident angle fluctuation relative to the magnet (Miura, Kuroda and Toyokawa, paragraph 9 line 8). Another crucial observation was the lower predicted electron energy than the exact energy when the QME beam’s emission direction and that of the X-ray beam were shifted leftwards from the main laser axis.
Also, the higher-energy electron beam produced an X-ray beam that had a smaller divergence angle. The experimental results agreed with the photon number. Large energy spread of the QME beam led a relatively large energy spread of the X-rays whose peak energy was approximated to be 86 keV for 60 MeV electrons (Miura, Kuroda, and Toyokawa, 2016 paragraph 10 line 7). The experiment revealed lower peak energy in the simulation result compared to a theoretically predicted value. Enhanced mass of electron effectiveness due to nonlinear interactions with a colliding pulse as well as normalized vector potential led to X-ray energy shifting to lower energies. Simulation results reveal the impact of nonlinear scattering.
Conclusion: The experiment showed that the laser-accelerated quasi-monochromatic electron emissions through LCS generate X-rays. It produced well-collimated X-ray emissions that had an estimate divergence angle of 5 mrad with an estimated photon number of 2×107 per pulse. Besides, the numerical simulation reveals that scattering of peak energy and photon number took place with an angle of 5 mrad were 60 keV and 1.8 × 107, and the photon number adequately agreed with the experimental outcomes. The results from the numerical simulation prove the experimental results.
The X-ray diffraction was to produce a well-collimated X-ray beam with relativist energies over a distance of a few millimeters and bunches of electrons, which are a few femtoseconds with perfect temporal synchronization. Also, the X-rays beams have a lower divergence angle and a large photon number. Ultrashort all-optical sources, in this case, the laser Compton scattering produced a well-collimate X-ray beam over a short distance of few millimeters with a bunch of electrons, which were a few femtoseconds, with lower divergence angle and large photon number. This implies that it accomplished the task since it produced a source that allowed studying ultrafast structural dynamics of objects. Fundamental innovative ideas are required in the future to improve the effectiveness of these ultrashort all-optical sources. For instance, the sources still experience background radiations, which lead to high-power laser interactions (R. Rakowski et al., 2017, paragraph 4, line 16). Future work should aim to reduce background radiations since the high power laser interactions significantly reduce the source’s effectiveness.