We present results of the first tunable Compton backscattering (CBS) x-ray source that is based o... more We present results of the first tunable Compton backscattering (CBS) x-ray source that is based on the easily aligned combination of a laser-plasma accelerator (LPA) and a plasma mirror (PM). The LPA is driven in the blowout regime by 30 TW, 30 fs laser pulses, and produces high-quality, tunable, quasi-monoenergetic electron beams. A thin plastic film near the gas jet exit efficiently retro-reflects the LPA driving pulse with relativistic intensity into oncoming electrons to produce 2 × 10 7 CBS x-ray photons per shot with 10-20 mrad angular divergence and 50 % (FWHM) energy spread without detectable bremsstrahlung background. The xray central energy is tuned from 75 KeV to 200 KeV by tuning the LPA e-beam central energy. Particle-in-cell simulations of the LPA, the drive pulse/PM interaction and CBS agree well with measurements.
ABSTRACT We introduce Frequency-Domain Tomography (FDT) for visualizing sub-ps evolution of light... more ABSTRACT We introduce Frequency-Domain Tomography (FDT) for visualizing sub-ps evolution of light-speed refractive index structures in a single shot. As a prototype demonstration, we produce single-shot tomographic movies of self-focusing, filamenting laser pulses propagating in a transparent Kerr medium. We then discuss how to adapt FDT to visualize evolving laser- or beam-driven plasma wakefields of current interest to the advanced accelerator community. For short (L ~ 1 cm), dense (ne ~ 1019 cm-3) plasmas, the key challenge is broadening probe bandwidth sufficiently to resolve plasma-wavelength-size structures. For long (L ~ 10 to 100 cm), tenuous (ne ~ 1017 cm-3) plasmas, probe diffraction from the evolving wake becomes the key challenge. We propose and analyze solutions to these challenges.
ABSTRACT We introduce frequency domain tomography (FDT) for single-shot visualization of time-evo... more ABSTRACT We introduce frequency domain tomography (FDT) for single-shot visualization of time-evolving refractive index structures (e.g. laser wakefields, nonlinear index structures) moving at light-speed. Previous researchers demonstrated single-shot frequency domain holography (FDH), in which a probe-reference pulse pair co- propagates with the laser-generated structure, to obtain snapshot-like images. However, in FDH, information about the structure's evolution is averaged. To visualize an evolving structure, we use several frequency domain streak cameras (FDSCs), in each of which a probe-reference pulse pair propagates at an angle to the propagation direction of the laser-generated structure. The combination of several FDSCs constitutes the FDT system. We will present experimental results for a 4-probe FDT system that has imaged the whole-beam self-focusing of a pump pulse propagating through glass in a single laser shot. Combining temporal and angle multiplexing methods, we successfully processed data from four probe pulses in one spectrometer in a single-shot. The output of data processing is a multi-frame movie of the self- focusing pulse. Our results promise the possibility of visualizing evolving laser wakefield structures that underlie laser-plasma accelerators used for multi-GeV electron acceleration.
ABSTRACT We identify three regimes of correlated GeV-electron/keV-betatron-x-ray generation by a ... more ABSTRACT We identify three regimes of correlated GeV-electron/keV-betatron-x-ray generation by a laser-plasma accelerator driven by the Texas Petawatt laser, and relate them to variations in strength of blowout, injection geometry and beam loading.
Electron self-injection into a laser-plasma accelerator (LPA) driven by the Texas Petawatt (TPW) ... more Electron self-injection into a laser-plasma accelerator (LPA) driven by the Texas Petawatt (TPW) laser is reported at plasma densities 1.7 -6.2 x 10 17 cm -3 . Energy and charge of the electron beam, ranging from 0.5 GeV to 2 GeV and tens to hundreds of pC, respectively, depended strongly on laser beam quality and plasma density. Angular beam divergence was consistently around 0.5 mrad (FWHM), while shot-to-shot pointing fluctuations were limited to ±1.4 mrad rms. Betatron x-rays with tens of keV photon energy are also clearly observed.
We visualize ps-time-scale evolution of an electron density bubble--a wake structure created in a... more We visualize ps-time-scale evolution of an electron density bubble--a wake structure created in atmospheric density plasma by an intense ultrashort laser pulse--from the phase "streak" that the bubble imprints onto a probe pulse that crosses its path obliquely. Phase streaks, recovered in one shot using frequency-domain interferometric techniques, reveal the formation, propagation, and coalescence of the bubble within a 3 mm long ionized helium gas target. 3D particle-in-cell simulations validate the observed density-dependent bubble evolution, and correlate it with the generation of a quasimonoenergetic ∼ 100 MeV electron beam. The results provide a basis for understanding optimized electron acceleration at a plasma density n(e) ≈ 2 × 10(19) cm(-3), inefficient acceleration at lower density, and dephasing limits at higher density.
ABSTRACT We visualize formation, propagation and collapse of laser-driven plasma bubbles using a ... more ABSTRACT We visualize formation, propagation and collapse of laser-driven plasma bubbles using a single-shot frequency domain streak camera, thereby identifying bubble dynamics that optimize electron injection and acceleration.
Conference on Lasers and Electro-Optics 2012, 2012
ABSTRACT We report electron acceleration to 1.25 GeV by laser-driven wakefield acceleration at pl... more ABSTRACT We report electron acceleration to 1.25 GeV by laser-driven wakefield acceleration at plasma density 5x1017 cm-3. Electron beams are dark-current-free, quasi-monoenergetic, highly collimated, contain tens of pC and have excellent pointing stability.
ABSTRACT We report self-injected quasi-monoenergetic (5% spread FWHM) acceleration of electrons t... more ABSTRACT We report self-injected quasi-monoenergetic (5% spread FWHM) acceleration of electrons to 2.0 ± 0.1 GeV by 0.6 PW-laser-driven wakefield acceleration in pure He plasma of density 5x10^17 cm^-3. Electron bunches diverge ~0.5mrad, and contain ~60 pC.
We demonstrate a frequency-domain streak camera (FDSC) that captures the picosecond time evolutio... more We demonstrate a frequency-domain streak camera (FDSC) that captures the picosecond time evolution of luminalvelocity refractive index structures in a single shot. In our prototype FDSC, a probe-reference pulse pair propagates obliquely to a subpicosecond pump pulse that creates an evolving nonlinear index structure in glass, supplementing a conventional frequency-domain holographic probe-reference pair that copropagates with the pump. A single spectrometer acquires data from both pairs via spatial or temporal multiplexing, demonstrating the feasibility of a compact frequency-domain tomographic system in which a single spectrometer processes data from multiple probing angles.
We demonstrate a single-shot method of visualizing the evolution of light-speed, laser-generated ... more We demonstrate a single-shot method of visualizing the evolution of light-speed, laser-generated structures as they propagate over hundreds of Rayleigh lengths (typically ≥10 cm) through a tenuous medium. An ultrashort probe pulse crosses the object's path at a small angle (θ<5°) and a specific time delay. Copies of the phase-modulated probe are then relay-imaged to separate detectors from selected object planes along the propagation path. A phase-contrast technique based on Kerr effect and nonlinear absorption converts phase to intensity modulation, improving sensitivity in tenuous media. A continuous record of the probe phase modulation along the propagation path is reconstructed.
Tomography--cross-sectional imaging based on measuring radiation transmitted through an object al... more Tomography--cross-sectional imaging based on measuring radiation transmitted through an object along different directions--enables non-invasive imaging of hidden stationary objects, such as internal bodily organs, from their sequentially measured projections. Here we adapt tomographic methods to visualize--in one laser shot--the instantaneous structure and evolution of a laser-induced object propagating through a transparent Kerr medium. We reconstruct 'movies' of a laser pulse's diffraction, self-focusing and filamentation from phase 'streaks' imprinted onto probe pulses that cross the main pulse's path simultaneously at different angles. Multiple probes are generated and detected compactly and simply, making the system robust, easy to align and adaptable to many problems. Our technique could potentially visualize, for example, plasma wakefield accelerators, optical rogue waves or fast ignitor pulses, light-velocity objects, whose detailed space-time dynamics are known only through intensive computer simulations.
Laser-plasma accelerators of only a centimetre's length have produced nearly monoenergetic electr... more Laser-plasma accelerators of only a centimetre's length have produced nearly monoenergetic electron bunches with energy as high as 1 GeV. Scaling these compact accelerators to multigigaelectronvolt energy would open the prospect of building X-ray free-electron lasers and linear colliders hundreds of times smaller than conventional facilities, but the 1 GeV barrier has so far proven insurmountable. Here, by applying new petawatt laser technology, we produce electron bunches with a spectrum prominently peaked at 2 GeV with only a few per cent energy spread and unprecedented sub-milliradian divergence. Petawatt pulses inject ambient plasma electrons into the laser-driven accelerator at much lower density than was previously possible, thereby overcoming the principal physical barriers to multi-gigaelectronvolt acceleration: dephasing between laser-driven wake and accelerating electrons and laser pulse erosion. Simulations indicate that with improvements in the laser-pulse focus quality, acceleration to nearly 10 GeV should be possible with the available pulse energy.
We present results of the first tunable Compton backscattering (CBS) x-ray source that is based o... more We present results of the first tunable Compton backscattering (CBS) x-ray source that is based on the easily aligned combination of a laser-plasma accelerator (LPA) and a plasma mirror (PM). The LPA is driven in the blowout regime by 30 TW, 30 fs laser pulses, and produces high-quality, tunable, quasi-monoenergetic electron beams. A thin plastic film near the gas jet exit efficiently retro-reflects the LPA driving pulse with relativistic intensity into oncoming electrons to produce 2 × 10 7 CBS x-ray photons per shot with 10-20 mrad angular divergence and 50 % (FWHM) energy spread without detectable bremsstrahlung background. The xray central energy is tuned from 75 KeV to 200 KeV by tuning the LPA e-beam central energy. Particle-in-cell simulations of the LPA, the drive pulse/PM interaction and CBS agree well with measurements.
ABSTRACT We introduce Frequency-Domain Tomography (FDT) for visualizing sub-ps evolution of light... more ABSTRACT We introduce Frequency-Domain Tomography (FDT) for visualizing sub-ps evolution of light-speed refractive index structures in a single shot. As a prototype demonstration, we produce single-shot tomographic movies of self-focusing, filamenting laser pulses propagating in a transparent Kerr medium. We then discuss how to adapt FDT to visualize evolving laser- or beam-driven plasma wakefields of current interest to the advanced accelerator community. For short (L ~ 1 cm), dense (ne ~ 1019 cm-3) plasmas, the key challenge is broadening probe bandwidth sufficiently to resolve plasma-wavelength-size structures. For long (L ~ 10 to 100 cm), tenuous (ne ~ 1017 cm-3) plasmas, probe diffraction from the evolving wake becomes the key challenge. We propose and analyze solutions to these challenges.
ABSTRACT We introduce frequency domain tomography (FDT) for single-shot visualization of time-evo... more ABSTRACT We introduce frequency domain tomography (FDT) for single-shot visualization of time-evolving refractive index structures (e.g. laser wakefields, nonlinear index structures) moving at light-speed. Previous researchers demonstrated single-shot frequency domain holography (FDH), in which a probe-reference pulse pair co- propagates with the laser-generated structure, to obtain snapshot-like images. However, in FDH, information about the structure's evolution is averaged. To visualize an evolving structure, we use several frequency domain streak cameras (FDSCs), in each of which a probe-reference pulse pair propagates at an angle to the propagation direction of the laser-generated structure. The combination of several FDSCs constitutes the FDT system. We will present experimental results for a 4-probe FDT system that has imaged the whole-beam self-focusing of a pump pulse propagating through glass in a single laser shot. Combining temporal and angle multiplexing methods, we successfully processed data from four probe pulses in one spectrometer in a single-shot. The output of data processing is a multi-frame movie of the self- focusing pulse. Our results promise the possibility of visualizing evolving laser wakefield structures that underlie laser-plasma accelerators used for multi-GeV electron acceleration.
ABSTRACT We identify three regimes of correlated GeV-electron/keV-betatron-x-ray generation by a ... more ABSTRACT We identify three regimes of correlated GeV-electron/keV-betatron-x-ray generation by a laser-plasma accelerator driven by the Texas Petawatt laser, and relate them to variations in strength of blowout, injection geometry and beam loading.
Electron self-injection into a laser-plasma accelerator (LPA) driven by the Texas Petawatt (TPW) ... more Electron self-injection into a laser-plasma accelerator (LPA) driven by the Texas Petawatt (TPW) laser is reported at plasma densities 1.7 -6.2 x 10 17 cm -3 . Energy and charge of the electron beam, ranging from 0.5 GeV to 2 GeV and tens to hundreds of pC, respectively, depended strongly on laser beam quality and plasma density. Angular beam divergence was consistently around 0.5 mrad (FWHM), while shot-to-shot pointing fluctuations were limited to ±1.4 mrad rms. Betatron x-rays with tens of keV photon energy are also clearly observed.
We visualize ps-time-scale evolution of an electron density bubble--a wake structure created in a... more We visualize ps-time-scale evolution of an electron density bubble--a wake structure created in atmospheric density plasma by an intense ultrashort laser pulse--from the phase "streak" that the bubble imprints onto a probe pulse that crosses its path obliquely. Phase streaks, recovered in one shot using frequency-domain interferometric techniques, reveal the formation, propagation, and coalescence of the bubble within a 3 mm long ionized helium gas target. 3D particle-in-cell simulations validate the observed density-dependent bubble evolution, and correlate it with the generation of a quasimonoenergetic ∼ 100 MeV electron beam. The results provide a basis for understanding optimized electron acceleration at a plasma density n(e) ≈ 2 × 10(19) cm(-3), inefficient acceleration at lower density, and dephasing limits at higher density.
ABSTRACT We visualize formation, propagation and collapse of laser-driven plasma bubbles using a ... more ABSTRACT We visualize formation, propagation and collapse of laser-driven plasma bubbles using a single-shot frequency domain streak camera, thereby identifying bubble dynamics that optimize electron injection and acceleration.
Conference on Lasers and Electro-Optics 2012, 2012
ABSTRACT We report electron acceleration to 1.25 GeV by laser-driven wakefield acceleration at pl... more ABSTRACT We report electron acceleration to 1.25 GeV by laser-driven wakefield acceleration at plasma density 5x1017 cm-3. Electron beams are dark-current-free, quasi-monoenergetic, highly collimated, contain tens of pC and have excellent pointing stability.
ABSTRACT We report self-injected quasi-monoenergetic (5% spread FWHM) acceleration of electrons t... more ABSTRACT We report self-injected quasi-monoenergetic (5% spread FWHM) acceleration of electrons to 2.0 ± 0.1 GeV by 0.6 PW-laser-driven wakefield acceleration in pure He plasma of density 5x10^17 cm^-3. Electron bunches diverge ~0.5mrad, and contain ~60 pC.
We demonstrate a frequency-domain streak camera (FDSC) that captures the picosecond time evolutio... more We demonstrate a frequency-domain streak camera (FDSC) that captures the picosecond time evolution of luminalvelocity refractive index structures in a single shot. In our prototype FDSC, a probe-reference pulse pair propagates obliquely to a subpicosecond pump pulse that creates an evolving nonlinear index structure in glass, supplementing a conventional frequency-domain holographic probe-reference pair that copropagates with the pump. A single spectrometer acquires data from both pairs via spatial or temporal multiplexing, demonstrating the feasibility of a compact frequency-domain tomographic system in which a single spectrometer processes data from multiple probing angles.
We demonstrate a single-shot method of visualizing the evolution of light-speed, laser-generated ... more We demonstrate a single-shot method of visualizing the evolution of light-speed, laser-generated structures as they propagate over hundreds of Rayleigh lengths (typically ≥10 cm) through a tenuous medium. An ultrashort probe pulse crosses the object's path at a small angle (θ<5°) and a specific time delay. Copies of the phase-modulated probe are then relay-imaged to separate detectors from selected object planes along the propagation path. A phase-contrast technique based on Kerr effect and nonlinear absorption converts phase to intensity modulation, improving sensitivity in tenuous media. A continuous record of the probe phase modulation along the propagation path is reconstructed.
Tomography--cross-sectional imaging based on measuring radiation transmitted through an object al... more Tomography--cross-sectional imaging based on measuring radiation transmitted through an object along different directions--enables non-invasive imaging of hidden stationary objects, such as internal bodily organs, from their sequentially measured projections. Here we adapt tomographic methods to visualize--in one laser shot--the instantaneous structure and evolution of a laser-induced object propagating through a transparent Kerr medium. We reconstruct 'movies' of a laser pulse's diffraction, self-focusing and filamentation from phase 'streaks' imprinted onto probe pulses that cross the main pulse's path simultaneously at different angles. Multiple probes are generated and detected compactly and simply, making the system robust, easy to align and adaptable to many problems. Our technique could potentially visualize, for example, plasma wakefield accelerators, optical rogue waves or fast ignitor pulses, light-velocity objects, whose detailed space-time dynamics are known only through intensive computer simulations.
Laser-plasma accelerators of only a centimetre's length have produced nearly monoenergetic electr... more Laser-plasma accelerators of only a centimetre's length have produced nearly monoenergetic electron bunches with energy as high as 1 GeV. Scaling these compact accelerators to multigigaelectronvolt energy would open the prospect of building X-ray free-electron lasers and linear colliders hundreds of times smaller than conventional facilities, but the 1 GeV barrier has so far proven insurmountable. Here, by applying new petawatt laser technology, we produce electron bunches with a spectrum prominently peaked at 2 GeV with only a few per cent energy spread and unprecedented sub-milliradian divergence. Petawatt pulses inject ambient plasma electrons into the laser-driven accelerator at much lower density than was previously possible, thereby overcoming the principal physical barriers to multi-gigaelectronvolt acceleration: dephasing between laser-driven wake and accelerating electrons and laser pulse erosion. Simulations indicate that with improvements in the laser-pulse focus quality, acceleration to nearly 10 GeV should be possible with the available pulse energy.
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Papers by Zhengyan Li