J.-Y. Lin

Efficient generation of extended plasma waveguides with the axicon ignitor-heater scheme

By using an axicon lens in conjunction with the ignitor-heater scheme, a 1.2-cm-long high-quality plasma waveguide is generated efficiently, which can extend the range of laser-plasma interaction much beyond the limit of Rayleigh range

Enhancement of injection and acceleration of electrons in a laser wakefield accelerator by using an argon-doped hydrogen gas jet and optically preformed plasma waveguide

A systematic experimental study on injection of electrons in a gas-jet-based laser wakefield accelerator via ionization of dopant was conducted. The pump-pulse threshold energy for producing a quasi-monoenergetic electron beam was significantly reduced by doping the hydrogen gas jet with argon atoms, resulting in a much better spatial contrast of the electron beam. Furthermore, laser wakefield electron acceleration in an optically preformed plasma waveguide based on the axicon-ignitor-heater scheme was achieved. It was found that doping with argon atoms can also lower the pump-pulse threshold energy in this experimental configuration.

Induction of electron injection and betatron oscillation in a plasma-waveguide-based laser wakefield accelerator by modification of waveguide structure

By adding a transverse heater pulse into the axicon ignitor-heater scheme for producing a plasma waveguide, a variable three-dimensionally structured plasma waveguide can be fabricated. With this technique, electron injection in a plasma-waveguide-based laser wakefield accelerator was achieved and resulted in production of a quasi-monoenergetic electron beam. The injection was correlated with a section of expanding cross-section in the plasma waveguide. Moreover, the intensity of the X-ray beam produced by the electron bunch in betatron oscillation was greatly enhanced with a transversely shifted section in the plasma waveguide. The technique opens a route to a compact hard-X-ray pulse source.

Fabrication of three-dimensionally structured plasma waveguide and application to induction of electron injection and betatron oscillation in a laser wakefield electron accelerator

By adding a transverse heater pulse with controlled intensity distribution into the axicon ignitor-heater scheme for optically producing a plasma waveguide, three-dimensionally structured plasma waveguide can be fabricated. The additional heater pulse generates further heating of the plasma filament produced by the axicon pulses in a spatially and temporally controlled manner. The succeeding evolution of the plasma leads to a properly structured plasma waveguide that suits for targeted application. With this technique, induction of electron injection in a plasma-waveguide-based laser wakefield accelerator was achieved and resulted in production of a quasi-monoenergetic electron beam with an electron energy reaching 280 MeV and an energy spread as low as 1% in a 4-mm-long gas jet by properly setting the transverse heater pulse delay with respect to the axicon pulses. Furthermore, strong hard X-ray beam was observed upon further increase of transverse heater delay so that the irradiated section in the plasma waveguide acts as a plasma kicker to enhance betatron oscillation.

High Brightness Optical-Field-Ionization X-Ray Lasers Driven in Plasma Waveguides

We experimentally demonstrate that the plasma waveguide can be efficiently produced in a pure Xe, Kr, and Ar cluster jet operated at a high backing pressure with axicon ignitor-heater scheme. The lasing photon number of Ni-like Kr laser at 32.8 nm generated in waveguide is dramatically enhanced by about three orders of magnitude in comparison to that without plasma waveguide, resulting in a photon number of 8×1010 and an energy conversion efficiency of 2×10−6 with a pump pulse of just 235 mJ. Due to the high atom density and long gain length provided by the plasma waveguide, Ne-like Ar laser at 46.9 nm is achieved in OFI plasma channel at the first time and simultaneous lasing in two ion species is also generated in a 10-mm gas jet with Kr and Ar mixtures. With sufficient x-ray photons, we show that x-ray digital holographic microscopy can be achieved in single-shot measurement providing a spatial resolution of >500 nm and a temporal resolution of >10 ps.

Experimental Investigation of the Parameter Space ofOptical-Field-Ionization Collisional-Excitation X-RayLasers in a Cluster Jet

Optical-field-ionization collisional-excitation soft x-ray lasers in clustered gas jets were experimentally investigated in detail. A tomographic measurement based on laser machining technique was used to resolve the growth of x-ray lasing intensity as a function of position in a cluster jet, exploring the origins of the dependences of x-ray lasing intensity on atom density and laser polarization. The presence of optimal atom density was found to be a result of ionizationinduced refraction. An unexpected observation is that circular polarization appears to be not the optimal polarization ellipticity, which may be a manifestation of effects of pre-ionization at the laser-cluster ionization front.

Tomography of the Injection and Acceleration Processes of Monoenergetic Electrons in a Laser Wakefield Accelerator

A tomographic method based on laser machining was used to resolve the electron injection and acceleration processes in a laser wakefield accelerator. It was found that all the electrons in the monoenergetic electron beam are injected at the same location in the plasma column and then accelerated with an acceleration gradient exceeding 2 GeV/cm. In addition, it was observed that there is no significant deceleration of the monoenergetic electron bunch after reaching the maximum energy and the injection position shifts with change of the position of pump‐pulse focus. The results are consistent with the model of transverse wave‐breaking and beam loading for injection of monoenergetic electrons. With this method the details of the underlying physical processes in a laser wakefield accelerator can be resolved and compared directly to the observations in particle‐in‐cell simulations.

Fabrication of transient plasma density structures for plasma photonic devices

Fabrication of a periodic plasma density structure in a gas jet with a boundary scale length approaching 10 μm was demonstrated. This technique has potential applications in various transient high-field plasma photonic devices. The capability of fabrication of gas/plasma density structures is the key to the development of plasma photonic devices such as plasma-based particle accelerators, x-ray lasers, high-harmonic generation, plasma nonlinear optics, etc. For instance, in the development of laser wakefield electron accelerator, it was proposed that by creating a sharp downward density ramp in the plasma, background electrons can be injected into the plasma wave at the ramp. In the development of high harmonic generation from gases, quasi-phase matching by using periodically interlaced layers of highand low-density gas or plasma was proposed to increase the conversion efficiency. The main obstacle for the realization of these ideas is that a gas/plasma density structure with a scale length of below 100 μm cannot be achieved by mechanical shaping of the gas jet nozzle, neither can a three-dimensional structure. In this work, we show that a density structure in a gas jet with a boundary scale length approaching 10 μm can be fabricated by direct optical machining. Although the desired structure only exists transiently (~10 ns), it is effectively static as far as the action of the photonic device is concerned (<100 ps). We demonstrated this by fabricating a periodically modulated gas/plasma density distribution. The idea for the fabrication of a periodic plasma density structure in a gas jet is as follows. When a spatially periodic laser intensity pattern is produced in the neutral gas jet, if the average laser intensity is set at the intensity threshold of optical-field ionization, plasma is formed only at the high intensity regions. At several nanoseconds after the heating of the plasma by the same pulse through above-threshold-ionization heating or by a subsequent long heater pulse though inverse bremsstrahlung heating, the plasma has dissipated as a result of hydrodynamic expansion. This leads to the creation of a periodic distribution of interlacing layers of high-density neutral gas and low-density plasma. The capability of producing a very sharp structure results from that optical-field ionization (multi-photon ionization) is a highly nonlinear process. A 10 TW, 55 fs, 810 nm, and 10 Hz Ti:sapphire laser system based on chirped-pulse amplification was used in this experiment. A compressed laser beam was split into two. One with 80% energy served as a longitudinal probe beam, and the other with 20% energy was used as the machining pulse. The machining pulse was set to be 7-ns earlier than the longitudinal probe and its energy was tunable with a half-wave plate and a thin-film polarizer. The longitudinal probe beam was focused by an f/7.2 off-axis parabolic mirror to a focal spot of 7 μm diameter in full width at half maximum (FWHM) with 80% energy enclosed in a Gaussian-fit profile. It was set to propagate in the same path as the pump pulse that will be used to drive various plasma photonic devices. Perpendicular to the probe pulse, the machining pulse right after passing through the mask was imaged horizontally onto the interaction plane by a cylindrical lens of 20 cm focal length with a demagnification factor of 8, and focused vertically by a cylindrical lens of 30 cm focal length to produce a line width of 20 μm FWHM. The line-shaped machining beam overlapped the propagation path of the longitudinal probe beam. An imaging system was used to measure the intensity distribution of the machining beam on the interaction plane. The intensity patterns of this machining pulse for using masks of various periods are shown in Fig. 1. With masks of different periods, we can produce a machining beam with a horizontal intensity modulation of 250 μm, 100 μm, and 40 μm period, respectively. The gas target was produced from a pulsed valve with a supersonic conical nozzle. The density profile has a 1-mm flat-top region with 250-μm boundaries. The atom density increased linearly with increasing backing pressure and reached 2.5×10 cm at 800-psi backing pressure for hydrogen gas. Side imaging of Thomson scattering from electrons was used to observe the laser channel of the longitudinal probe pulse. Since the intensity of Thomson scattering is proportional to the product of laser intensity and electron density, it can be used to measure the electron density distribution when the laser intensity distribution is known.

Experimental observation of relativistic cross phase modulation

An intense pump pulse propagating in a plasma can produce relativistic cross phase modulation on an overlapping copropagating probe pulse to broaden its spectrum. The first experimental observation is presented. When the laser intensity approaches 10 W/cm2, the electron quivering velocity in the laser field approaches the speed of light, leading to relativistic increase of electron mass. This in turn results in change of plasma refractive index, which can be expressed as 2 2

The development of soft x-ray lasers in IAMS

An optical-field-ionization soft x-ray laser with prepulse-controlled nanoplasma expansion in a cluster gas jet was demonstrated. Pd-like xenon lasing at 41.8-nm with 95 nJ pulse energy and 5-mrad divergence was achieved, indicating near-saturation amplification. In addition, by using deflectometry of a longitudinal probe pulse to resolve the spatiotemporal distribution of the preformed plasma, we characterize and control the plasma density distribution near the target surface for the development of solid-target x-ray lasers. We show that the use of prepulses in an ignitor-heater scheme can increase the scale length of the preformed plasma and how the effect varies with target materials.