C. J. Zhang, J. F. Hua, Y. Wan, C.-H. Pai, B. Guo, J. Zhang, Y. Ma, F. Li, Y. P. Wu, H.-H. Chu, Y. Q. Gu, X. L. Xu, W. B. Mori, C. Joshi, J. Wang, and W. Lu
Physical Review Letters 119, 064801 (2017)
We show that a high-energy electron bunch can be used to capture the instantaneous longitudinal and
transverse field structures of the highly transient, microscopic, laser-excited relativistic wake with
femtosecond resolution. The spatiotemporal evolution of wakefields in a plasma density up ramp is
measured and the reversal of the plasma wake, where the wake wavelength at a particular point in space
increases until the wake disappears completely only to reappear at a later time but propagating in the
opposite direction, is observed for the first time by using this new technique.
C. J. Zhang, J. F. Hua, Y. Wan, B. Guo, C.-H. Pai, Y. P. Wu, F. Li, H.-H. Chu, Y. Q. Gu, W. B. Mori, C. Joshi, J. Wang, and W. Lu
Physical Review Accelerators and Beams 19, 062802 (2016)
A new method for diagnosing the temporal characteristics of ultrashort electron bunches with linear energy chirp generated from a laser wakefield accelerator is described. When the ionization-injected bunch interacts with the back of the drive laser, it is deflected and stretched along the direction of the electric field of the laser. Upon exiting the plasma, if the bunch goes through a narrow slit in front of the dipole magnet that disperses the electrons in the plane of the laser polarization, it can form a series of bunchlets that have different energies but are separated by half a laser wavelength. Since only the electrons that are undeflected by the laser go through the slit, the energy spectrum of the bunch is modulated. By analyzing the modulated energy spectrum, the shots where the bunch has a linear energy chirp can be recognized. Consequently, the energy chirp and beam current profile of those bunches can be reconstructed. This method is demonstrated through particle-in-cell simulations and experiment.
Gin-yih Tsaur and Jyhpyng Wang
European Journal of Physics 37, 045402 (2016)
The Green function method is a powerful technique for solving the initial value problem in quantum mechanics. Once the Green function is solved the whole wavefunction evolution is represented in a concise form and can be evaluated conveniently by numerical integration. We present a method for constructing the Green functions systematically which is different from the conventional methods of eigenfunction expansion or path integration. By using variable changing, function substitution, and Fourier transforms, the time dependent Schrödinger equations can be simplified and the solutions for the simplified equations can be easily derived. We then obtain the Green functions for the original equations by the reverse transforms. The method is demonstrated for the linear potential, the harmonic oscillator, the centrifugal potential, and the centripetal barrier oscillator, where the Green function for the centripetal barrier oscillator has not been solved previously by conventional methods. The method and examples illustrated in this paper can be utilised to strengthen undergraduate courses on quantum mechanics and/or partial differential equation.
Hsu-hsin Chu, Chi-Hsiang Yang, Shih-Cheng Liu, and Jyhpyng Wang
Optics Express 23, 34082 (2015)
Single-shot ultrashort extreme-UV(EUV) pulse waveform measurement is demonstrated by utilizing strong field ionization of H2 gas for transmission gating. A cross-propagating intense near-IR gate pulse ionizes the EUV absorbing H2 molecules into EUV-non-absorbing H2++ (two protons) and creates a time sweep of transmission encoded spatially across the EUV pulse. The temporal envelope is then retrieved from the lopsided spatial profile of the transmitted pulse. This method not only measures EUV temporal envelope for each single shot, but also determines timing jitter and envelope fluctuation statistically, thus is particularly useful for characterizing low-repetition-rate fluctuating EUV/soft x-ray sources.
Te-Sheng Hung, Chi-Hsiang Yang, Jyhpyng Wang, Szu-yuan Chen, Jiunn-Yuan Lin, Hsu-hsin Chu
Applied Physics B 117, 1189 (2014)
A Ti:sapphire laser system has been constructed with two synchronized main beams of 110 TW and 13 TW, and a 5-TW wavelength-tunable synchronized auxiliary beam for versatile control of laser-plasma interaction. The first main beam provides 3.3-J, 30-fs, 810-nm pulses, and the second 450-mJ, 34-fs, 805-nm pulses. The auxiliary beam comes from amplified spectral windows selected from a supercontinuum of high spatial coherence and provides 38-fs pulses with tunable wavelengths (870–920 nm). The two main beams can be focused down to M2 = 1.2 and 1.1, with 77 and 81% energy enclosed in the focal spots, respectively. The energy fluctuations are 1.1 and 1.8 %, and the pointing fluctuations are 4.5 and 4.8 μrad, respectively. By using a preamplifier and saturable absorber before the pulse stretcher to suppress amplified spontaneous emission, the temporal contrast of the 110-TW main beam reaches 4 ⨯1010 at the -100-ps timescale. Even though the auxiliary beam is generated from a highly nonlinear process, by confining the supercontinuum generation in a single self-trapping filament, a spatial coherence close to the main beams can be achieved. It can be focused down to M2 = 1.3, with 72 % energy enclosed in the focal spot. The energy fluctuation is 2.6 %, and the pointing fluctuation is 4.7 μrad. The versatility of synchronized multiple-beams with tunable wavelengths, good energy and pointing stability, and the spatiotemporal quality of the laser system has been essential to our experiments in high-harmonic generation, extreme-UV lasers, and laser-wakefield accelerators in which precision control of laser-plasma interaction is facilitated by a concerted sequence of driving pulses.
Tung-Chang Liu, Xi Shao, Chuan-Sheng Liu, Bengt Eliasson, WT Hill III, Jyhpyng Wang, and Shih-Hung Chen
New Journal of Physics 17, 023018 (2015)
Wepresent a theoretical and numerical study of a novel acceleration scheme by applying a combination of laser radiation pressure and shielded Coulomb repulsion in laser acceleration of protons in multi-species gaseous targets. By using a circularly polarized CO2 laser pulse with a wavelength of 10 μm—much greater than that of a Ti:Sapphire laser—the critical density is significantly reduced, and a high-pressure gaseous target can be used to achieve an overdense plasma. This gives us a larger degree of freedom in selecting the target compounds or mixtures, as well as their density and thickness profiles. By impinging such a laser beam on a carbon–hydrogen target, the gaseous target is first compressed and accelerated by radiation pressure until the electron layer disrupts, after which the protons are further accelerated by the electron-shielded carbon ion layer. An 80 MeV quasi-monoenergetic proton beam can be generated using a half-sine shaped laser beam with a peak power of 70 TW and a pulse duration of 150 wave periods.