Il Woo Choi

Laser prepulse dependency of proton-energy distributions in ultraintense laser-foil interactions with an online time-of-flight technique

Fast protons are observed by a newly developed online time-of-flight spectrometer, which provides shot-to-shot proton-energy distributions immediately after the irradiation of a laser pulse having an intensity of ∼1018W∕cm2 onto a 5-μm-thick copper foil. The maximum proton energy is found to increase when the intensity of a fs prepulse arriving 9ns before the main pulse increases from 1014 to 1015W∕cm2. Interferometric measurement indicates that the preformed-plasma expansion at the front surface is smaller than 15μm, which corresponds to the spatial resolution of the diagnostics. This sharp gradient of the plasma has the beneficial effect of increasing the absorption efficiency of the main-pulse energy, resulting in the increase in the proton energy. This is supported by the result that the x-ray intensity from the laser plasma clearly increases with the prepulse intensity.

Strong extreme ultraviolet emission from a double-stream xenon/helium gas puff target irradiated with a Nd:YAG laser

Abstract Extreme ultraviolet (EUV) emission in the 8–18 nm wavelength range from a new double-stream gas puff target irradiated with a Nd:YAG laser has been studied for the first time. The target was formed by pulsed injection of a xenon gas stream into a hollow gas stream from helium by using a double-nozzle setup. Strong EUV production near 11 nm from the double-stream xenon/helium target exceeding emissions from iron, copper, and tin solid targets was observed. This new result may be useful to design a laser-produced radiation source for EUV lithography.

Focusing and spectral enhancement of a repetition-rated, laser-driven, divergent multi-MeV proton beam using permanent quadrupole magnets

A pair of conventional permanent magnet quadrupoles is used to focus a 2.4 MeV laser-driven proton beam at a 1 Hz repetition rate. The magnetic field strengths are 55 and 60 T/m for the first and second quadrupoles, respectively. The proton beam is focused to a spot with a size of less than ∼3×8 mm2 at a distance of 650 mm from the source. This result is in good agreement with the Monte Carlo particle trajectory simulation.

Efficient production of a collimated MeV proton beam from a polyimide target driven by an intense femtosecond laser pulse

High-flux energetic protons whose maximum energies are up to 4MeV are generated by an intense femtosecond titanium:sapphire laser pulse interacting with 7.5, 12.5, and 25μm thick polyimide tape targets. Laser pulse with an energy of 1.7J and with a duration of 34fs is focused with an f/3.4 parabolic mirror giving an intensity of 3×1019Wcm−2. The main pulse to amplified spontaneous emission (ASE) intensity contrast ratio is 2.5×107. The conversion efficiency from the laser energy into the proton kinetic energies is achieved to be ∼3%, which is comparable to or even higher than those achieved in the previous works; using nanometer-thick targets, in combination with the short-pulse lasers that have almost the same pulse width and the intensity but different main pulse to ASE intensity contrast of ∼1010 [Neely et al., Appl. Phys. Lett. 89, 021502 (2006)], in which the authors claim that the main mechanism is target normal sheath acceleration; or using the 7.5μm thick polyimide target, in combination with the ...

Detailed space-resolved characterization of a laser-plasma soft-x-ray source at 13.5-nm wavelength with tin and its oxides

Space-resolved soft-x-ray spectra of laser-produced plasmas of pure-Sn metal and its oxides were measured in the spectral range 7–23 nm. We established a comprehensive spectroscopic database of the emission characteristics of the transition array of highly ionized Sn near 13.5-nm wavelength by varying the incident laser energy and the angle between the observation axis and the target normal. We examined the narrow spectral bandwidth of the transition array obtained by use of a gas-mixed fine-particle (SnO2 powder) target proposed by Matsui [Proc. SPIE3886, 610 (2000) ]. We selected pure-Sn metal, SnO and SnO2 powder, and SnO2 thin-film targets with which to clarify the roles of additional constituent ions, such as O and Ar, in plasmas of the gas-mixed fine-particle targets. The space-resolved spectra show that the bandwidth of the transition array broadens dramatically and that the wavelength at peak intensity shifts slightly toward longer wavelengths with increasing distance from the original target surface or with decreasing incident laser energy. The origins of the broadening and the wavelength shift can be explained in terms of an increase in the range of ion stages that contribute to the transition array and in terms of transfer of the dominant ion stages to lower stages. The narrow bandwidth of the gas-mixed fine-particle target is probably due to the presence of a narrow range of moderate ion stages.

Space-resolving flat-field extreme ultraviolet spectrograph system and its aberration analysis with wave-front aberration

The Nam aberration of a flat-field extreme ultraviolet spectrograph system, composed of a varied line-spacing concave grating and a toroidal mirror, was analyzed by calculating the wave-front aberration with respect to an astigmatic reference surface. The toroidal mirror was used to compensate for the astigmatism that was due to the grazing incidence of light at the concave grating. The spectrograph system could form a space-resolved spectrum along the sagittal direction. The spectral and spatial resolutions of the spectrograph system were estimated from the root-mean-square spot size. The actual spectral resolution of the spectrograph system was measured from extreme ultraviolet spectra obtained from plasmas produced by an iodine laser having an energy of 0.5 J in a 4-ns duration, and it was compared with the calculated value.

Radiation pressure acceleration of protons to 93 MeV with circularly polarized petawatt laser pulses

The radiation pressure acceleration (RPA) of charged particles has been a challenging task in laser-driven proton/ion acceleration due to its stringent requirements in laser and target conditions. The realization of radiation-pressure-driven proton acceleration requires irradiating ultrathin targets with an ultrahigh contrast and ultraintense laser pulses. We report the generation of 93-MeV proton beams achieved by applying 800-nm 30-fs circularly polarized laser pulses with an intensity of 6.1×1020 W/cm2 to 15-nm-thick polymer targets. The radiation pressure acceleration was confirmed from the obtained optimal target thickness, quadratic energy scaling, polarization dependence, and three-dimensional particle-in-cell simulations. We expect this clear demonstration of RPA to facilitate the realization of laser-driven proton/ion sources delivering energetic and short-pulse particle beams for novel applications.

Precise and long-term stabilization of the carrier-envelope phase of femtosecond laser pulses using an enhanced direct locking technique

We demonstrate a long-term operation with reduced phase noise in the carrier-envelope-phase (CEP) stabilization process by employing a double feedback loop and an improved signal detection in the direct locking technique [Opt. Express 13, 2969 (2005)]. A homodyne balanced detection method is employed for efficiently suppressing the dc noise in the f-2f beat signal, which is converted into the CEP noise in the direct locking loop working at around zero carrier-envelope offset frequency (f(ceo)). In order to enhance the long-term stability, we have used the double feedback scheme that modulates both the oscillator pump power for a fast control and the intracavity-prism insertion depth for a slow and high-dynamic-range control. As a result, the in-loop phase jitter is reduced from 50 mrad of the previous result to 29 mrad, corresponding to 13 as in time scale, and the long-term stable operation is achieved for more than 12 hours.

Ion spectrometer composed of time-of-flight and Thomson parabola spectrometers for simultaneous characterization of laser-driven ions.

An ion spectrometer, composed of a time-of-flight spectrometer (TOFS) and a Thomson parabola spectrometer (TPS), has been developed to measure energy spectra and to analyze species of laser-driven ions. Two spectrometers can be operated simultaneously, thereby facilitate to compare the independently measured data and to combine advantages of each spectrometer. Real-time and shot-to-shot characterizations have been possible with the TOFS, and species of ions can be analyzed with the TPS. The two spectrometers show very good agreement of maximum proton energy even for a single laser shot. The composite ion spectrometer can provide two complementary spectra measured by TOFS with a large solid angle and TPS with a small one for the same ion source, which are useful to estimate precise total ion number and to investigate fine structure of energy spectrum at high energy depending on the detection position and solid angle. Advantage and comparison to other online measurement system, such as the TPS equipped with microchannel plate, are discussed in terms of overlay of ion species, high-repetition rate operation, detection solid angle, and detector characteristics of imaging plate.

Absolute energy calibration for relativistic electron beams with pointing instability from a laser-plasma accelerator

The pointing instability of energetic electron beams generated from a laser-driven accelerator can cause a serious error in measuring the electron spectrum with a magnetic spectrometer. In order to determine a correct electron spectrum, the pointing angle of an electron beam incident on the spectrometer should be exactly defined. Here, we present a method for absolutely calibrating the electron spectrum by monitoring the pointing angle using a scintillating screen installed in front of a permanent dipole magnet. The ambiguous electron energy due to the pointing instability is corrected by the numerical and analytical calculations based on the relativistic equation of electron motion. It is also possible to estimate the energy spread of the electron beam and determine the energy resolution of the spectrometer using the beam divergence angle that is simultaneously measured on the screen. The calibration method with direct measurement of the spatial profile of an incident electron beam has a simple experimental layout and presents the full range of spatial and spectral information of the electron beams with energies of multi-hundred MeV level, despite the limited energy resolution of the simple electron spectrometer.