G. Fubiani

Gamma-neutron activation experiments using laser wakefield accelerators

Gamma-neutron activation experiments have been performed with relativistic electron beams produced by a laser wakefield accelerator. The electron beams were produced by tightly focusing (spot diameter ≈6 μm) a high power (up to 10 TW), ultra-short (⩾50 fs) laser beam from a high repetition rate (10 Hz) Ti:sapphire (0.8 μm) laser system, onto a high density (>1019 cm−3) pulsed gasjet of length ≈1.5 mm. Nuclear activation measurements in lead and copper targets indicate the production of electrons with energy in excess of 25 MeV. This result was confirmed by electron distribution measurements using a bending magnet spectrometer. Measured γ-ray and neutron yields are also found to be in reasonable agreement with simulations using a Monte Carlo transport code.

Terahertz radiation from laser accelerated electron bunches

Coherent terahertz and millimeter wave radiation from laser accelerated electron bunches has been measured. The bunches were produced by tightly focusing (spot diameter ≈6 μm) a high peak power (up to 10 TW), ultra-short (⩾50 fs) laser pulse from a high repetition rate (10 Hz) laser system (0.8 μm), onto a high density (>1019 cm−3) pulsed gas jet of length ≈1.5 mm. As the electrons exit the plasma, coherent transition radiation is generated at the plasma-vacuum boundary for wavelengths long compared to the bunch length. Radiation in the 0.3–19 THz range and at 94 GHz has been measured and found to depend quadratically on the bunch charge. The measured radiated energy for two different collection angles is in good agreement with theory. Modeling indicates that optimization of this table-top source could provide more than 100 μJ/pulse. Together with intrinsic synchronization to the laser pulse, this will enable numerous applications requiring intense terahertz radiation. This radiation can also be used as a...

Terahertz radiation as a bunch diagnostic for laser-wakefield-accelerated electron bunches

Experimental results are reported from two measurement techniques (semiconductor switching and electro-optic sampling) that allow temporal characterization of electron bunches produced by a laser-driven plasma-based accelerator. As femtosecond electron bunches exit the plasma-vacuum interface, coherent transition radiation (at THz frequencies) is emitted. Measuring the properties of this radiation allows characterization of the electron bunches. Theoretical work on the emission mechanism is presented, including a model that calculates the THz wave form from a given bunch profile. It is found that the spectrum of the THz pulse is coherent up to the 200 μm thick crystal (ZnTe) detection limit of 4THz, which corresponds to the production of sub-50fs (rms) electron bunch structure. The measurements demonstrate both the shot-to-shot stability of bunch parameters that are critical to THz emission (such as total charge and bunch length), as well as femtosecond synchronization among bunch, THz pulse, and laser beam.

Powerful, pulsed, THz radiation from laser accelerated relativistic electron bunches

Coherent THz radiation was produced from relativistic electron bunches of subpicosecond duration. The electron beam was produced by strongly focused (≈ 6 μm), high peak power (up to 10 TW), ultra-short (≥50 fs) laser pulses of a 10 Hz repetition rate Ti:sapphire chirped pulse amplification (CPA) laser system via self-modulated laser wakefield acceleration (SM-LWFA) in a high density (> 1019 cm-3) pulsed gas jet. As the electrons exit the plasma, coherent transition radiation is generated at the plasma-vacuum boundary for wavelengths long compared to the bunch length. Radiation yield in the 0.3 to 19 THz range and at 94 GHz has been measured and found to depend quadratically on the bunch charge. The measured total radiated energy in the THz range for two different collection angles is in good agreement with theory. Modeling indicates that optimization of this table-top source could provide more than 100 μJ/pulse. Together with intrinsic synchronization to the laser pulse, this will enable numerous applications requiring intense terahertz radiation. This radiation can also be applied as a useful tool for measuring the properties of laser accelerated bunches at the exit of the plasma accelerator.

Semi‐Analytical 6D Space Charge Model for Dense Electron Bunches with Large Energy Spreads

Laser driven accelerators are capable of producing multi nC, multi MeV electron beams with transverse and longitudinal sizes on the order of microns. To investigate the transport of such electron bunches, a fast and fully relativistic space charge code which can handle beams with arbitrarily large energy spread has been developed. A 6‐D macroparticle model for the beam is used to calculate the space charge fields at each time step. The collection of macroparticles is divided into longitudinal momentum bins, each with a small spread in relative momentum. The macroparticle distribution in each momentum bin is decomposed into ellipsoidal shells in position space. For each shell, an analytical expression for the electrostatic force in the bin rest frame is used. The total space charge force acting on one macroparticle in the lab frame is then the vector sum of the Lorentz‐transformed forces from all the momentum bins. We have used this code to study the evolution of typical beams emerging from the plasma in t...

Laser Triggered Injection of Electrons in a Laser Wakefield Accelerator with the Colliding Pulse Method

An injection scheme for a laser wakefield accelerator that employs a counterpropagating laser (colliding with the drive laser pulse, used to generate a plasma wake) is discussed. The threshold laser intensity for electron injection into the wakefield was analyzed using a heuristic model based on phase‐space island overlap. Analysis shows that the injection can be performed using modest counterpropagating laser intensity a1 ⩽ 0.5 for a drive laser intensity of a0 ≃ 1.0. Preliminary experiments were preformed using a drive beam and colliding beam. Charge enhancement by the colliding pulse was observed. Increasing the signal‐to‐noise ratio by means of a preformed plasma channel is discussed.

Enhancement of Laser Injection Methods by Plasma Density Gradients

Plasma density down‐ramps are proposed as a method for improving the performance of laser injection schemes. A decrease in density implies an increase in plasma wavelength, which can shift a relativistic electron from the defocusing to the focusing region of the accelerating wakefield. Also, a decrease in density leads to a decrease in wake phase velocity, which lowers the trapping threshold. The specific method of two‐pulse colliding pulse injector was examined using a 3D test particle tracking code. A density down‐ramp of 30% led to an order of magnitude enhancement of the trapping fraction compared to the uniform plasma case, without degrading other bunch properties.

Laser wakefield accelerator experiments at LBNL

The status is presented of the laser wakefield acceleration research at the l’OASIS laboratory of the Center for Beam Physics at LBNL. Experiments have been performed on laser driven production of relativistic electron beams from plasmas using a high repetition rate (10 Hz), high power (10 TW) Ti:sapphire (0.8 μm) laser system. Large amplitude plasma waves have been excited in the self-modulated laser wakefield regime by tightly focusing (spot diameter 8 μm) a single high power (⩽10 TW), ultra-short (⩾50 fs) laser pulse onto a high density (>1019 cm−3) pulsed gasjet (length 1.2 mm). Nuclear activation measurements in lead and copper targets indicate the production of electrons with energy in excess of 25 MeV. This result was confirmed by electron distribution measurements using a bending magnet spectrometer. Progress on implementing the colliding pulse laser injection method is also presented. This method is expected to produce low emittance (<1π mm-mrad), low energy spread (<1%), ultrashort (fs), 40 MeV ...

Studies of space-charge effects in ultrashort electron bunches

Laser-driven plasma-based accelerators are capable of producing ultrashort electron bunches in which the longitudinal size is much smaller than the transverse size. We present theoretical studies of the transport of such electron bunches in vacuum. Space charge forces acting on the bunch are calculated using an ellipsoidal bunch shape model. The effects of space charge forces and energy spread on longitudinal and transverse bunch properties are evaluated for various bunch lengths, energies and amount of charge.