Terahertz-driven linear electron acceleration

Abstract

The cost, size and availability of electron accelerators are dominated by the achievable accelerating gradient. Conventional high-brightness radio-frequency accelerating structures operate with 30-50 MeV m(-1) gradients. Electron accelerators driven with optical or infrared sources have demonstrated accelerating gradients orders of magnitude above that achievable with conventional radio-frequency structures. However, laser-driven wakefield accelerators require intense femtosecond sources and direct laser-driven accelerators suffer from low bunch charge, sub-micron tolerances and sub-femtosecond timing requirements due to the short wavelength of operation. Here we demonstrate linear acceleration of electrons with keV energy gain using optically generated terahertz pulses. Terahertz-driven accelerating structures enable high-gradient electron/proton accelerators with simple accelerating structures, high repetition rates and significant charge per bunch. These ultra-compact terahertz accelerators with extremely short electron bunches hold great potential to have a transformative impact for free electron lasers, linear colliders, ultrafast electron diffraction, X-ray science and medical therapy with X-rays and electron beams.

Figure 1. Terahertz-driven linear accelerator.

(a) Schematic of the THz LINAC. Top right: a linearly polarized THz pulse is converted into a radially polarized pulse by a segmented waveplate before being focused into the THz waveguide. The THz pulse is reflected at the end of the waveguide to co-propagate with the electron bunch, which enters the waveguide through a pinhole (lower left). The electron bunch is accelerated by the longitudinal electric field of the co-propagating THz pulse. The electron bunch exits the THz waveguide and passes through a hole in the focusing mirror (right) for the THz pulse. (b) Photograph of the compact millimetre scale THz LINAC. (c) The time-domain waveform of the THz pulse determined with electro-optic sampling (see Methods: Electro-optic sampling). Insert: corresponding frequency-domain spectrum. (d) The time-domain waveform of the THz pulse at the exit of a THz waveguide 5 cm in length, including two tapers. (e) Normalized intensity of the focused THz beam.

Figure 2. Demonstration of terahertz acceleration.

Transverse electron density of the electron bunch as recorded by a micro-channel plate (MCP) at 59 kV for (a) THz off and (b) THz on. The bimodal distribution is due to the presence of accelerated and decelerated electrons, which are separated spatially by the magnetic dipole energy spectrometer. The images are recorded over a 3-s exposure at 1 kHz repetition rate. (c) Comparison between simulated (red) and measured (black) energy spectrum of the electron bunch measured at the MCP due to the deflection of the beam by a magnetic dipole. At 59 keV and with 25 fC per bunch, the simulation predicts a σ_⊥=513 μm and ΔE=1.25 keV. The observed ΔE/E appears larger due to the large transverse size of the beam. After the pinhole, the transverse emittance is 25 nm rad and the longitudinal emittance is 5.5 nm rad. (d) Comparison between simulated (red) and measured (black) electron bunch at MCP after acceleration with THz. Decelerated electrons are present due to the long length of the electron bunch, as well as the slippage between the THz pulse and the electron bunch. Error bars in c and d correspond to one s.d. in counts over the data set of 10 integrated exposures.

Figure 3. Acceleration gradient and terahertz phasing.

(a) Scaling of energy gain for accelerated electrons as a function of the initial electron energy at the entrance of the THz LINAC. Black dots with one s.d. error bars are measured values and the red line is a single-particle model. (b) The temporal profile for the mean energy gain of accelerated electrons comparing the THz on and THz off signal against the simulated electron bunch. The initial electron energy was set at 55 keV to ensure stable performance of the d.c. electron gun over the acquisition time of the data set.

Figure 4. TM₀₁ THz LINAC parameters.

(a) Normalized magnitude of the longitudinal electric field and perpendicular magnetic field for the TM₀₁ mode at 450 GHz in a circular copper waveguide with dielectric loading. The inner diameter of the copper waveguide is 940 μm with a vacuum radius r_v=200 μm and a dielectric wall thickness of d=270 μm. The dimensions r_v and d are labelled in the E_z plot. The solid black line indicates the boundary between the vacuum core and the quartz capillary. (b) The dispersion relation for the TM₀₁ mode with and without dielectric loading. The black line indicates the speed of light in vacuum. (c) The group and phase velocity of the THz pulse as a function of frequency with dielectric loading.

Figure 5. Relativistic THz LINAC design.

Performance parameters as a function of vacuum radius and dielectric wall thickness for a relativistic THz LINAC operating in the TM₀₁ mode with a 10 mJ single-cycle THz pulse and an initial electron energy of 1 MeV. The phase velocity is c for the nominal frequency of operation. The (a) frequency of operation, (b) energy gain, (c) acceleration gradient, (d) group velocity and (e) interaction length for the THz LINAC. (f) The electron energy as a function of distance for two cases, which operate with a frequency of 0.45/1 THz, a vacuum space with a radius of r_v=105/105 μm and a dielectric wall thickness of 90/30 μm.

Figure 6. Electromagnetic field distribution.

The field distribution in the THz waveguide for (a) the non-relativistic design described in Fig. 4 with r_v=200 μm and d=270 μm, (b) the relativistic design in Fig. 5f for operation at 0.45 THz with r_v=105 μm and d=90 μm and (c) the relativistic design in Fig. 5f for operation at 1 THz with r_v=105 μm and d=30 μm.

Figure 7. Coupling and dispersion of THz pulses in waveguides.

(a) HFSS simulation of the coupling of the free-space radially polarized mode into the TM₀₁ mode through a dielectric-loaded taper. (b) The time-domain waveform of the THz pulse determined with electro-optic sampling before being coupled into the THz waveguide. (c) The measured versus the modelled time-domain waveform of the THz pulse at the exit of a 5-cm (including tapers) dielectric-loaded THz waveguide.