Plasma Undulator Research Articles

Chromatic matching in a plasma undulator

The principle of chromatic matching, i.e., using two laser modes of different geometric mode numbers and different colors so that they copropagate at the same group velocity, is proposed in order to realize the plasma undulator concept. Chromatic matching allows one to overcome group velocity slippage, whereby lower-order modes outrun higher-order ones, allowing for extended interaction lengths. The dephasing limit can be mitigated by using a special tapering of the plasma channel such that the electron bunch propagates in phase with the laser mode in addition to maintaining a constant undulator frequency. Furthermore, controlled dephasing is proposed as a means to induce chirp in the generated X-ray spectrum.

Flexible x-ray source with tunable polarization and orbital angular momentum from Hermite-Gaussian laser modes driven plasma channel wakefield

A plasma channel undulator/wiggler may be created through the plasma wakefield excited by the beating of several Hermite-Gaussian laser modes propagating in a parabolic plasma channel. Control over both the betatron and undulator forces is conveniently achieved by tuning the amplitude ratios, colors, and order numbers of the modes. A special structure of the undulator/wiggler field without the focusing force near the propagation axis is generated inside the plasma wakefield by matching the strengths of the fundamental and first-order Hermite-Gaussian modes. The electron beam only experiences forced undulator oscillations in such a field, which significantly improves the quality of the emitted radiation. Since the value of the undulator strength parameter could be in a wide range, less or larger than unity, it is capable of generating narrow bandwidth x-ray, as well as the synchrotronlike high-energy $\mathrm{x}/\ensuremath{\gamma}$-ray, radiation by harmonics. Additionally, controlling the relative phases between the laser modes allows for polarization control of the plasma undulator. High-order harmonics produced from a circularly polarized plasma undulator clearly show the vortex nature and carry well-defined orbital angular momentum.

Plasma channel undulator excited by high-order laser modes

The possibility of utilizing plasma undulators and plasma accelerators to produce compact ultraviolet and X-ray sources, has attracted considerable interest for a few decades. This interest has been driven by the great potential to decrease the threshold for accessing such sources, which are mainly provided by a few dedicated large-scale synchrotron or free-electron laser (FEL) facilities. However, the broad radiation bandwidth of such plasma devices limits the source brightness and makes it difficult for the FEL instability to develop. Here, using multi-dimensional particle-in-cell (PIC) simulations, we demonstrate that a plasma undulator generated by the beating of a mixture of high-order laser modes propagating inside a plasma channel, leads to a few percent radiation bandwidth. The strength of the undulator can reach unity, the period can be less than a millimeter, and the number of undulator periods can be significantly increased by a phase locking technique based on the longitudinal tapering. Polarization control of such an undulator can be achieved by appropriately choosing the phase of the modes. According to our results, in the fully beam loaded regime, the electron current in the plasma undulator can reach 0.3 kA level, making such an undulator a potential candidate towards a table-top FEL.

Generation of a wakefield undulator in plasma with transverse density gradient

We show that a short relativistic electron beam propagating in a plasma with a density gradient perpendicular to the direction of motion generates a wakefield in which a witness bunch experiences a transverse force. A density gradient oscillating along the beam path would create a periodically varying force—an undulator, with an estimated strength of the equivalent magnetic field more than 10 T. This opens an avenue for the creation of a high-strength, short-period undulator, which eventually may lead to all-plasma, free electron lasers where a plasma wakefield acceleration is naturally combined with a plasma undulator in a unifying, compact setup.

A compact tunable polarized X-ray source based on laser-plasma helical undulators.

Laser wakefield accelerators have great potential as the basis for next generation compact radiation sources because of their extremely high accelerating gradients. However, X-ray radiation from such devices still lacks tunability, especially of the intensity and polarization distributions. Here we propose a tunable polarized radiation source based on a helical plasma undulator in a plasma channel guided wakefield accelerator. When a laser pulse is initially incident with a skew angle relative to the channel axis, the laser and accelerated electrons experience collective spiral motions, which leads to elliptically polarized synchrotron-like radiation with flexible tunability on radiation intensity, spectra and polarization. We demonstrate that a radiation source with millimeter size and peak brilliance of 2 × 1019 photons/s/mm2/mrad2/0.1% bandwidth can be made with moderate laser and electron beam parameters. This brilliance is comparable with third generation synchrotron radiation facilities running at similar photon energies, suggesting that laser plasma based radiation sources are promising for advanced applications.

Plasma effects on the free-electron laser gain with a plasma wave undulator

Employing a magnetized plasma medium in the interaction region of a free-electron laser (FEL) offers the possibility of generating short wavelengths using moderate energy beams. Plasma in the presence of static magnetic field supports right and left circularly polarized electromagnetic modes. By superposition of these two modes, a linearly polarized electromagnetic mode is generated which can be employed as a plasma undulator in a FEL. This configuration has a higher tunability by controlling the plasma density on top of the -tubability of the conventional FELs. The roles of the axial magnetic field and plasma on the laser gain and the electron trajectories of an e-beam propagating through the plasma medium have been studied and new orbits of group (I, II, and III) have been found. Moreover, the stability of these orbits for different values of plasma frequencies has been investigated. It is shown that by increasing the axial guide magnetic field strength, the gain for orbits of group I trivially increase, while a decrease in gain has been obtained for orbits of group II and group III. In addition, it is found that with increasing the plasma frequency (or plasma density) the gain for orbits of group I and group II trivially decreases and shift to the lower cyclotron frequencies, while an increase in gain has been obtained for orbits of group III.

Plasma undulator based on laser excitation of wakefields in a plasma channel.

An undulator is proposed based on the plasma wakefields excited by a laser pulse in a plasma channel. Generation of the undulator fields is achieved by inducing centroid oscillations of the laser pulse in the channel. The period of such an undulator is proportional to the Rayleigh length of the laser pulse and can be submillimeter, while preserving high undulator strength. The electron trajectories in the undulator are examined, expressions for the undulator strength are presented, and the spontaneous radiation is calculated. Multimode and multicolor laser pulses are considered for greater tunability of the undulator period and strength.

An ultracompact X-ray source based on a laser-plasma undulator.

The capability of plasmas to sustain ultrahigh electric fields has attracted considerable interest over the last decades and has given rise to laser-plasma engineering. Today, plasmas are commonly used for accelerating and collimating relativistic electrons, or to manipulate intense laser pulses. Here we propose an ultracompact plasma undulator that combines plasma technology and nanoengineering. When coupled with a laser-plasma accelerator, this undulator constitutes a millimetre-sized synchrotron radiation source of X-rays. The undulator consists of an array of nanowires, which are ionized by the laser pulse exiting from the accelerator. The strong charge-separation field, arising around the wires, efficiently wiggles the laser-accelerated electrons. We demonstrate that this system can produce bright, collimated and tunable beams of photons with 10-100 keV energies. This concept opens a path towards a new generation of compact synchrotron sources based on nanostructured plasmas.

Plasma wave undulator for laser-accelerated electrons

Laser-plasma accelerators have become compact sources of ultrashort electron bunches at energies up to the gigaelectronvolt range thanks to the remarkable progress made over the past decade. A direct application of these electron bunches is the production of short pulse x-ray radiation sources. In this letter, we study a fully optically driven x-ray source based on the combination of a laser-plasma accelerator and a plasma wave undulator. The longitudinal electric field of a laser-generated plasma wave is used to wiggle electrons transversally. The period of this plasma undulator being equal to the plasma wavelength, tunable photon energies in the 10 keV range can be achieved with electron energies in the 100–200 MeV range. Considering a 10s TW class femtosecond laser system, undulators with a strength parameter K∼0.5 and with about ten periods can be combined with a laser-plasma accelerator, resulting in several 10−2 emitted x-ray photons per electron.

A table-top X-ray FEL based on the laser wakefield accelerator-undulator system

Laser wakefield acceleration makes it possible to build a compact electron linac. While acceleration is attributed to longitudinal wakefields, transverse wakefields simultaneously generated by a short laser pulse can serve as a plasma undulator with a very short wavelength equal to half of the plasma wavelength. We propose a new FEL concept for X-rays based on a laser pulses delivered from a table-top terawatt laser. The system is composed of the accelerator and undulator stages in a table-top size. A low energy electron beam is accelerated due to laser wakefields in the accelerator stage. A bunched electron beam travelling to the opposite direction of driving laser pulses produces a coherent X-ray radiation in the undulator stage. A practical configuration and its analysis are presented.

Conceptual design of a plasma undulator

A micropitched plasma undulator is proposed for the first time. It is composed of an array of hundreds of discharged and pinched slender columns of plasma. Two kinds of undulating forces on the electron beam can be considered: one is a periodic magnetic field generated by the electric currents flowing through the array of the small radius plasma columns and the other is a periodic electrostatic field generated by the interaction of the relativistic electron beam with the rippled density of the plasma. Methods for both producing and stabilizing the array of the plasma columns are discussed. The effects of collisions between the plasma particles and the electron beam on the orbit and the momentum of the electrons are also discussed, leading to the conclusion that they can be negligibly small, as far as the gas pressure in the chamber leaking from the plasma region does not disturb the performance of the electron accelerator.

The plasma undulator

On the basis of the theory of magnetic undulator for free electron laser (FEL) we analyze the use of a large amplitude plasma wave as an electrostatic undulator.We review the original idea of the plasma undulator and consider some particular plasma undulator configurations. By using low energy electron beams we show the possibility to generate very small wavelength electromagnetic radiation.The fluid theory for the FEL with a plasma undulator, in complete analogy with the ordinary ones, is presented and the thresholds for the FEL action are discussed. An analysis for the intrinsic efficiency, with respect to the emittance and the plasma wake field effects, is presented and applied to some new configurations.