Capillary Discharge Waveguide Research Articles

Multi-GeV cascaded laser wakefield acceleration in a hybrid capillary discharge waveguide

Based on a 6 cm-long two-segment hybrid capillary discharge waveguide, a multi-GeV electron beam with energy up to 3.2 GeV and 9.7% rms energy spread was achieved in a cascaded laser wakefield acceleration scheme, powered by an on-target 210 TW laser pulse. The electron beam was trapped in the first segment via ionization-induced injection, and then seeded into the second segment for further acceleration. The long-distance stable guiding of the laser pulse and suppression of the dark current inside the second-segment capillary played an important role in the generation of high-energy electron beams, as demonstrated by quasi-three-dimensional particle-in-cell simulations.

Impact of electron transport models on capillary discharge plasmas

Magnetohydrodynamics (MHD) can be used to model capillary discharge waveguides in laser-wakefield accelerators. However, the predictive capability of MHD can suffer due to poor microscopic closure models. Here, we study the impact of electron heating and thermal conduction on the capillary waveguide performance as part of an effort to understand and quantify uncertainties in modeling and designing next-generation plasma accelerators. To do so, we perform two-dimensional high-resolution MHD simulations using an argon-filled capillary discharge waveguide with three different electron transport coefficients models. The models tested include (i) Davies et al., (ii) Spitzer, and (iii) Epperlein–Haines (EH). We found that the EH model overestimates the electron temperature inside the channel by over 20% while predicting a lower azimuthal magnetic field. Moreover, the Spitzer model, often used in MHD simulations for plasma-based accelerators, predicts a significantly higher electron temperature than the other models suggest.

Beam-based characterization of plasma density in a capillary-discharge waveguide

Next-generation plasma-based accelerators can push electron bunches to gigaelectronvolt energies within centimeter distances. In these devices, the accelerating force is provided by a driver pulse, either a laser pulse or a particle bunch, that loses its energy into the plasma generating huge electric fields up to tens of GV/m. The stability of such fields strongly depends on plasma density, whose exact value should be precisely known and controlled. However, currently available methods based on spectroscopic or interferometric techniques find it very difficult to measure plasma density lower than 1015–16 cm−3 in capillary-discharge waveguides. Here, we present a novel diagnostic tool that allows us to estimate the average density of a plasma capillary by probing it with an ultra-relativistic electron beam. The plasma density and the generated accelerating field are inferred by analyzing the beam longitudinal phase space after its interaction with the plasma. The results are validated by simulations showing excellent agreement.

Radial density profile and stability of capillary discharge plasma waveguides of lengths up to 40 cm

Abstract We measured the parameter reproducibility and radial electron density profile of capillary discharge waveguides with diameters of 650 $\mathrm{\mu} \mathrm{m}$ to 2 mm and lengths of 9 to 40 cm. To the best of the authors’ knowledge, 40 cm is the longest discharge capillary plasma waveguide to date. This length is important for $\ge$ 10 GeV electron energy gain in a single laser-driven plasma wakefield acceleration stage. Evaluation of waveguide parameter variations showed that their focusing strength was stable and reproducible to $<0.2$ % and their average on-axis plasma electron density to $<1$ %. These variations explain only a small fraction of laser-driven plasma wakefield acceleration electron bunch variations observed in experiments to date. Measurements of laser pulse centroid oscillations revealed that the radial channel profile rises faster than parabolic and is in excellent agreement with magnetohydrodynamic simulation results. We show that the effects of non-parabolic contributions on Gaussian pulse propagation were negligible when the pulse was approximately matched to the channel. However, they affected pulse propagation for a non-matched configuration in which the waveguide was used as a plasma telescope to change the focused laser pulse spot size.

Modeling of capillary discharge plasmas for wakefield acceleration and beam transport

Next generation accelerators demand sophisticated beam sources to produce ultra-low emittances with large gradients. The subsequent beamline optics are equally critical to transporting these beams between accelerating stages or to interaction points. Capillary discharge plasmas may address each of these challenges. Capillaries have been demonstrated as sources capable of increasing the peak energy and beam quality of laser wakefield accelerators, and as active plasma lenses featuring orders-of-magnitude increases in peak magnetic field. These systems are sensitive to energy deposition, heat transfer, ionization dynamics, and magnetic field penetration; therefore, improved modeling will enable advances in capillary design. We present simulations of capillary discharge waveguides and active plasma lenses in using FLASH, a publicly-available multi-physics code in development at the University of Chicago. We report on the implementation of a 2D, cylindrically symmetric capillary model for capturing plasma density and temperature evolution with realistic conductivities and magnetic fields. We then illustrate the use of laser energy deposition to model low density channel formation for the matching and guiding of intense laser pulses. Lastly, we discuss simulations of active capillary plasmas with different fill species, which show agreement with experimental observations of nonlinearities in the current density profile and magnetic field.

Laser-heated capillary discharge waveguides as tunable structures for laser-plasma acceleration

Laser-heated capillary discharge waveguides are novel, low plasma density guiding structures able to guide intense laser pulses over many diffraction lengths and have recently enabled the acceleration of electrons to 7.8 GeV by using a laser-plasma accelerator (LPA). These devices represent an improvement over conventional capillary discharge waveguides, as the channel matched spot size and plasma density can be tuned independently of the capillary radius. This has allowed the guiding of petawatt-scale pulses focused to small spot sizes within large diameter capillaries, preventing laser damage of the capillary structure. High performance channel-guided LPAs require control of matched spot size and density, which experiments and simulations reported here show can be tuned over a wide range via initial discharge and laser parameters. In this paper, measurements of the matched spot size and plasma density in laser-heated capillary discharges are presented, which are found to be in excellent agreement with simulations performed using the MHD code MARPLE. Strategies for optimizing accelerator performance are identified based on these results.

Laser-heated capillary discharge plasma waveguides for electron acceleration to 8 GeV

A plasma channel created by the combination of a capillary discharge and inverse Bremsstrahlung laser heating enabled the generation of electron bunches with energy up to 7.8 GeV in a laser-driven plasma accelerator. The capillary discharge created an initial plasma channel and was used to tune the plasma temperature, which optimized laser heating. Although optimized colder initial plasma temperatures reduced the ionization degree, subsequent ionization from the heater pulse created a fully ionized plasma on-axis. The heater pulse duration was chosen to be longer than the hydrodynamic timescale of ≈1 ns, such that later temporal slices were more efficiently guided by the channel created by the front of the pulse. Simulations are presented which show that this thermal self-guiding of the heater pulse enabled channel formation over 20 cm. The post-heated channel had lower on-axis density and increased focusing strength compared to relying on the discharge alone, which allowed for guiding of relativistically intense laser pulses with a peak power of 0.85 PW and wakefield acceleration over 15 diffraction lengths. Electrons were injected into the wake in multiple buckets and times, leading to several electron bunches with different peak energies. To create single electron bunches with low energy spread, experiments using localized ionization injection inside a capillary discharge waveguide were performed. A single injected bunch with energy 1.6 GeV, charge 38 pC, divergence 1 mrad, and relative energy spread below 2% full-width half-maximum was produced in a 3.3 cm-long capillary discharge waveguide. This development shows promise for mitigation of energy spread and future high efficiency staged acceleration experiments.

Petawatt Laser Guiding and Electron Beam Acceleration to 8GeV in a Laser-Heated Capillary Discharge Waveguide.

Guiding of relativistically intense laser pulses with peak power of 0.85 PW over 15 diffraction lengths was demonstrated by increasing the focusing strength of a capillary discharge waveguide using laser inverse bremsstrahlung heating. This allowed for the production of electron beams with quasimonoenergetic peaks up to 7.8GeV, double the energy that was previously demonstrated. Charge was 5pC at 7.8GeV and up to 62pC in 6GeV peaks, and typical beam divergence was 0.2mrad.

Measurement of the matched spot size in a capillary discharge waveguide with a collimated laser

Measurement of the matched spot size in the hydrogen-filled capillary discharge waveguide based on the spot size oscillation of a collimated laser is presented in this paper. The spot size oscillation trace is retrieved from the laser modes measured at the exits of discharged capillaries of different lengths under the same discharge conditions. With the gas pressure, peak discharge electric current and capillary radius fixed, the radial density profiles are identical in all the discharged capillaries. The measured laser modes are equivalent to the evolution at discrete positions in a long plasma channel. Compared to former researches based on the spot size at the capillary exit, this method is not affected by the multiple solution problem. The use of a collimated laser eliminates the influences of the divergence angle on the fitting accuracy. By this means, the matched spot sizes of hydrogen-filled capillary discharge waveguides under different gas pressures (5-20mbar) are measured. The results can provide a spot size reference for the laser wakefield accelerator guided in a plasma channel.

Hybrid capillary discharge waveguide for laser wakefield acceleration

A hybrid capillary discharge waveguide formed by injecting low-pressure hydrogen (<3.8 Torr) into a pure ablative capillary is presented to supply the stable guiding for multi-GeV laser wakefield acceleration. The injected low-pressure gas only provides the seed plasma for ablative discharge breakdown, like the adsorbed gas in the inner wall of the ablative capillary. With this hybrid capillary, a stable discharge with low jitter (∼5 ns) can be achieved in a simple way, and the plasma density inside the plasma channel can also be controlled in the range of ∼0.7×1018cm−3–1.2×1018cm−3 within a 150-ns temporal window. Furthermore, the hybrid capillary can also be easily extended to a longer length by adding multiple segments, and femtosecond laser pulses can be well guided in both the single and multiple segment modes. With these advantages, the hybrid capillary may provide an attractive plasma channel for multi-GeV-scale laser wakefield acceleration.

Optimization of gas-filled quartz capillary discharge waveguide for high-energy laser wakefield acceleration

A hydrogen-filled capillary discharge waveguide made of quartz is presented for high-energy laser wakefield acceleration (LWFA). The experimental parameters (discharge current and gas pressure) were optimized to mitigate ablation by a quantitative analysis of the ablation plasma density inside the hydrogen-filled quartz capillary. The ablation plasma density was obtained by combining a spectroscopic measurement method with a calibrated gas transducer. In order to obtain a controllable plasma density and mitigate the ablation as much as possible, the range of suitable parameters was investigated. The experimental results demonstrated that the ablation in the quartz capillary could be mitigated by increasing the gas pressure to ∼7.5–14.7 Torr and decreasing the discharge current to ∼70–100 A. These optimized parameters are promising for future high-energy LWFA experiments based on the quartz capillary discharge waveguide.

Guiding of charged particle beams in curved capillary-discharge waveguides

A new method able to transport charged particle beams along a curved path is presented. It is based on curved capillary-discharge waveguides where the induced azimuthal magnetic field is used to focus the beam and, at the same time, keep it close to the capillary axis. We show that such a solution is highly tunable, it allows to develop compact structures providing large deflecting angles and, unlike conventional solutions based on bending magnets, preserves the beam longitudinal phase space. Such a feature, in particular, is very promising when dealing with ultra-short bunches for which non-trivial manipulations on the longitudinal phase spaces are usually required when employing conventional deflecting devices.

Plasma equilibrium inside various cross-section capillary discharges

Plasma properties inside a hydrogen-filled capillary discharge waveguide were modeled with dissipative magnetohydrodynamic simulations to enable analysis of capillaries of circular and square cross-sections implying that square capillaries can be used to guide circularly symmetric laser beams. When the quasistationary stage of the discharge is reached, the plasma and temperature in the vicinity of the capillary axis have almost the same profile for both the circular and square capillaries. The effect of cross-section on the electron beam focusing properties was studied using the simulation-derived magnetic field map. Particle tracking simulations showed only slight effects on the electron beam symmetry in the horizontal and diagonal directions for square capillary.

Transient behavior of a supersonic three-dimensional micronozzle with an intersecting capillary

An analysis of the interaction between a pulsed, supersonic microjet and an intersecting gas-filled capillary is presented, which enables a direct measurement of the pressure evolution inside the nozzle of the microjet. Plasma-emission spectroscopy was used to resolve, on a sub-microsecond timescale, the build-up and decay of pressure in the nozzle, which are shown to be correlated to the volume of the plenum supplying the nozzle and to the nozzle-throat size, respectively. The microjet, which was integrated with a capillary-discharge waveguide in a sapphire structure, was used to create a small, tunable region of high density gas within a centimeter-scale plateau of lower-density for use in a laser-plasma accelerator. The resultant longitudinally structured gas-density profile has been used to provide control of electron trapping and acceleration, but its evolution has not previously been directly quantified. The results presented here pave the way for improved control of laser-plasma accelerators and are also relevant to applications such as miniature satellites and lab-on-a-chip where precise knowledge of microjet pressure evolution is critical.

Demonstration of a high repetition rate capillary discharge waveguide

A hydrogen-filled capillary discharge waveguide operating at kHz repetition rates is presented for parameters relevant to laser plasma acceleration (LPA). The discharge current pulse was optimized for erosion mitigation with laser guiding experiments and MHD simulation. Heat flow simulations and measurements showed modest temperature rise at the capillary wall due to the average heat load at kHz repetition rates with water-cooled capillaries, which is promising for applications of LPAs such as high average power radiation sources.

Dynamics and density distributions in a capillary-discharge waveguide with an embedded supersonic jet

We present an analysis of the gas dynamics and density distributions within a capillary-discharge waveguide with an embedded supersonic jet. This device provides a target for a laser plasma accelerator which uses longitudinal structuring of the gas-density profile to enable control of electron trapping and acceleration. The functionality of the device depends sensitively on the details of the density profile, which are determined by the interaction between the pulsed gas in the jet and the continuously-flowing gas in the capillary. These dynamics are captured by spatially resolving recombination light from several emission lines of the plasma as a function of the delay between the jet and the discharge. We provide a phenomenological description of the gas dynamics as well as a quantitative evaluation of the density evolution. In particular, we show that the pressure difference between the jet and the capillary defines three regimes of operation with qualitatively different longitudinal density profiles and show that jet timing provides a sensitive method for tuning between these regimes.

Generation and pointing stabilization of multi-GeV electron beams from a laser plasma accelerator driven in a pre-formed plasma waveguidea)

Laser pulses with peak power 0.3 PW were used to generate electron beams with energy >4 GeV within a 9 cm-long capillary discharge waveguide operated with a plasma density of ≈7×1017 cm−3. Simulations showed that the super-Gaussian near-field laser profile that is typical of high-power femtosecond laser systems reduces the efficacy of guiding in parabolic plasma channels compared with the Gaussian laser pulses that are typically simulated. In the experiments, this was mitigated by increasing the plasma density and hence the contribution of self-guiding. This allowed for the generation of multi-GeV electron beams, but these had angular fluctuation ≳2 mrad rms. Mitigation of capillary damage and more accurate alignment allowed for stable beams to be produced with energy 2.7±0.1 GeV. The pointing fluctuation was 0.6 mrad rms, which was less than the beam divergence of ≲1 mrad full-width-half-maximum.

Multi-GeV electron beams from capillary-discharge-guided subpetawatt laser pulses in the self-trapping regime.

Multi-GeV electron beams with energy up to 4.2 GeV, 6% rms energy spread, 6 pC charge, and 0.3 mrad rms divergence have been produced from a 9-cm-long capillary discharge waveguide with a plasma density of ≈7×10¹⁷ cm⁻³, powered by laser pulses with peak power up to 0.3 PW. Preformed plasma waveguides allow the use of lower laser power compared to unguided plasma structures to achieve the same electron beam energy. A detailed comparison between experiment and simulation indicates the sensitivity in this regime of the guiding and acceleration in the plasma structure to input intensity, density, and near-field laser mode profile.

The role of the gas/plasma plume and self-focusing in a gas-filled capillary discharge waveguide for high-power laser-plasma applications

The role of the gas/plasma plume at the entrance of a gas-filled capillary discharge plasma waveguide in increasing the laser intensity has been investigated. Distinction is made between neutral gas and hot plasma plumes that, respectively, develop before and after discharge breakdown. Time-averaged measurements show that the on-axis plasma density of a fully expanded plasma plume over this region is similar to that inside the waveguide. Above the critical power, relativistic and ponderomotive self-focusing lead to an increase in the intensity, which can be nearly a factor of 2 compared with the case without a plume. When used as a laser plasma wakefield accelerator, the enhancement of intensity can lead to prompt electron injection very close to the entrance of the waveguide. Self-focusing occurs within two Rayleigh lengths of the waveguide entrance plane in the region, where the laser beam is converging. Analytical theory and numerical simulations show that, for a density of 3.0 × 1018 cm−3, the peak normalized laser vector potential, a0, increases from 1.0 to 1.85 close to the entrance plane of the capillary compared with a0 = 1.41 when the plume is neglected.

Investigation of GeV-scale electron acceleration in a gas-filled capillary discharge waveguide

The generation of GeV-scale electron beams in a gas-filled capillary discharge waveguide with good reproducibility is discussed. Beams of electrons with energies above 900 MeV, and with root-mean-square divergences of 3.5 mrad, are observed for a plasma density of 2.2 × 1018 cm−3 and a peak input laser power of 55 TW. The variation of the maximum electron energy with the plasma density is measured and found to agree well with simple models. Injection and acceleration of electrons at the to date lowest plasma density of 3.2 × 1017 cm−3 are reported. The energy spectra of the generated electron beams exhibit good shot-to-shot reproducibility, with the observed variations attributable to the measured shot-to-shot jitter of the laser parameters. Two methods for correcting the effect of beam pointing variations on the measured energy spectrum are described.