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

Abstract

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 that show 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 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 percent 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.

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

“…At n e = 2 × 10 17 electrons/cm 3 the peak accelerating field (estimated based on the cold plasma wave-breaking limit) is 43 GV/m. A 1% variation in n e (0) translates to changes of ∼0.22 GV/m or ∼0.5%, much smaller than the variations that are typically observed in experiments [42,43] .…”

“…Matched spot size variations were largest ( w m =0.2%) for the longest capillary (40 cm). We project that these quantities are not sufficient to cause the large electron beam parameter variations that are typically observed in laser-driven plasma wakefield acceleration experiments to date [42,43] . Improvements of the quality and reproducibility of particle bunches accelerated in laser-driven plasma wakefields therefore require improvements of the laser pulse parameter variations [44] .…”

Guiding of high-intensity laser pulses in 100-mm-long hydrodynamic optical-field-ionized plasma channels

“…Further, employing methods to increase the channel depth [47] is expected to increase the attenuation length above 1 m, leading to negligible propagation losses. Channels of this type would have an energy transmission which would equal or exceed that demonstrated by gas-filled capillary discharge waveguides [48], which, for example, have demonstrated a transmission above 80% for low-intensity pulses guided in 90 mm long channels.…”

Meter-scale conditioned hydrodynamic optical-field-ionized plasma channels

“…A power attenuation length of L att ≈ 0.15 m has been reported for capillary discharge waveguides [18] and that for the lowest-order mode of hollow capillary waveguides [11,40] is L att ≈ 0.9 m for a 50-μmdiameter capillary. Gonsalves et al [23] have reported an energy transmission of 85% for low-intensity pulses guided in 90-mm-long laser-assisted capillary discharges, corresponding to L att ≈ 0.6 m, assuming perfect coupling.…”

“…Many methods for guiding high-intensity laser pulses have been investigated. These include grazing-incidence guiding in capillaries [11] and many varieties of plasma channels generated by hydrodynamic expansion [12][13][14][15], capillary discharges [16][17][18], Z pinches [19,20], open-geometry discharges [21], and laser-heated capillary discharges [22,23]. To date, the most successful approaches for driving laser-plasma accelerators are capillary discharges and its laser-heated variant, which have been used to generate electron beams with energies up to 4.2 and 7.8 GeV respectively [24,25].…”