Low-density hydrodynamic optical-field-ionized plasma channels generated with an axicon lens

Abstract

We demonstrate optical guiding of high-intensity laser pulses in long, low density hydrodynamic optical-field-ionized (HOFI) plasma channels. An axicon lens is used to generate HOFI plasma channels with on-axis electron densities as low as ne(0) = 1.5 × 10 17 cm −3 and matched spot sizes in the range 20 µm WM 40 µm. Control of these channel parameters via adjustment of the initial cell pressure and the delay after the arrival of the channel-forming pulse is demonstrated. For laser pulses with a peak axial intensity of 4 × 10 17 W cm −2 , highly reproducible, high-quality guiding over more than 14 Rayleigh ranges is achieved at a pulse repetition rate of 5 Hz, limited by the available channel-forming laser and vacuum pumping system. Plasma channels of this type would seem to be well suited to multi-GeV laser wakefield accelerators operating in the quasi-linear regime.Many applications of high-intensity laser-plasma interactions require the propagation of high-intensity laser pulses through plasmas which are orders of magnitude longer than the Rayleigh range. One example of particular current interest is the laser wakefield accelerator (LWFA) [1], in which a laser pulse with an intensity of order 10 18 W cm −2 propagates though a plasma, driving a trailing density wave. The electric fields generated within this plasma wave are of the order of the wave-breaking field E 0 = m e ω p c/e, where ω p = (n e e 2 /m e ǫ 0 ) 1/2 and n e is the electron density [2,3]. For plasma densities in the range n e = 10 17 −10 18 cm −3 , E 0 ≈ 30 − 100 GV m −1 , which is several orders of magnitude higher than the fields generated in radio-frequency machines.Plasma accelerators can drive compact sources of femtosecond-duration [4-6] radiation via betatron emission [7,8], undulator radiation [9, 10], and Thomson scattering [11][12][13], with many potential applications in ultrafast science. In the longer term they could provide a building block for future high-energy particle colliders [14].For LWFAs operating in the quasilinear regime [2], the energy gain per stage varies as W ∝ E 0 L acc ∝ 1/n e , and the required length of the stage varies as L acc ∝ 1/n 3/2 e . Hence reaching higher energy gains requires the drive laser to propagate over longer lengths of lower density plasma.To date, the highest reported electron energy generated in a LWFA is 7.8 GeV, which was achieved by guiding intense laser pulses through a 200-mm-long plasma channel with an axial electron density of 2.7 × 10 17 cm 3 [15].Laser-plasma accelerators providing 10 GeV energy gain per stage will require laser guiding through 100s of millimetres of plasma of electron density n e ≈ 10 17 cm −3 [16,17]. Further, for many of the applications identified above it will be necessary to operate at pulse repetition rates, f rep , several orders of magnitude above the few hertz typical of today's GeV-scale LWFAs.To date, the workhorse waveguide for LWFAs has been the capillary discharge waveguide [18,19]. Capillary discharge waveguides have generated plasma channels up to 150...

Positron transport and acceleration in beam-driven plasma wakefield accelerators using plasma columns

“…The generation of plasma columns with a radial dimension of a few tens of microns and an on-axis density of ∼10 17 cm −3 (parameters near those considered in this work) have also been demonstrated, for the purpose of laser guiding over cm-scale plasmas [25,26]. The generation of meter-scale plasma columns, with a few tens of microns radius and an on-axis density of ∼10 17 cm −3 , using an axicon lens is possible with current laser technology, requiring on the order of a few mJ of laser energy per cm of plasma [26]. On the other hand, creating the plasma column by means of beam-field-induced ionization from the drive beam, instead of relying on an ionizing laser, has the advantage that the column is inherently aligned with the drive beam itself; however, since the ionization rate is strongly coupled to the drive beam parameters and to the gas density, the viable parameter space is limited.…”

“…Production of meter-scale plasma columns by laser-induced ionization using an axicon lens [23] has been experimentally demonstrated [24]. The generation of plasma columns with a radial dimension of a few tens of microns and an on-axis density of ∼10 17 cm −3 (parameters near those considered in this work) have also been demonstrated, for the purpose of laser guiding over cm-scale plasmas [25,26]. The generation of meter-scale plasma columns, with a few tens of microns radius and an on-axis density of ∼10 17 cm −3 , using an axicon lens is possible with current laser technology, requiring on the order of a few mJ of laser energy per cm of plasma [26].…”

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

“…The channel-forming beam was passed through a pulse energy control system, its wavefront flattened by reflection from a deformable mirror (DM1), and its polarization converted to circular [31,36]. It was then sent to a retroreflecting timing stage, reflected by a second holed mirror (HM2), and focused into the target gas cell by a fused silica axicon lens of base angle ϑ ¼ 5.6°.…”

“…Previously we demonstrated [36] the formation of, and guiding in, 16 mm long HOFI channels produced by an axicon lens, with an on-axis density as low as n e ð0Þ ¼ 1.5 × 10 17 cm −3 . Control of the channel parameters by variation of the initial gas pressure, and the delay after the arrival of the channel-forming pulse, was demonstrated.…”

Compact Multistage Plasma-Based Accelerator Design for Correlated Energy Spread Compensation

“…The two identical LWFAs have a length L p = 8 cm and a parabolic transverse density profile for laser pulse guiding n p = n p,0 + r 2 /πr e w 4 0 , where n p,0 = 10 17 cm −3 is the on-axis plasma density, r the radial coordinate, r e the classical electron radius and w 0 the spot size of the laser driver. Plasma cells in this range of parameters have been recently demonstrated [67,68]. For simplicity, a longitudinal flat-top plasma density profile has been considered, although the presence of smooth plasma-to-vacuum transitions would be beneficial for electron beam matching [69] and emittance growth minimization [70].…”