Comparative study of active plasma lenses in high-quality electron accelerator transport lines

Abstract

Electrically discharged active plasma lenses (APLs) are actively pursued in compact highbrightness plasma-based accelerators due to their high-gradient, tunable, and radially symmetric focusing properties. In this manuscript, the APL is experimentally compared with a conventional quadrupole triplet, highlighting the favorable reduction in the energy dependence (chromaticity) in the transport line. Through transport simulations, it is explored how the non-uniform radial discharge current distribution leads to beam-integrated emittance degradation and a charge density reduction at focus. However, positioning an aperture at the APL entrance will significantly reduce emittance degradation without additional loss of charge in the high-quality core of the beam. An analytical model is presented that estimates the emittance degradation from a short beam driving a longitudinally varying wakefield in the APL. Optimizing laser plasma accelerator operation is discussed where emittance degradation from the non-uniform discharge current (favoring small beams inside the APL) and wakefield effects (favoring larger beam sizes) is minimized.

Emittance Preservation in an Aberration-Free Active Plasma Lens

“…Although APLs provide kT/m focusing fields, orders of magnitude stronger focusing compared to conventional quadrupole magnets, they can suffer from aberrations that increase the emittance of the beam being focused [13,14]. One such aberration is caused by plasma temperature gradients in the capillary (colder plasma closer to the wall), which leads to a radially nonlinear magnetic field distribution [15,16] with enhanced focusing closer to the axis.…”

Density characterization of discharged gas-filled capillaries through common-path two-color spectral-domain interferometry

“…More recently, discharged plasma structures have been applied to focus relativistic electron beams, exploiting the advantages of radial symmetry, tunability, and strong multi kilo-Tesla/meter focusing gradients [11]. Knowledge of the on-axis plasma density is of critical importance to accelerator applications since the density dominates the plasma response time, self-injection threshold, accelerating field strength, electron beam dephasing length [6], and beam-driven wakefield effects [12].…”

Focusing of High-Brightness Electron Beams with Active-Plasma Lenses

“…Radially symmetric focusing with even larger magnetic gradient (of the order of kT=m) has been demonstrated for both electron [18][19][20][21] and ion [22][23][24] beams by means of plasma-based lenses [25][26][27]. Several results have been also obtained with the so-called "active" plasma lens (APL) [28], showing the focusing of relativistic electron beams both from laser-plasma [29,30] and rf [31][32][33][34] accelerators. These devices consist of a discharge current (flowing through a capillary) that generates an azimuthally magnetic field with radially increasing strength [35,36].…”

“…The nonlinearities of the overall focusing can be minimized by manipulating both the bunch shape and the capillary-discharge setup. The strength of the radial plasma wakefield is governed both by n b and n p [53] and its effect can be reduced by decreasing both, i.e., by operating with low plasma densities or by entering into the plasma with a large transverse spot (corresponding to a lower n b [30]). On the contrary, the linearity of the APL field is guaranteed only at small radii since the magnetic field toward the capillary walls bends down, as shown in Fig.…”