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

Plasma-based technology promises a tremendous reduction in size of accelerators used for research, medical, and industrial applications, making it possible to develop tabletop machines accessible for a broader scientific community. By overcoming current limits of conventional accelerators and pushing particles to larger and larger energies, the availability of strong and tunable focusing optics is mandatory also because plasma-accelerated beams usually have large angular divergences. In this regard, active-plasma lenses represent a compact and affordable tool to generate radially symmetric magnetic fields several orders of magnitude larger than conventional quadrupoles and solenoids. However, it has been recently proved that the focusing can be highly nonlinear and induce a dramatic emittance growth. Here, we present experimental results showing how these nonlinearities can be minimized and lensing improved. These achievements represent a major breakthrough toward the miniaturization of next-generation focusing devices.

Figure 1

Experimental layout. Several screens allow us to measure the beam envelope along the path. The first two, located at $z=0$ (capillary entrance) and $z=23\mathrm{cm}$, are mounted on movable actuators. Three electromagnetic quadrupoles (at $z=0.8\mathrm{m}$, $z=1.1\mathrm{m}$, and $z=1.3\mathrm{m}$) are used to measure the beam emittance on the last screen, located at $z=5.2\mathrm{m}$.

Figure 2

(a) Discharge current waveform obtained by averaging 20 consecutive shots. The red cross indicates the effective current that produces the smallest spot size on the first screen downstream of the capillary. (b) Calculated radial profiles of the azimuthal magnetic field (blue) and temperature (red) across the capillary. The solid (dotted) lines are obtained when plasma is produced by a 230 A (100 A) discharge current.

Figure 3

Resulting emittance as a function of the beam spot size at capillary entrance. The black (red) data points refer to the experimentally measured $X(Y)$ emittances. The blue line reports the expected emittance obtained with numerical simulations. The gray line shows the unperturbed ($X$) beam emittance without APL.

Figure 4

Bunch spot measured on the first screen downstream of the capillary with the discharge turned off [(a) ${\sigma }_{x,y}=105\pm 4\mu \mathrm{m}$] and on [(b) ${\sigma }_{x,y}=17.5\pm 0.3\mu \mathrm{m}$]. (c) Experimental spot sizes (circles) obtained on the same screen for different discharge currents. The envelopes computed by simulations (solid lines) are also reported. The dashed line refers to the beam envelope obtained with the discharge turned off.

Figure 5

Measurement of the normalized emittance with quadrupole-scan technique of the bunch exiting from the APL. The horizontal axis reports the quadrupole currents while the vertical one shows the horizontal (a) and vertical (b) spot sizes. The red line shows the computed fit used to evaluate the emittance value. The dotted green line reproduces the quadrupole scan obtained with GPT simulation.

Figure 6

(a) Simulation of the perturbed plasma density (top) profile at the entrance of the plasma and computed radial ($W_r$, center) and longitudinal ($W_z$, bottom) wakefields. (b) Bunch envelope (blue) and normalized emittance (red) evolution along the plasma. The red dashed line shows the simulated plasma density profile (3 cm–long capillary with 1 cm input and exit ramps).