LNF
Users Site

Plasma lenses

The active-plasma lens consists of a neutral gas confined in a narrow structure like a capillary. The gas is then ionized into a plasma by a current-discharge flowing through the capillary itself. According to the Ampère law, the discharge induces an azimuthal magnetic field whose strength is directly proportional to the flowing current. As a reference, a 100 A current flowing into a 100 μm hole diameter capillary is able to generate magnetic gradients as large as 1 kT/m, i.e. approximately one order of magnitude larger than the strongest available quadrupoles. Such a magnetic field can then be used to focus particle beams and replace conventional devices (like solenoids and quadrupoles) with more compact structures.

So far, permanent-magnet quadrupoles represent the state of the art of strong focusing providing gradients of the order of 600 T/m [e.g. in L. K. Lim et al. and R. Pompili et al.]. Nevertheless, their focusing is nonsymmetric and the gradient fixed thus nontrivial movable systems (consisting of at least three lenses) must be implemented to produce round beams with (slightly) adjustable focal length.
Plasma-based lenses [P. Chen et al., J. B. Rosenzweig et al., H. Nakanishi et al.] are been used for radially symmetric focusing with even larger magnetic gradient (of the order of kT/m) for both electrons [J. J. Su et al., G. Hairapetian et al., C. Thaury et al.] and ions [E. Boggasch et al., E. Boggasch et al., A. Tauschwitz et al.]. Several results have been also obtained with the so-called “active” plasma lens [W. K. H. Panofsky and W. R. Baker], showing the focusing of relativistic electron beams both from laser-plasma [J. van Tilborg et al., J. van Tilborg et al.] and RF [R. Pompili et al., A. Marocchino et al., E. Chiadroni et al., C. A. Lindstrøm et al.] accelerators.

Schematic setup of plasma lens at SPARC_LAB. Movable screens can be inserted in different positions to measure the beam dimension. A triplet of permanent quadrupoles can also be inserted to recollimate the beam after the capillary.

The SPARC_LAB team has developed an active-plasma lens consisting of a 3 cm-long capillary with 1 mm hole diameter. The plasma is produced by ionizing hydrogen gas with a 20 kV discharge. The experimental setup has been installed at the end of the SPARC linac and the first tests started in 2016. At that time the main finding was the observation of a strong degradation of the beam emittance [R. Pompili et al., A. Marocchino et al.]. Such an effect limited the overall beam focusing to minimum spot sizes of 24 μm. Further tests and theoretical investigations determined that such a behavior was mainly due to non-uniformities on the plasma when driven by low currents (~100 A). With the aim to overcome such a limit, the peak current has been increased up to 230 A by developing a new discharge pulser. With the new setup, the data acquired at the end of 2017 proved that a better focusing was achieved (17 μm spot sizes) and emittance preserved downstream the plasma lens [R. Pompili et al.].

These results represent a fundamental step toward the development of next-generation focusing optics and demonstrate their effective usability in view of new compact facilities.

 

Publication Highlights:

  • “Experimental characterization of active plasma lensing for electron beams “, R. Pompili, et al., Applied Physics Letters vol. 110 (2017) pag. 104101, doi: 10.1063/1.4977894
  • “Experimental characterization of the effects induced by passive plasma lens on high brightness electron bunches”, A. Marocchino et al., Applied Physics Letters vol. 111 (2017) pag. 184101, doi: 10.1063/1.4999010
  • “Focusing of High-Brightness Electron Beams with Active-Plasma Lenses”, R. Pompili et al., Phys. Rev. Lett. vol. 121 (2018) pag. 174801, doi: 10.1103/PhysRevLett.121.174801

 

Plasma experiments
Last update: 02/2019