Abstract
Electron beams from laser-plasma wakefield accelerators have low transverse emittance, comparable to those from conventional radio frequency accelerators, which highlights their potential for applications, many of which will require the use of quadrupole magnets for optimal electron beam transport. We report on characterizing electron bunches where double bunches are observed under certain conditions. In particular, we present pepper-pot measurements of the transverse emittance of 120–200 MeV laser wakefield electron bunches after propagation through a triplet of permanent quadrupole magnets. It is shown that the normalized emittance at source can be as low as 1 π mm mrad (resolution limited), growing by about five times after propagation through the quadrupoles due to beam energy spread. The inherent energy-dependence of the magnets also enables detection of double electron bunches that could otherwise remain unresolved, providing insight into the self-injection of multiple bunches. The combination of quadrupoles and pepper-pot, in addition, acts as a diagnostic for the alignment of the magnetic triplet.
Highlights
Laser-wakefield accelerators (LWFAs) are table-top devices capable of delivering high quality electron beams with energies up to GeV levels by exploiting the large electric field gradients created when intense laser pulses interact with plasma [1]
The small size of the accelerator and the promising properties of electron beams make LWFAs attractive tools in many fields, including as potential drivers of a new generation of compact synchrotron-like [2,3,4] and freeelectron laser (FEL) [5,6,7] light sources. The requirement of such applications to transport beams over long distances with minimum degradation has prompted the development of beam lines and diagnostic systems tailored for laser-produced beams, which so far suffer from larger instabilities than conventional radio frequency (RF) accelerators
Experiments for the characterization of laser-produced electron beams using a permanent magnet quadrupole (PMQ) triplet coupled to a pepper-pot mask have been performed at the Advanced Laser-Plasma High-energy Accelerators towards X-rays (ALPHA-X) beam line [3] with the setup shown in figure 5
Summary
Laser-wakefield accelerators (LWFAs) are table-top devices capable of delivering high quality electron beams with energies up to GeV levels by exploiting the large electric field gradients created when intense laser pulses interact with plasma [1]. The small size of the accelerator and the promising properties of electron beams make LWFAs attractive tools in many fields, including as potential drivers of a new generation of compact synchrotron-like [2,3,4] and freeelectron laser (FEL) [5,6,7] light sources The requirement of such applications to transport beams over long distances with minimum degradation has prompted the development of beam lines and diagnostic systems tailored for laser-produced beams, which so far suffer from larger instabilities than conventional radio frequency (RF) accelerators. To avoid electron beam blowup over long drift propagation, miniature permanent magnet quadrupole (PMQ) lenses are typically installed close to the accelerator [6, 7] These have very high magnetic field gradients (∼ 500 T m−1) for collimation and focusing of high energy beams over short distances. Fine tuning and beam transport capabilities of the PMQs have been studied in detail [8, 9], leading to further control of the electron beam divergence and pointing stability [10]
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