Abstract
Diagnosing plasma conditions can give great advantages in optimizing plasma wakefield accelerator experiments. One possible method is that of photon acceleration. By propagating a laser probe pulse through a plasma wakefield and extracting the imposed frequency modulation, one can obtain an image of the density modulation of the wakefield. In order to diagnose the wakefield parameters at a chosen point in the plasma, the probe pulse crosses the plasma at oblique angles relative to the wakefield. In this paper, mathematical expressions relating the frequency modulation of the laser pulse and the wakefield density profile of the plasma for oblique crossing angles are derived. Multidimensional particle-in-cell simulation results presented in this paper confirm that the frequency modulation profiles and the density modulation profiles agree to within 10%. Limitations to the accuracy of the measurement are discussed in this paper. This technique opens new possibilities to quantitatively diagnose the plasma wakefield density at known positions within the plasma column.
Highlights
When a driver is fired into an underdense plasma, it will generate a large amplitude longitudinal wave in the electron density profile, widely known as a plasma wakefield [1,2,3]
By propagating a laser probe pulse through a plasma wakefield and extracting the imposed frequency modulation, one can obtain an image of the density modulation of the wakefield
Multidimensional particle-in-cell simulation results presented in this paper confirm that the frequency modulation profiles and the density modulation profiles agree to within 10%
Summary
When a driver is fired into an underdense plasma, it will generate a large amplitude longitudinal wave in the electron density profile, widely known as a plasma wakefield [1,2,3]. In order to avoid the averaging effect, a new study is presented here where the probe pulse is allowed to propagate with an oblique angle relative to the wakefield This oblique crossing angle makes it possible to obtain the density modulation profile of the wakefield at certain positions and diagnose the evolution of the wakefield along the propagation distance, thereby overcoming one of the limitations of the previous methods. This technique is complementary to the frequency domain streak camera technique [25] by providing the quantitative information of the electron density modulation in the wakefield.
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