| dc.description.abstract | Compare to traditional linear electron accelerator, Laser Wakefield Accelera-
tor (LWFA) have higher acceleration gradient which is about hundreds of MeV
per millimeter. If we can overcome the challenge of high stability, low energy
spread, and low emittance, it is a good electron source to construct laboratory-
scale Free Electron laser (FEL).
The wakefield is generated by the ionization of the gas by a high intensity
laser. The ionized electrons are expelled by the ponderomotive force and then
attracted by the ions. Thus, there will form an ion cavity which is called a bub-
ble behind the laser. The bubble can trap electrons and accelerate them, so the
way to make the electrons move into the bubble is called injection method. We
conducted the experiment of LWFA by three different injection methods: ioniza-
tion injection, self-injection, and shock-front injection with 100 TW laser system
in NCU. The ionization injection is a simple way to generate electron beam by
a gas target with high atomic number. The electrons at inner shell are only ion-
ized at the center of the laser beam, so these electrons are easy to be injected.
This property can help us to find the preliminary shooting parameters, such like
laser focus position and group delay dispersion (GDD).
Self-injection occurs when the laser intensity is high enough, the plasma wave
may become broken. Thus, the electrons may be injected by itself throughout the
plasma channel. The shock-front injection happens when the laser pulse passes
through a shock front with high density gradient in micrometer scale. The sud-
denly dropped density makes the bubble length increase, so the electrons at the
back of the bubble may be injected. If the self-injection doesn’t occur, the in-
jection volume is only near the shock front. As a result, the energy spread is
the smallest among these three injection meachanisms although the aceleration
length is reduced by the shock front position.
In our results, the monoenergetic electron beams with the peak energy of
125 ± 12.2 MeV, FWHM energy spread are only 7.5 ± 1.68%, charge of 5.2 ±
2.2 pC, FWHM divergence of 2.5 ± 0.8 mrad, are generated by the shock-front
injection. However, the probability of these monoenergetic shots is only 15 %.
The stability of the electron means should be further improved by improving
the gas jet design and the laser quality. | en_US |