Optimization of Electron Bunch Characteristics in Bubble Wakefield: Role of Different Linear Upward Density Ramp Profiles

Laser wakefield acceleration (LWFA) technique is used for the generation of electron beam of high quality. In future, these beams may be used to develop compact X-ray sources. In the present work, using a linearly polarized Gaussian laser pulse for electron acceleration, we have explored different linear upward density ramp structures for achieving better characteristics of the injected electron bunch employing Fourier–Bessel particle-in-cell simulations (FBPIC) code. Laser parameters and plasma density ramp profile are optimized for maximizing energy gain (E), minimizing injected electron bunch charge
${(\text {Q})}$
, and normalizing the rms emittance within the wake bubble. The optimized values of emittances are observed as
$\epsilon _{x}=3.69 \times 10^{-5} \text {mrad}$
and
$\epsilon _{y}=0.83 \times 10^{-5} \text {mrad}$
in 2.23-mm-long plasma channel for ramp lengths of
$\text {Z}_{\text {ramp}}=100\,\,\mu \text {m}$
and
$\text {Z}_{\text {ramp}}=20\,\,\mu \text {m}$
, respectively. Simulation results show weaker particle trapping in bubble for increasing the values of density ramp length, revealing that the injected electron bunch charge is strongly related to the plasma density ramp length. A minimum charge of ~277.26 pC trapped in the bubble and a maximum energy gain of 0.21 GeV are obtained for the plasma channel of 2.23 mm length.