A Low Flow Rate Micro Gas Cell for Laser Wakefield Acceleration

After forty years of tremendous advances, Laser wakefield acceleration (LWFA), in which an ultra-intense femtosecond laser interacts with a gas target to produce energetic electrons, is becoming more and more mature. Acceleration with a high repetition rate will be an important topic in the near future. When operating at a high repetition rate, the impact of the gas load on the vacuum system cannot be neglected. Among the widely used gas targets, gas cells have a lower flow rate compared to supersonic gas nozzles. However, most gas cells were several centimeters long, not suitable for a moderate-size laser facility. In this paper, we designed a kind of micro gas cell with a sub-centimeter length. The flow rate of the micro gas cell and the supersonic nozzle were compared by hydromechanics simulations. Compared to the supersonic nozzle, the flow rate of the micro gas cell was reduced by 97 percent. Moreover, the gas cell sustained a longer flattop region. The decreased flow rate is attributed to 2 reasons. 1, the area of the nozzle exit decreased significantly. In the case of the supersonic nozzle, the laser interacted with the gas jet outside the nozzle exit. Therefore, the exit size was determined by the interaction length. In the case of the micro gas cell, the laser interacted with the gas inside the gas cell. The exit only needs to be larger than the laser focal, which is much smaller than the interaction length. 2, the velocity of the gas jet was decreased. When using a supersonic nozzle, the velocity at the nozzle exit had to be high enough to generate a flattop density distribution, which is required by LWFA. As a comparison, in the micro gas cell, the gas was confined by the cell wall. As a consequence, the gas velocity had little influence on the density distribution inside the cell. By changing the inner radius of the cell, (1-4) mm long flattop regions can be generated while keeping a low flow rate. Experiments using the micro gas cell were conducted on a 45TW femtosecond laser facility at Laser Fusion Research Center. Stable electron beams with a maximum energy of 250MeV were generated. This study will contribute to the stable and high-frequency laser wakefield acceleration.