Application of low flow rate micro gas cell nozzle in laser wakefield acceleration*

After forty-year 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 influence of the gas load on the vacuum system cannot be neglected. Among the widely used gas targets, gas cells have a lower flow rate than supersonic gas nozzles. However, most of gas cells are several centimeters long, unsuitable for a moderate-size laser facility. In this work, we design a kind of micro gas cell with a sub-centimeter length. The flow rate of the micro gas cell and the supersonic nozzle are compared by hydromechanics simulations. Comparing with the supersonic nozzle, the flow rate of the micro gas cell is reduced by 97%. Moreover, the gas cell sustains a longer flattop region. The reduced flow rate is attributed to two reasons. The first reason is that the area of the nozzle exit decreases significantly. In the case of the supersonic nozzle, the laser interacts with the gas jet outside the nozzle exit. Therefore, the exit size is determined by the interaction length. In the case of the micro gas cell, the laser interacts 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. The second reason is that the velocity of the gas jet decreases. When using a supersonic nozzle, the velocity at the nozzle exit must be high enough to generate a flattop density distribution, which is required by LWFA. As a comparison, in the micro gas cell, the gas is confined by the cell wall. As a consequence, the gas velocity has 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 are conducted on a 45 TW femtosecond laser facility at the Laser Fusion Research Center. The stable electron beams with maximum energy of 250 MeV are generated. This study will contribute to the investigation of stable and high-frequency laser wakefield acceleration.