  An investigation of the multi-hundred MeV electron beam yield (charge) form
helium, nitrogen, neon and argon gas jet plasmas in a laser-plasma wakefield
acceleration experiment was carried out. The charge measurement has been made
via imaging the electron beam intensity profile on a fluorescent screen into a
14-bit charge coupled device (CCD) which was cross-calibrated with
nondestructive electronics-based method. Within given laser and plasma
parameters, we found that laser-driven low Z- gas jet targets generate
high-quality and well-collimated electron beams with reasonable yields at the
level of 10-100 pC. On the other hand, filamentary electron beams which were
observed from high-Z gas jets at higher densities reached much higher yield.
Evidences for cluster formation were clearly observed in high-Z gases,
especially in the argon gas jet target where we received the highest yield of ~
3 nC
