Experimental investigation of interactions of long CO$_2$ laser pulse with plasma at ATF

Laser wakefield accelerators (LWFAs) can sustain accelerating gradients that greatly surpass those of conventional accelerators. Long ($\sim$ps) and intense ($>$TW) laser pulses have been employed in LWFAs to generate bright, hard X-rays which are of interest for imaging and diagnosing warm-dense matter. The CO$_2$ laser at the ATF facility of the Brookhaven National Laboratory is a unique source, which can generate $~$2 ps-long, multi-TW laser pulses in the mid-IR (9.2 $\mu$m) regime. The properties of the laser-plasma interactions were characterized by imaging the plasma wakefields with the linac-produced short (150-250 fs) relativistic electron beam at ATF. The evolution of a self-modulated laser wake in an underdense plasma has been directly observed and analyzed. Experimental results as well as simulations exploring the properties of this regime will be presented.