We summarize the progress achieved at LSAI in the understanding and optimization of the transient collisional pumping of x-ray lasers using an ultra-short picosecond heating pulse. A strong ×300-400 enhancement of the 13.9 nm Ni-like 4d-4p lasing emission is obtained when a travelling-wave short pulse irradiates the preformed plasma. The temporal history of the 13.9 nm laser pulse is measured with a high-resolution streak camera. A very short, 2 ps x-ray laser pulse is directly demonstrated for the first time.