A collinear wakefield test facility using two linacs with a common tes
t section is constructed. Beams from one linac excite wakefields in a
test medium such as a plasma, while beams from the other linac witness
the wakefields. The time interval between the two beams is controllab
le with an accuracy of similar to 1 psec. Plasma wakefield acceleratio
n experiments were conducted using this facility.