A large volume expansion chamber (25‐cm diameter, 25 cm deep) was constructed to investigate prebreakdown ionization processes in gaseous discharge. A homogeneous field electrode system was mounted within the chamber and provision was made for irradiation of the cathode. The chamber could be operated automatically at pressures from 40 to 68 cm Hg with a recycling time of about 5 min. Associated circuits for the production of millimicrosecond pulses up to 70 kvp are described. Some preliminary photographs of point‐plane breakdown and of electron avalanches in a homogeneous field electrode system are shown.