The development of high-frequency oscillators and power amplifiers from the 1940s, especially the klystron, was essential for these two acceleration techniques . The first larger linear accelerator with standing waves - for protons - was built in 1945/46 in the Lawrence Berkeley National Laboratory under the direction of Luis W. Alvarez. The frequency used was 200 MHz.  The first electron accelerator with traveling waves of around 2 GHz (gigahertz) was developed a little later at Stanford University by W.W. Hansen and colleagues.