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A picosencond time measurement method is described based on the microwave properties of beams from electron linear accelerators or from possible remodulated external proton beams at proton synchrotrons. A conventional RF separator (RFS) is used operating at the same frequency as the microwave structure of beams and driven by the same master oscillator which modulates the beam. For example, the 2856 MHz microwave at SLAC corresponds to 350 psec between beam RF buckets; accelerated electrons with an RF phase bite of about 5 degrees are time structured and correspondingly have about 5 psec time spread in every 350 psec interval. Charged particles produced in interactions or decays originate with this time structure. An RFS is placed at the last stage of a double focussing spectrometer, providing a lever arm between it and the final (few mm) small spot size focus. Reference calibration charged particles are timed by setting the RFS relative phase to null deflections. Other observed particles arriving at different times than this standardare deflected a few tenths ofa anm per psec of delay. A multi-wire proportional chamber (IMPC) having a 0.05 mm space resolution can provide time measurements with sub-picosecond resolution. The technique is used in an experiment to search for changes in the velocity of light with high energy photons at SLAC. The devised timing method is presented and the properties of a new, fine space resolution MWPC is demonstrated.

Over the past several years the Stanford superconducting linac has been evolving from an experimental accelerator towards an operational facility capable of providing a high duty cycle electron beam for users. The linac has recently been fitted with bending magnets and other transport elements to allow its beam to be accelerated several times by the same linac sections (recirculated1). At present, with one orbit of recirculation, an 84 MeV beam has been extracted. By the end of this year we expect to have a 170 MeV beam and by the end of 1978, with the completion of four orbits, the beam energy will be 280 MeV. The beam duty cycle will be between 20% and 100%, and the maximum beam current will be between 20 ?A and 100 ?A.