Production of Far-infrared Coherent Synchrotron Radiation on Electron Storage Ring

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Normally, synchrotron radiation intensity is proportional to number of electrons which contribute to the radiation. However, when the electrons are bunched into a short pulse whose length is shorter than the radiation wave length, the radiation from individual electrons is emitted coherently and the radiation intensity is proportional to the square of the number of the electrons. This phenomenon is called coherent synchrotron radiation(CSR). Since the typical number of the electrons in a storage ring is 10^<10>, the intensity of CSR can be larger than the normal synchrotron radiation by many orders of magnitudes.We have been trying to produce CSR on the electron beam circulating in a storage ring. Our approach is to create a density modulation whose typical length is shorter than the radiation wavelength, in stead of shortening the electron pulse length. As a result, we have detected very intense bursts of terahertz radiation, whose peak intensity is larger than normal radiation by 4 orders of magnitudes. These bursts were observed only in single bunch filling mode with the beam current higher than a certain threshold current. We suppose that a density modulation whose typical scale is in millimeter range, is created on an electron beam as a result of beam instability. This modulation forces the electron beam to emit such intense bursts. The typical duration of these bursts are 100 micro-seconds. The interval of the bursts is typically 10 milliseconds and varied on the beam current. This phenomenon is essentially unstable and chaotic. To utilize this intense radiation, we must control the occurrence of the bursts. We are trying to produce density modulation on an electron beam by utilizing the interaction between the electrons and laser pulses. We have already succeeded in producing intense bursts whose occurrence is controlled by an external trigger.