New Relativistic Electron Beam Diagnostic Developed

The Accelerator Physics Group at the Stanford Free-Electron Laser Center (located on campus in the Hansen Experimental Physics Laboratory) has successfully demonstrated a new technique for making direct measurements of the temporal structure of relativistic electron beams. As shown in the accompanying figure, the new technique has already been used to observe free-electron laser generated microbunching of an electron beam, thus demonstrating a temporal resolution of about 100 femtoseconds (1 femtosecond = 10^-15 seconds). Further development is expected to improve the resolution to the 10 femtosecond level. Even at the current state of development the resolution is roughly an order of magnitude faster than that of any other existing method.

Time resolved image of a relativisitic electron beam showing the microbunching generated by a free-electron laser operating at a wavelength of 60 micrometers. The scale factor for the time axis, which is horizontal, can be determined by the fact that the width of a light/dark pair of the vertical stripes corresponds to 200 femtoseconds (one optical cycle).

The new method works by adding a subtle modification to the well know procedure of using microwave cavities to modulate the energy of a pulsed charged particle beam in such a way that a correlation is established between the energy of a particle and its location within the pulse. Then an ordinary energy spectrometer is used to obtain temporal information. Temporal resolution in the standard implementation is limited by two factors: 1) there is always some intrinsic spread in energy of the particles forming the beam and unless the energy difference imparted by the cavity is greater than this spread there is no unique correlation between energy and time; 2) there is a practical limit to the rate of change of voltage in the cavity and thus a limit to the difference in voltage which can be imparted to two particles separated by some given time interval. The modification implemented by physics graduate student Ken Ricci consists of having the electron beam pass through a temporally dispersive system of magnets just before entering the energy modulation cavity. The magnet system has the property that a high energy electron takes less time to pass through it than a low energy electron, even though they are both travelling at essentially the speed of light. Adjusting the current used to excite the magnets can control the time difference between the two. This property of temporal dispersion, when properly adjusted and coupled with the energy modulation of the microwave cavity, leads to the remarkable result that the output energy difference between any two electrons is a function only (to first order) of the temporal separation between them when they entered the magnets. One of the limitations mentioned in the previous paragraph has been eliminated! The Accelerator Physics/ FEL Physics group is actively preparing to exploit the unprecedented time resolution provided by the new technique. Many of the frontier accelerator projects around the world, such as laser accelerators, plasma accelerators and fourth generation light sources, rely on understanding and manipulating ultra-short bunches of charge. While there is no shortage of theories and computer simulations providing guidance for these projects, direct experimental confirmation of many predictions has been impossible. As an example, it has been assumed by virtually everyone in the field that the operation of a free-electron laser had to be accompanied by density modulation of the driving electron beam at the same wavelength as the optical output of the laser. However, because the density modulation occurs on such a short time scale, its direct observation was not possible before Ricci's development of the new system. It's now possible to begin studying some of the predictions in detail.

By Todd Smith

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