Principle of Electron Microbunch Formation

The source of electrons in a linear accelerator (linac) is typically either from a thermionic cathode, where the electrons “boil” off a hot surface, or a laser-driven photocathode, where laser light causes the electrons to emit off a non-heated surface.  For a number of reasons, laser-driven photocathodes are preferred in laser-driven electron acceleration (laser acceleration) and this is the type of electron emitter used by the BNL Accelerator Test Facility (ATF) linac.

In the ATF linac, the electrons are emitted from the photocathode with a typical pulse length of approximately 3 ps (1s).  These electrons are then accelerated using conventional microwave-driven cavities to an energy of 45 MeV for the STELLA experiment. [1]   In the experiment, these electrons interact with the ATF CO₂ laser light, which has a pulse length of »180 ps, inside the inverse free electron laser (IFEL) undulators.  Since the 3-ps electron beam (e-beam) pulse length is much shorter than the laser pulse length, the electrons experience essentially a uniform electric field from the laser light.

The electric field in the laser pulse consists of a train of oscillating waves with a period equal to the laser wavelength, i.e., 10.6 mm.  The 3-ps e-beam pulse corresponds to 900 mm in length.  Hence, there are roughly 85 wave periods within the 3-ps e-beam pulse.  This means some of the electrons will experience an accelerating electric field, some a decelerating field, and some little field along the length of the e-beam pulse.  Graphically, this can be represented by creating an energy-phase plot, in which the energy shift of each electron from the mean e-beam energy is plotted as a function of laser field phase within one wavelength period.  Phase is equivalent to position along a laser wavelength where 2p phase equals 10.6 mm.  Figure 1 shows the energy-phase plot for the electrons initially entering the STELLA experiment from the ATF linac.  This plot is created by a computer simulation of the experiment and contains 5,000 particles representing the electrons distributed initially uniformly over all phases of the laser light.  (Recall the e-beam pulse is much longer than the wavelength of the laser light.)  Thus, except for an intrinsic energy spread or width about the mean energy, all the electrons are near zero energy shift.

Figure 2 shows what happens after the electrons interact with the laser beam in the first IFEL.  Within each of the wavelength periods covered by the e-beam pulse, there are accelerated and decelerated electrons.  (The distribution shown Fig. 2 repeats itself 85 times over the length of the e-beam pulse.)

As these electrons continue traveling downstream of the first IFEL, the faster electrons catch up in phase or longitudinal position with the slower ones as depicted in Fig. 3.  This process is called bunching because now the electrons become grouped together into a microbunch as shown in Fig. 4, which is a plot of the electron density distribution of the energy-phase plot given in Fig. 3.  (Note, the abscissa of Fig. 4 is plotted in units of microns rather than radians.)

The microbunch shown in Fig. 4 is approximately 1 mm long, which is equivalent to 3 fs in time duration.  Since Fig. 4 represents only one of 85 wavelengths within the e-beam pulse envelope, this means a train of 85 1-mm-long microbunches is formed separated by 10.6 mm.

A shortcut for causing the electrons to bunch sooner after the e-beam energy is modulated within the IFEL undulator is to replace the drift space after the undulator with a magnetic device called a chicane.  The chicane used during the STELLA experiment (see Fig. 5) has both a fixed magnetic field (i.e., permanent magnets) and a variable magnetic field (i.e., electromagnets).  The fixed field forces the electrons to travel through a “V-shaped” trajectory in which the faster electrons generated by the IFEL traverse a shorter path than the slower ones, thereby causing the electrons to bunch together.  This is trajectory is depicted in Fig. 6 for an electron at the mean e-beam energy, i.e., 45 MeV.