In a storage ring, as in linac-based free-electron lasers (FELs),
the electromagnetic field associated with the synchrotron radiation
emitted by a bunch passing through an insertion device and/or a
bending magnet interacts with the electrons in the same bunch,
modulating their energy. Above a threshold current (number of electrons
per bunch) in storage rings, the intensity becomes strong enough
to exponentially amplify modulations in the bunch distribution,
resulting in an effect known as the microbunching instability (MBI).
Such density modulations have characteristic lengths smaller or
comparable to typical terahertz radiation wavelengths and thus
radiate powerful CSR pulses in that frequency range.
The FEL gain described above can amplify either small fluctuations
(i.e., noise) in the electron-beam density or a small modulation
in the density induced by interaction with a laser beam. The second
process is referred to as laser seeding. Until now, the MBI was
always observed in storage rings as a stochastic process starting
from noise and associated with the emission of powerful CSR bursts
with random repetition rates and large fluctuations in amplitude.
But experiments performed at ALS Beamline
1.4.3 now show that by laser-inducing
a modulation on the electron bunch above the MBI current threshold,
it is possible to seed the instability in a controlled way. In
this case, the CSR bursts become synchronous with the modulating
laser, and the instability gain can be exploited for generating
high-power terahertz CSR pulses.
Above the current threshold for the microbunching
instability (MBI), the slicing process seeds the instability.
Top: Random bursts of high-intensity, coherent synchrotron radiation
(CSR) associated with the MBI, as measured at the ALS. Bottom:
When the slicing laser is turned on at a 1-kHz repetition rate,
the bursts become synchronous with the slicing laser, as the
burst onset positions match the vertical lines at 1-ms intervals
on the horizontal scale.
The bunch-current dependence of the terahertz
detector's Fourier (frequency) spectra shows two prominent features:
the onset of the MBI at around 15 mA and the match between the
1-kHz line and its harmonics with the laser repetition frequency.
The inset shows two frequency spectra for currents above the
MBI threshold with the slicing laser on (red) and off (blue).
The 1-kHz line and harmonics clearly disappear when the laser
is switched off.
The researchers then showed that their observations can be explained
in the framework of the MBI theory. Quantitative comparison of
the model with the ALS data showed good agreement up to currents
of about 19 mA, thus indicating that the model can account for
the CSR power growth. At higher currents, a saturation effect takes
control as the MBI goes into a nonlinear regime where the theory
breaks down.
Above the MBI threshold (15 mA), the total average
CSR power (red) and the power associated with the 1-kHz line
(green) both grow exponentially with the single-bunch current,
thanks to the gain process that increases the number of electrons
emitting coherently. If the power at 1 kHz were due only to the
particles in the bunch that were modulated in energy by the slicing
laser, then the current dependence would be quadratic, as observed
below the MBI threshold. The dashed blue curves represent theoretical
calculations: the exponential curve predicted by the MBI model
in the top graph and a quadratic comparison in the bottom graph.
The laser-seeding phenomenon could be the basis
for a THz source. Pump–probe and other experiments not requiring
shot-to-shot intensity stability could benefit from the several-orders-of-magnitude
increase in power that the seeded MBI offers. In a more speculative
scenario, a fraction of the THz signal can be brought back into
the ring to co-propagate in an insertion device or in a bending
magnet with a subsequent electron bunch, modulating its energy
and seeding the MBI that generates a new burst that is then used
in the loop for seeding a fresh bunch. By this process, one can
in principle bring the CSR emission to a stable high-power saturation
regime where all the bunches radiate coherently.
Research conducted by J.M. Byrd, D.S. Robin, F. Sannibale, A.A.
Zholents, and M.S. Zolotorev (Accelerator and Fusion Research Division,
Berkeley Lab); Z. Hao and M.C. Martin (ALS); and R.W. Schoenlein
(Materials Sciences Division, Berkeley Lab).
Research funding: U.S. Department of Energy, Office of High Energy
Physics and Office of Basic Energy Sciences (BES). Operation of
the ALS is supported by BES.
Publication about this research: J.M. Byrd, Z. Hao, M.C. Martin,
D.S. Robin, F. Sannibale, R.W. Schoenlein, A.A. Zholents, and M.S.
Zolotorev, "Laser seeding of the storage-ring microbunching
instability for high-power coherent terahertz radiation," Phys.
Rev. Lett. 97, 074802 (2006). |