|
|
|
Coherent Synchrotron Radiation at the ALS
When the wavelength of synchrotron radiation is comparable to the length
of an electron bunch in the storage ring, or the length of any structure
on the bunch, the radiation from multiple electrons is in phase, resulting
in a quadratic rather than the usual linear dependence of the power emitted
on the number of electrons. Because the number of electrons participating
in the coherence can be large (more than 1 million), the potential power
enhancement is very large, making coherent synchrotron radiation (CSR)
a subject of great interest to both synchrotron users and accelerator
designers. However, the electromagnetic field associated with CSR can
influence the motion of the electrons in the bunch, resulting in a self-amplified
instability. This instability increases the electron bunch length and
energy spread and represents a fundamental limitation on the performance
of an electron storage ring. A joint Advanced Light Source/Berkeley Lab/University
of California, Davis, team has now been able to observe and, for the
first time, explain this instability.
The interaction of an electron bunch and its synchrotron radiation begins
when an electron bunch bends through a magnetic field and emits a cone of synchrotron
radiation that has a transverse electromagnetic field. Because of their bent
trajectory, electrons in the front of the bunch sense a longitudinal component
of the radiation field that can either accelerate or decelerate the electron,
depending on its position. The interaction can give rise to a self-amplified
instability starting from a small modulation on the bunch profile. This modulation
radiates coherently, causing the bunch modulation to increase. Counteracting
this effect is the natural energy spread within the bunch, which tends to cause
any modulation to smear out. An instability occurs when the runaway amplification
beats out the damping effect of the energy spread.
Schematic
view of the interaction of an electron bunch with its own synchrotron radiation.
The curvature of the electron orbit allows the radiation field to interact
with the electron.
Computer simulations of the microbunching instability as the threshold for
the instability is passed were the first step. Above the instability threshold,
a ripple in the energy distribution is evident in the simulations, along with
a small ripple on the bunch profile. As the instability progressed, the disruption
in the bunch increased, giving a larger modulation in the bunch profile. Finally,
the instability reached saturation and the bunch profile smoothed over, although
with an increased length. After radiation damping returned the bunch distribution
to its original shape, the instability repeated. |
How to Corral T Rays
|
Evolution of a microbunching instability illustrated via a computer
simulation. The top row shows the electron density in coordinates
of relative position along the bunch and fractional energy offset.
The bottom row shows the projection of the top row on the longitudinal
charge density. The small modulation in the density (left)
is amplified until the instability reaches saturation (right).
During the instability, the microbunching resulted in bursts of
CSR at the wavelength of the bunch modulation, which for ALS parameters
ranges from a few millimeters down to half a millimeter (far infrared
or terahertz). To observe these bursts experimentally, the team installed
detectors, such as bolometers and heterodyne receivers, at ALS infrared
Beamline 1.4.3. With one of the bolometers , the researchers found
that the bursts appear above a threshold single-bunch current. As
the bunch current increased further, the burst rate and amplitude
increased and eventually saturated the detector. At the highest bunch
currents achievable at the ALS, the researchers measured a 700-fold
enhancement in the power of the CSR emission over the normal incoherent
radiation. However, the bursting nature of the signal presents a
challenge for its use as a source of CSR.
Examples
of bursts of far-infrared synchrotron radiation measured with a
bolometer. Each of the bursts is associated with a microbunching of the
electron beam caused by interaction with the synchrotron radiation. At
larger bunch currents, the burst rate and amplitude increased.
To compare these results with a model recently developed
elsewhere, the team measured the bursting threshold as a function
of electron
beam energy. The data show good agreement with the model. The researchers
believe they have good a understanding of this instability and
can use the model to predict the performance of
future storage rings, particularly sources of CSR.
Comparison
of the measured microbunching instability threshold as a function of electron
beam energy with that predicted by a model. The comparison shows good
agreement between the two.
Research conducted by J.M. Byrd (ALS and University of California,
Davis); W.P. Leemans and B. Marcelis (Berkeley Lab); A. Loftsdottir
(Berkeley Lab and University of California, Davis); and M.C. Martin,
W.R. McKinney, F. Sannibale, T. Scarvie, and C. Steier (ALS). Research funding: U.S. Department of Energy, Office of Basic Energy
Sciences, and Berkeley Lab Laboratory Directed Research and Development.
Publication about this research: J.M. Byrd, W. Leemans, A. Loftsdottir,
B. Marcelis, M.C. Martin, W.R. McKinney, F. Sannibale, T. Scarvie,
and C. Steier, "Observation of broadband self-amplified spontaneous
coherent terahertz synchrotron radiation in a storage ring," Phys.
Rev. Lett. 89, 224801 (2002).
|
ALSNews Vol. 224, June 11, 2003
|