Back
To Research Facilities Home
The
ion beams produced by the SNICS and HIS
ion sources for injection into the FN Tandem accelerator
can be thought of as a continuous stream of particles,
and for most applications, this is precisely what is needed
to explore the nuclear physics being studied within the
laboratory. However, there are many experiments being
conducted within the laboratory where a continuous stream
of particles is not the best approach, and what is required
is that the beam be provided in short, discrete bursts,
or bunches. The purpose of a buncher system is to convert
the continuous stream of ions from the ion source into
a beam comprised of short pulses of ions, and the most
common application of this technique is to allow a timing
signal to be used to measure the time that elapses between
the bunch striking the target and the reaction products
striking various detectors. This technique is generally
referred to as a Time of Flight (ToF) measurement, and
provides additional information regarding the parameters
of the reaction that would not be available with a continuous
beam of ions. To
ensure high resolution in the timing, the beam bursts
should be of a very short duration, typically a few nanoseconds,
with a few hundred nanoseconds between bursts. This precludes
the use of a chopper system, which simply turns the beam
on and off in a timed sequence, as only a very small fraction
of the original beam intensity would ever reach the target.
 |
A buncher system
is designed to operate so that the original continuous
beam is concentrated into "packets" which contain a significant
fraction of the original beam. This is done by alternately
accelerating and decelerating the beam on a very short
time scale. To visualize this situation, imagine a line
of 5 evenly spaced cars going down the highway at constant
speed of 50 mph. Suppose the first car slows down to 48
mph, the second car slows down to 49 mph, the middle car
maintains its speed at 50 mph, the fourth car speeds up
to 51 mph and the last car speeds up to 52 mph. Assuming
they don't collide, at some point down the road they will
all be abreast of each other. The cars, originally spread
out along the road, are now "bunched" together. The buncher
system works on the same principle.
The buncher
system consists of three parts, being the buncher, the
sweeper and the pulse selector. The buncher is located
near the entrance to the FN Tandem accelerator and consists
of two mesh screens which the beam must pass through,
spaced about an inch apart. A very rapidly alternating
high voltage is placed across the screens, so that when
the voltage on one screen is going positive, the voltage
on the other is going negative. The effect is to alternately
accelerate and decelerate portions of the beam, causing
the beam to become bunched. Our buncher operates at fixed
frequency of 10 MHZ, producing a bunch every 100 ns, but
the voltage level applied to the screens can be adjusted
to ensure that as the bunch strikes the target, it has
coaslesced into the smallest possible packet.
Our buncher is comparatively
simple, using a sinusoidal voltage to drive the buncher
screens. Only the relatively linear portion of the sine
wave near the zero crossing can be effectively used to
bunch the beam, and as a result, there is a substantial
portion of the original continuous beam that passes through
the buncher which cannot be effectively bunched. If these
particles were allowed to reach the target, the timing
information provided by the bunch would be destroyed,
so that it is necessary to eliminate this portion of the
beam, referred to as "dark current". Our system makes
use of a device known as a sweeper, located near the exit
of the FN Tandem accelerator. It consists of two horizontal
plates between which the beam passes. A sinusoidal voltage
is applied to these plates at one-half the frequency of
the buncher and the phase is then adjusted so that the
center of the bunch passes through the plates during the
zero crossing of the plate voltage, when there is no deflection
of the beam. The sweeper plate voltages rise very quickly,
effectively deflecting away any beam not contained in
the bunches. After exiting the sweeper, approximately
30% of the original beam intensity remains in the bunch
and the width of the bunch in time is approximately 1.5
ns.
In our system,
the time between bunches is fixed at 100 ns, but for those
experiments that require more time between bunches, a
pulse selector is available. The pulse selector is located
upstream of the buncher and consists of a pair of deflector
plates that can be biased so as to deflect the beam from
the ion source onto a set of slits, preventing the beam
from reaching the buncher. Again, the biasing of these
plates is synchronized with the operation of the buncher,
and the experimentor can set the pulse selector so that
beam passes through the buncher only at times which are
multiples of 100 ns. In practice, the pulse selector is
set to allow for 1/n bunches, where n is an integer. For
example, if the pulse selector is set to 1/3, then only
every third bunch will be produced, and the timing between
the bunches will be 300 ns.
You may contact
the lab at nsl@nd.edu for more information regarding Pulsed and Bunched
Beams.