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.
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