A fast chopper for programmed population of the longitudinal phase space of the AGS Page: 2 of 3
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v-ero: 220.6 Kev/two 1dot: 6.5 T/sc inj turns: 76 1 bea:2f
R.E.: 17.5 seV Fdot: 33.6 kiiz/ns 0I1.1: 30.0 de
sevine Rt !pm Stat ionry Slt
Aree: 0.93 aves 6.76 eVs 1.27 *Vs Ol stable: 6.57 dog
Height: 2.22 NaY 1.8 WeW 2.51 seV
Heigt: 16.35 iz 3.27 k~z 11.75 kHz delta R: -8.05 en
mjeaetton time: 361.6 ise Synch rp: 5.640 Kr ISgnch osc: 2.11
fillfraction: 13.0 2 "r bunch height Ifnj particles: 2.32e'13
Figure 1. Chopper control program output; bucket,
bunch, and area into which particles
Numerical outputs are given for the parameters of
phase space and several other relevant quantities,
such as: the variation of radius of the synchronous
particle, the total number of particles injected,
and the synchrotron oscillation frequency. Indi-
vidual pulses can be switched on and off by changing
values of the elements of a bunch-X-turn matrix from
one to zero.
Construction of the Chopper
The fuasibility of Implement ing a chopper with
the necessary flaxibiltty follows from the change of
the AGS preinjector from a Cockcroft-Walton to an
RFQ linac.' This made the beam auccesmible at ground
potential while it is at a low enough energy (35
keV) so that it can be effectively deflected with
practical voltagaa. The chopper is a pulsed elec-
trostatic deflector, located between the ion scurce
and the RFQ, midway between two focusing solenoids
that match the beam to the RFQ acceptance. The beam
diameter is 80 ma at that point. Unwanted beam is
rejected by deflecting it outside the opening of a
14 me diameter circular aperture which is located at
the entrance of the RFQ, 22 mm upstream from the
vane tips. The beam is rapidly converging at this
point with a waist inside the RFQ. The Twiss
Figure 2. Chopper slow-wave structure.
Figure 3. Outputs from high voltage
pulsers. Center is zero volts,
parameters are a -' 5.0 and B - 0.27 m and the beam
emittance is 133 w me mrad. With a parallel beam in
the chopper, the deflection that is needed to dis-
place the beam outside the aperture is 44 mrad, five
times what would be needed if the aperture were at
the waist. Furthermore, when the chopper is in
nperoton, the space charge neutralization by toni-
zatton of the residual gas is lost, and the beam
becomes divergent in the chopper, increasing the
renirpA deFlfoctton to 65 mrad. Civen the size of
the beam and the desire to keep voltagus low, the
array of deflecting plates was made 380 mm long ..n
the beam direction.
A drawbauck of Lhu low inurdy of thEo beam to thi
long time of flight through the deflecting plates,
150 nm. In order to got an acceptable riqe/fall
time on the beam pulse, it was necessary to segment
the deflection plates into strips that are 17 mm
long in the beam direction and 160 mm long trans-
verse. The strips are located on 26 mm centers.
The voltage is applied to the strips sequentially At
a rate that matches the beam velocity as a slow-wave
structure. The delay between the strips is obtained
by connecting them one to the next with the appro-
priate length of coaxial cable. Figure 2 shows the
mechanical assembly of the chopper. One can see
Lhat Lhere are two nets of strips, located abovo and
below the beam. There are two advantages to this
arrangement; one, the magnitude of the voltage need-
ed is halved and two, the extent of thu Eringe Ciold
in the beam direction for each strip is also halved.
This results in reduced rise/fall time of the beam
hulcn. Octatlf of the toning of the impedanct of
the strips to the impedance of the coaxial cables
have been published elsewhere.5
'rle slow wave structures uru driven by a com-
plementary pair of high voltage pulse generators.
The pulse generators are commercial units and
reflect the state of the art of fast high voltage
pulse technology using power MOSFETS.6 The genera-
tors have both pull-up and pull-down transistors in
the output stage and, hence, have equal rise and
fall times of the output pulse. The outputs are dc
coupled to the MOSFETS so that the voltage level in
the "off" state is always zero, independent of the
duty factor of the waveform. This is an essential
feature for the chopper since the usable beam is
taken when the pulsers are "off". The output wave-
forms are shown in Figure 3, measured with a Tek-
tronix 2467 oscilloscope via a 40.9 dB power attenu-
ator. The rise and fall times (10% to 90%) are all
less than 9 ns. The pulse amplitudes are 1 760
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Brennan, J.M.; Ahrens, L.; Alessi, J.; Brodowski, J.; Kats, J. & van Asselt, W. A fast chopper for programmed population of the longitudinal phase space of the AGS, article, January 1, 1989; Upton, New York. (digital.library.unt.edu/ark:/67531/metadc1111791/m1/2/: accessed January 16, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.