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SLAC-PUB-12275
January 2007
e-/e+ ACCELERATING STRUCTURE
WITH CYCLIC VARIATION OF AZIMUTH ASYMMETRY
Anatoly Krasnykh, Stanford Linear Accelerator Center, Menlo Park, California 94025Abstract
A classical electron/positron accelerating
structure is a disk-loaded cylindrical waveguide.
The accelerator structure here has azimuth
symmetry. The proposed structure contains a disk-
loaded cylindrical waveguide where there is a
periodical change of RF-field vs. azimuth. The
modulation deforms the rf-field in such a manner
that the accelerated particles undergo transverse
focusing forces. The new class of accelerator
structures covers the initial part of e+/e- linacs
where a bunch is not rigid and additional transverse
focusing fields are necessary. We discuss a bunch
formation with a high transverse aspect ratio in the
proposed structure and particularly in the
photoinjector part of a linac.
INTRODUCTION
The proposed structure is a new type of e-/e+
accelerating structure, which combines a
longitudinal acceleration with rf- transverse
focusing/defocusing attributes. The working
principle of the new accelerator structure is the
same as the classical RFQ-structure that is widely
used for the proton or heavy ion machines. The
main difference is that the new structure runs with
the electron/positron beams. The cross section of
the first cell in the vertical plane is a typical cross
section for a classical accelerator structure. The
cross section in the horizontal plan for the same
cell is different. If the bunch passes the structure
from left to right, the first cavity wall has a slope
for the horizontal plane. The slope of the iris wall
is periodically changed as a function of azimuth 0<
0 < 22. The left side of the iris surface wall has no
azimuth variation and looks like a classical disk-
loaded waveguide surface. The inner surfaces in
the second cell are the same as in the first cell
except that the second cell is azimuthally rotated
for 0 =2/2. The slope of the vertical plane may be
slightly different compared to the slope in the
horizontal plane of the first cell. We are not
discussing the common case when the following
cell is azimuthally rotated for 0 < 2/2 (adiabatic
rotation). The role of the azimuth wall modulation
is a creation of the required potential distribution in
the region where the bunch is propagated. To
create the required potential distribution wepropose employing a wall modulation that can be
described (in polar coordinate system) in the next
form
x(O) = r " cos O
y(O) = r " sin O
z(O)=z +h-jsinO[
where a<r<b are the cell radius, z0 is a local wall
cell coordinate, h is the depth of the wall
modulation in the longitudinal direction, and n is
the degree of modulation (in practical case n=2).
The depth and degree of modulation are cell
parameters, which can create the required potential
distribution in the cells. Fig. 1 shows one quarter of
a model with two cells.Fig. 1 Two cells
of accelerating
structure with
cyclic variation
of azimuth
asymmetry
(quarter model)A level of modulation vs. azimuth is shown in
Fig. 2. It has been shown that we can get the
modulation of cell vacuum space as a function of n
amount.l' 'I
16= _Fig. 2
Another way to get a spatial modulation of RF
power in an acceleration structure is the
employment of cyclic variation of elliptical
coupling holes in the iris array. In classic
accelerating structures, the coupling holes are
round. To produce a spatial RF focusing effect we
propose the accelerating structure that is shown inContributed to Partical Accelerator Conference 2007, 25-29 June 2007, Albuquerque, NM, USA
Work supported in part by the US Department of Energy contract DE-AC02-76SF00515-
-r
f+.
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Krasnykh, A. e-/e+ Accelerating Structure with Cyclic Variation of Azimuth Asymmetry, article, March 5, 2007; [Menlo Park, California]. (https://digital.library.unt.edu/ark:/67531/metadc887857/m1/1/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.