Transformer ratio enhancement using a ramped bunch train in a collinear wakefield accelerator. Page: 2 of 13
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ramp (see dotted line in Fig. lb) to produce R >2. In this later scheme, the individual
bunches in the train are symmetric (e.g. gaussian) separated by a distance d. The
charge is then ramped up such that the first bunch in the train has the lowest charge
and the last bunch the highest. From the figure we see that all four drive bunches in
the drive train experience the same maximum decelerating field W just like in the case
of a single triangular ramped bunch. Thus, the fundamental condition for both of these
schemes is that the trailing particles (bunches) in the drive bunch (train) are positioned
in the accelerating phase of the leading particles (bunches) so that all the driving
particles experience the same maximum decelerating field.
w+ p(z) W+ p(z)
zW zW
d d -. d
(a) (b)
FIGURE 1. Two schemes that have been proposed to generate R = W/W 2. The height of the
shaded area, p(z), represents the total amount of charge in the bunch at location z while the solid, sine-
like, line is the amplitude of the wakefield driven by the beam. (a) A single drive bunch with a
triangular axial current distribution moving to the left. (b) A train of gaussian drive bunches with an
overall triangular pattern of the train (see dotted line) moving to the left.
The difficulty with the schemes that propose to use asymmetric axial current
distribution to achieve R > 2, arises from the lack of suitable techniques to tailor the
axial distribution of the drive beam. For example, for nearly 10 years it has been
known that beam dynamics codes (such as PARMELA) predict improved beam
quality from rf photoinjectors when driven by a 'flat top' axial current distribution.
Despite this knowledge, no one has successfully produced a 'flat top' although several
experimenters are getting close. Since the 'flat top' pulse is most likely easier to
generate than the 'triangular top' pulse it may be some time before this method is used
to obtain R > 2.
In this paper we consider the later method, here termed, the 'ramped' bunch train
(RBT) method of transformer ratio enhancement. Since the AWA facility has already
generated a 'flat' bunch train [4] it should be easy to generate a RBT. How the RBT is
generated and the difficulties we are likely to encounter will be discussed in a later
section.
We begin by reviewing the concept of transformer ratio for a single, symmetric
beam in a collinear wakefield accelerator and studying the trade off between the
acceleration gradient and the transformer ratio. We then examine the ramped bunch
train method and present an algorithm for choosing the spacing of the bunch and
charge of the different bunches in the train. Finally, we describe a proof of principle
experiment where we propose to send a train of 4 electron bunches, with a ramped
charge distribution through a dielectric lined waveguide.
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Power, J. G.; Gai, W. & Kanareykin, A. Transformer ratio enhancement using a ramped bunch train in a collinear wakefield accelerator., article, October 10, 2000; Argonne, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc716919/m1/2/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.