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profile at (a) z = 4.5 cm is the result of the long flattop
pulse near the DWA entrance and the 1-cm Bulmlein
block configuration. As protons being accelerated, the
time delay between the turn-on times of the neighboring
Blumlein blocks reduces and the voltage pulses approach
Gaussian shape. Consequently, the net accelerating field
waveform on the axis becomes smoother and Gaussian
like as shown in Figs. 1 (b) and (c).
(a)z=4.5 cm (bz=55.5 cm
0.8 0.8
0.6 0.6
0.4- I 0.4
0.0 0.0
-4 -2 0 2 4 -4 -2 0 2 4
Time (ns) Time (ns)
1.0 ->--'
(c)z=8.5cm
0.8-
0.6
0.4
0.2
-4 -2 o z a
Time (ns)
Figure 1: On-axis accelerating field waveforms and
proton bunch temporal profiles at (a) z =4.5 cm, (b) 8.5
cm and (c) 55.5 cm.
There is no grid or foil at the entrance of the DWA.
Also, no external focusing is used in the DWA. The
transport strategy is to employ the entrance fringe fields,
switch timing and accelerating voltage. Due to the
relatively long accelerating pulse respect to the 200-ps
proton bunch at the DWA entrance, the bunch is
transversely focused by the fringe field and longitudinally
stable at the same time. As the voltage waveform losing
its flattop along the machine, the bunch position, or phase,
with respect to the virtual traveling wave is controlled by
changing the switch timing and the accelerating voltage.
After being transversely focused by the frindge field
initially, the bunch is positioned in the leading side of the
virtual traveling accelerating voltage wave so that the
bunch is being longitudinally compressed and
transversely defocused. In the second part of the DWA,
the proton bunch position is moved to the trailing side of
the voltage wave. By alternating the bunch's relative
phase respect to the virtual traveling wave, the net
focusing could be achieved both transversely and
longitudinally.
The LSP simulations for beam transport in the
strawman DWA start at 10 cm upstream from the DWA
entrance. The initial proton bunch is at its waist with a 5-
mm radius. Since the emittance of the initial proton bunch
provided by the RFQ is expected to be much smaller than
the emittance goal, the initial proton bunch is cold in the
simulations. The phase spaces of the LSP simulated beam
at the DWA exit are presented in Fig. 2. The r.m.s. beam
radius is 2 mm, and the edge radius is 5.5 mm. The r.m.s.
beam envelope slope is -0.5 mrad. The normalizedLapostalle emittance is 1.5 mm-mrad. The proton bunch's
energy distribution in Figure 3 shows that the exit beam
energy is 136 MeV, higher than the goal of 120 MeV, due
to that fact that we managed the proton bunch's phase
along the DWA by changing the individual Blumlein's
voltage in these set of simulations. However, it has
demonstrated the feasibility of beam transport without any
beam loss and without using any external focusing
element. The 1-sigma energy spread is 1.7 MeV.(a
) v_ (c
0 4
-04m)
(b) x(rad)
0.002
xx(cm) - .(cm)
.0.4 -0.4 4
-0.002Figure 2: Phase space plots at the DWA exit.
[30 132 134 136 13S
Energy (MeV)
Figure 3: Proton bunch's energy distribution at the DWA
exit.0t o5 -N- Unnormal Ized
E -N- Normalized -
04-
Fj 032
101
r otm
, ern ti eisE0.6 -% .
S04
2
0 50 10 150 200 250
-N--rsqt22
200O
5s o so 100 so 200 250
jitterng time (:)138 . -
136
m132
F 3 -%- Average Energy
10F -%-- Max mm~ Erergy-
128
126
aterneg time Ins~)
23
iso
10
05
-50 0 50 1o0 1s0 200 2s0
*itt gerngne ls)Figure 4: Proton bunch's Lapostoll emittance, beam
radius, beam slope, energy and energy variation at the
DWA exit versus injector timing jitter .Total charge = 32.04 pC
2.o
1.5
1.0-
0 0U
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Chen, Y.; Caporaso, G.; Blackfield, D.; Hawkins, S.; Nelson, S. & Poole, B. Beam Transport in a Proton Dielectric Wall Accelerator, article, August 18, 2010; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc863959/m1/4/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.