Electron Cloud Effects in High Intensity Proton Accelerators Page: 3 of 12
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Sh n are he rhI of ( -
the ehaniber wall) to average
proton bent lin den yl f
for 5.5,Cotloredprolono
Figure 5: Distribution of the electron flux measured on the
wall of the vacuum pipe at the PSR. The circumference of
the ring is 90.2 m. The kinetic energy of the proton beam
is 800 MeV. The flux ratio fe, varying around the ring,
is about 30% downstream of the extraction septum, about
25% downstream of the injection stripping-foil, about 4%
in section 4, and within the noise level in the TiN-coated
section 5.
a high temperature around 2000 K, emits thermionic elec-
trons at low energy. All these electrons may back-scatter
from the stripped-electron collector and the surrounding
surfaces [12]. As an example, Table 1 lists the sources of
production, yield, and energy-range of the electrons at the
PSR's injection region [13].
Figure 6 illustrates the collection of stripped-electrons at
the SNS accumulator ring. The electrons are guided by a
magnetic field and collected by a water-cooled device of
heat-resistant material. The electron collector uses a car-
bon material attached to water-cooled copper plate [14].
Selecting a low-charge-state material for the collector also
reduces the number of back-scattered electrons. Figure 7
shows the temperature distribution at the electron collector
when the stripped-electron beam of 3 kW power strikes the
surface of about 1 cm2 area.
Table 1: Estimated yield and kinetic energy of the electrons
produced by the injected H- beam at the PSR. The yield is
defined as the ratio of total number of electrons produced
during the accumulation period per injected H- particle.
The average number of foil traversal is about 50. The ki-
netic energy of the injecting beam is 800 MeV. The average
H- beam current is 100pA (courtesy M. Plum).
Source Yield Kinetic energy
Stripped e- 2.0 430 keV
Secondary e- 1.0 up to 20 eV
Knock-on e- 0.4 up to 2.4 MeV
Thermionic e~ < 0.002 -0.24 eV
Ionization 0.02 up to 2.4 MeVtion ofj r H- beamw at+ th SS cmlaorrig
t ^r-;,I' ='
We-oirate poiy3roo
>77
'^
Figure 7: Temperature (F) distribution at the stripped-
electron collector at the SNS ring in units of Fahrenheit (or
5F/9 + 255.37 K, Courtesy C. J. Liaw and J. Brodowski).
3.2 Collimation Region
The region near the scrapers and collimators is suscepti-
ble to a high beam-loss and, potentially, is another loca-
tion of high electron-concentration. Protons incident on
the collimators' surfaces produce secondary electrons. De-
pending on the energy of the beam and the incident angle,
the secondary electron-to-proton yield can greatly exceed
1 when the incident beam is nearly parallel to the surface
(i.e., grazing angle Q, ~ r/2). Experiments were per-
formed with different ions at Brookhaven's Tandem accel-
erator to verify the angular dependence of electron yield
[15]. As shown in Figure 8, the proton-induced yield Yep
has a 1/ cos b - dependence on the grazing angle rq, sim-
ilar to the electron-induced secondary-emission yield as
predicted by the Seiler model based on experimental fits
[16, 17, 18, 7]Ye =
1.11 Y,. 1 -exp [2.3(F)>3]
(E)O36 cos0
\ E"" a:1 CS(1)
where Ek is the kinetic energy of the primary proton, and
the proton energy that corresponds to the maximum yield,
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Wei, J. & Macek, R. J. Electron Cloud Effects in High Intensity Proton Accelerators, article, April 14, 2002; Upton, New York. (https://digital.library.unt.edu/ark:/67531/metadc741722/m1/3/: accessed April 22, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.