Observation of Magnetic Resonances in Electron Clouds in a Positron Storage Ring Page: 3 of 15
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lectors, 76.2 mm long and 2.54 mm wide, were placed on an horizontal plane
length-wise along the beam direction with 0.51 mm gaps. An array of 2 mm
diameter holes in the chamber wall, covering 15% of the local surface area,
allowed shielding of the beam fields and detection of the electron cloud with
minimal disturbance. Each collector was independently biased at +45 V. The
detected signal current returned to ground via a load resistor. At the highest
observed signal, this caused the bias voltage to "droop" by up to 1 volt; its
effect on the RFA's collection efficiency was found to be negligible.
A photograph of the apparatus in the first chicane dipole is shown in Figure 1.
The chamber wall exposed to direct synchrotron radiation beam was located
on the x > 0 side; y is vertical. For the data presented here, PEP-II operated
with 1722 bunches, with 6.65x 1010 positrons per bunch at an average beam
current of 2500 mA. The beam energy was 3.1 GeV. The beam bunches were
11.5 mm long (rms), with a spacing of Tb = 4.2 ns. In the study of magnetic
resonances in electron clouds in a dipole field, a useful quantity is the ratio of
Tb to the cyclotron period, n = Tb/rT, where T, = 27rmey/eBy, and me is the
electron's mass, e its charge, <y its Lorentz factor.
3 Electron Cloud Build-up
The number of electrons emitted from the surface is determined by the sec-
ondary electron yield (SEY). The SEY scales approximately as 1/cos(O), where
O is the incident angle with respect to the surface normal. For a fixed 0, SEY
increases rapidly as a function of incident energy until it reaches a maxi-
mum, and then decreases slowly at higher energies. The SEY parameters were
measured in the laboratory using test samples, before and after exposure to
positron beams in a setup installed at an upstream beamline location. The
SEY maximum for uncoated aluminum surface was determined to be 3.2 at
an incident energy of 300 eV, decreasing to 2.4 after beam exposure. While for
a TiN-coated aluminum substrate, the maximum was 1.8 at 500 eV, reducing
to 0.95 after beam exposure [7-9]. Both the laboratory measurement and the
beam conditioning were done in a field-free environment.
During electron cloud build-up, low energy secondary electrons emitted from
the surface were accelerated by the passing positron bunch. In the magnetic
field-free case, the electrons would oscillate about the beam axis for 4 to 5
bunch crossings on average before impinging on the chamber wall. In the dipole
field, the electrons were transversely localized, and they were constrained to
move predominantly vertically along helical tracks. The cloud density sta-
bilized within approximately 100 bunch crossings when the rate of electron
production reached an equilibrium with the rate of loss due to re-absorption.
The cloud electron flux at the chamber wall was measured by sampling the
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Pivi, M.T.F.; Ng, J.S.T.; Cooper, F.; Kharakh, D.; King, F.; Kirby, R.E. et al. Observation of Magnetic Resonances in Electron Clouds in a Positron Storage Ring, article, August 24, 2011; United States. (digital.library.unt.edu/ark:/67531/metadc931780/m1/3/: accessed December 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.