New positions for PhD students are available in the CesrTA group
and the g-2 group. Contact me for additional information.
Research projects
Halo Monitor
Beam halo is the source of an often debilitating backgrounds in experiments at proton colliders like the LHC. It is notoriously difficult to measure since the population in the halo is almost by
definition many orders of magnitude smaller than the population of the core. Synchrotron light detectors are typically sensitive to the 1 sigma core of the beam. The usefullness of the
higher event rate anticipated with the high luminosity upgrade of the LHC will be limited by halo. We are collaborating with CERN to develop a monitor that will use x-ray detetors with
five or more orders of magnitude of dynamic range in order to see the halo in the glare of the beam core. The device will be tested in the CESR where the distribution
of particles in an electron bunch is not so different from that of a proton bunch in the LHC.
Wakefields and Impedance
Intra-beam scattering
In high energy electron/positron storage rings, the beam
size is limited by single particle effects,
that is the equilibrium of radiation damping
and excitation. At very low emittance, and
therefore very high density beams,
intra-beam scattering plays an increasingly
important role. We have been able to achieve
low enough single particle emittance in CESR
to begin to explore the effects of intrabeam
scattering on the equilibrium beam size. We
are developing instrumentation to allow a
simultaneous measurement of the horizontal,
vertical, and longitudinal phase
space. Given the flexibility of the storage
ring to vary energy and lattice parameters,
we will have an unprecedented window on IBS
and will be able to test the theory.(Ehrlichman)
Fast Ion Instability
In electron storage rings, the residual gas in the vacuum chamber is
ionized and oscillate in the potential well of
the beam. If the bunches are spaced far
apart, then there is time for the ions to
dissipate. But if the bunches are closely
spaced in a long train, then the ion density
can grow from the head of the train to the
tail and couple the motion of lead bunches
to trailing bunches. The spectrum of
the turn by turn and bunch by bunch positon
data can in principle included frequencies
peculiar to particular ion
masses. Dependence of bunch size, or
the amplitude of centrode motion on distance
from the head of the train, could be a
signature of the fast ion instability.
Touschek scattering
Touschek scattering is characterized by the large change in
particle energy that may arise from
scattering in the transverse plane in a low
emittance high density particle beam. Loss of off energy particles
limits lifetime. Because CESR can operate at low energy and very low
emittance, we are in a position to explore sensitivites and
characterize the phenomona. The superconducting RF system allows the
possibility of large variations in the accelerating voltage, and bunch
length and Touschek lifetime.
Electron cloud (growth of the cloud)
Growth and evolution of the electron cloud depends on many beam parameters
including bunch charge, species, spacing, current, energy, and
vertical emittance, in addition to details of the environment such as
vacuum chamber surface chemistry, surface roughness, quantum
efficiency, reflectivity, etc. Retarding field analyzers are used to
measure time averaged dependence of cloud density. Shielded pickups
can be used to characterize the cloud on the nanosecond time scale.
(Calvey)
Electron cloud beam dynamics
The electron cloud focuses the positron beam, couples the head and
tail of the positron bunches, and couples one bunch to the next. We
observe bunch dependent tune shifts, single bunch emittance growth and
single and multibunch instabilities. The existing theory only incompletely
characterizes the data, and there is an opportunity to
develop theoretical and computational tools to model and interpret the
phenomona, and to extend the reach of the measurements.
Low Emittance Tuning
We have developed instrumentation and techniques that permit routine
correction of emittance diluting misalignments to less than 10 pm-rad
vertical emittance. We continue to investigate the systematic
measurement errors that limit our ability to resolve residual
dispersion. The minimum achieved to date (at CESR or any other storage ring)
is an order of magnitude greater than the theoretical lower limit, the so-called quantum limit.
What new phenomena will emerge as we approach that limit?
(Shanks)
g-2 beam dynamics
Model the transport of muons through the
inflector that brings the particles into the g-2 storage ring central
region, past the kicker and then to their ultimate decay to electrons,
or loss to scattering. Coherent oscillations of the stored muons is an
important systematic that must be minimized in order that we achieve
the 0.1 ppm precision that is the goal of the experiment.
Instrumentation
x-ray beam size monitor
Coded apertures for imaging white beams from single bunches.
Develop techniques for measuring intra-bunch motion
visible synchrotron light beam size monitor
Develop bunch by bunch electronics for single bunch measurement of height and width. Interferometry and angular distribution of polarized synchrotron radiation for measurement of vertical size of 15 micron bunch.
Time resolving electron cloud detectors
Properties of the surface of the vacuum chamber determine the
time development of the electron cloud.
g-2 injection kicker
Cornell has assumed responsibility for designing, building, and
testing the fast injection kicker that directs 3.1GeV muons onto the
central orbit of the g-2 storage ring. The single turn kicker magnet
requires >4000 A in a pulse with 100ns duration and a repitition rate of
100 Hz. We will model the kicker magnetic field and pulser, then
prototype and test the new system. We will also develop
instrumentation for measuring the magnetic field on the nanosecond
time scale.