ACCELERATOR PHYSICS
CEBAF, at Jefferson Lab, is the largest superconducting,
recirculating linac in the world. Jefferson Lab is also the
site of a free electron laser where high-efficiency energy
recovery has been demonstrated and is being used routinely.
These concepts are now part of an increasing number of proposed
ideas both here and at other laboratories. Before they become
reality these new ideas will have to be investigated further
and a number of challenging accelerator physics issues will
need to be resolved.
Beam-beam instabilities in electron-ion colliders
In an electron-ion collider, the interaction between the two
beams due to the relative lateral offset between the head of
the proton bunch and the electron bunch will deflect the electrons;
the deflected electrons will then interact with the tail of
the proton bunch. Thus, the electron beam acts as transverse
impedance to the proton bunch, potentially leading to instability.
The linear theory of this beam-beam instability is fairly
well developed but a full nonlinear theory is still lacking.
High-energy electron cooling
Electron beam cooling has, until now, been limited to low energy
beams. New concepts for electron-ion colliders will rely of
electron cooling at ultra relativistic energies. Theoretical
understanding of high-energy electron cooling, as well as means
to experimentally tests the theory are needed for the electron-ion colliders.
Beam loss mechanisms and control in high-current linacs
As superconducting linacs are being designed for higher and
higher current, beam loss can become a serious issues or even
impose a limit on the current that can be accelerated.
A better understanding of all the mechanisms that can lead
to beam loss are needed as well as means to control them.
Collective effects in energy recovering linacs
In high-current, energy- recovering linacs, the feedback between
the beam and the cavities is closed and, at sufficiently high
currents, instabilities can occur. These instabilities can
result from interaction between the beam and the fundamental
modes (beam loading instabilities), high-order monopoles
(Longitudinal beam breakup) or high-order dipoles (transverse
beam breakup). It is expected that transverse BBU will be the
main limitation to the maximum current that can be accelerated/decelerated
in an ERL but a complete investigation of these instabilities
is needed to assess the full potential of ERLs.
Higher-order modes losses analysis and mitigation in
high-current superconducting linacs
In energy-recovering linacs with beams of several 100mA the beams
will deposit power in the higher-order modes of each cavity of
the order of several kW. It is expected that only a small fraction
of that power will be dissipated in the cavity walls but could
still be excessive. A better understanding of the mechanisms
of high-order mode excitation, of where the power deposited is
dissipated, and of how to extract that power is needed for the
development of high-current energy-recovering linacs.
RF control of high-Q superconducting cavities in high-current
energy recovering linacs
In energy-recovering linacs the beam is accelerated and subsequently
decelerated in the same cavity. In the case of perfect energy
recovery (current of the same magnitude, 1800 out of phase) the
cavity sees no net current and thus can operate at a high loaded Q,
thus improving the efficiency of the accelerator. Any deviation
from perfect energy recovery, either in phase or amplitude, will
cause a net beam loading in the cavity that the rf control system
will have to manage. Rf control systems and algorithms needed to
deal with random beam loading of high-Q cavities will need to be developed.
Simulation and visualization tools for the design and
operation of accelerators
The design of accelerators relies increasingly in the use of complex
computer codes. The interpretation of the output is often difficult
and the creation of simulation and visualization tools to display
and interpret the output, preferably in real time and in an interacting
manner would facilitate and aid in optimizing the design of accelerators.
Similarly, the operation of such machines relies on the monitoring
and display of a large number of parameters; tools for the visualization
and optimization of the setting of operational parameters would
benefit operation of accelerators, and improve their performance
and reliability.
Control Theory of Accelerators
The CEBAF accelerator requires complex control algorithms for
maintaining stability of the electron beams. Both feedback and
feedforward systems are used at present, but improvements are
needed in the short and long term stability, as well as in
decoupling the different systems. The goal is to be able to
provide high-reliability, high performance systems that will
control every important beam parameter. The studies will be
both theoretical and experimental.