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Physics and Technology of Linear Collider Facilities
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“A roller-coaster ride through the subject of linear colliders – the
Next BIG Thing!”
“I hear the roar of the Big Machine
Two worlds and in-between…”
The Sisters of Mercy
Course Material (as
presented at Santa Barbara
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Unit 1 |
introduction and overview |
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Unit 2 |
linac technology |
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Unit 3 |
linac technology cont. |
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Unit 4 |
damping rings |
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Unit 5 |
bunch compressors |
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Unit 6 |
beam delivery systems |
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Unit 7 |
beam-beam effects |
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Unit 8 |
stability issues and feedback |
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Unit 9 |
a) beam-based
alignment b) SLC
and the alternatives c) course
review |
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Additional
material (simulations, lab. activities, animations etc) for units 6, 7 and 8
can be found here.
NEW: Nick Walker’s lecture
emittance preservation in electron linacs given at the CAS school in Triest
(2005):
Nick Walker’s lectures on
S.C. electron linacs given at the CAS school in Zeuthen (2003):
June 2003 Syllabus
What’s in Store
The lecture
series is divided into nine units designed to cover in various levels of detail
all the major issues facing the design and realization of a high-energy
electron-positron linear collider:
- A Linear Collider Overview
- Accelerating Structures and Linac Beam Dynamics
- Production of High Powered Microwaves
- Damping Rings
- Bunch Compression
- Beam Delivery System
- Beam-Beam and other Interaction Region
Effects
- Stability Issues and Beam-Based Feedback
Systems
- The SLC and the Alternatives
We first
introduce the reasons behind the various sub-systems of a linear collider via
the important parameter of luminosity (lecture 1). Lecture 2 discusses
the luminosity issue further in the framework of the important beam-beam
interaction. Lectures 3-8 then cover each of the machine sub-systems in detail,
with an emphasis on both the fundamental concepts (accelerator physics and engineering),
and the particular challenges facing that sub-system. Finally, lecture 9
summarizes all that we have learnt by reviewing both the only existing linear
collider (the SLC), and the various proposals for the next generation machines.
Every single sub-system of a Linear Collider pushes accelerator physics and
engineering beyond the current state-of-the-art! In other words, we are
boldly going where no man has gone before.
Course Structure
Each unit
will take the form of an informal lecture session, ranging from two to three
hours (with a break for much needed coffee), and an associated tutorial
session. The tutorial sessions will consist of
worked examples and/or computer simulations.
We also intend to schedule additional work-group sessions as the need arises to
discuss further topics of interest. It is our hope that the course will be
dynamic in the sense that we will react to the needs of the participants. To
that end we encourage the participants to ask
questions and contribute to ‘round-table’ discussions on the topics
presented.
The course
will also include problem sets to be solved in the afternoon/evening (during
the tutorial and/or after dinner), and a final exam on the last day of
class. Grades, for those who take the
class for credit, will be based on the final exam score (60%) and the
cumulative score on problem sets (40%).
Books and Course Material
As with any
subject “on the cutting edge”, there is a lack of single source text books
covering the subjects. Instead, the relevant information is distributed across a legion of conference and
workshop publications, review articles and – in a few exceptional cases – some
text books. We will, however, produce a concise bibliography of those source
materials that we feel cover the relevant topics (i.e. the ones we used
ourselves). In addition, there will be written course notes provided for each
lecture unit.
Your Lecturers
We are (in
alphabetical order)
- Andrei Seryi SLAC
- Peter Tenenbaum SLAC
- Nick Walker DESY,
- Andrzej
Wolski LBL
Between us,
we represent something like 50 man-years of active R&D on linear collider
design. We are all relatively young and dynamic, and all of us are extremely
excited about the concept of building such a challenging machine (if not just a
little crazy). You can be assured that we are putting a lot of work into trying
to infect you with the same excitement and enthusiasm!
If you have
any questions, comments or even suggestions, please don’t hesitate to contact
one or all of us.
We look
forwarded to seeing you in Santa Barbara.
Course Unit Summaries
Unit
1: A Linear Collider Overview
This
introductory lecture will set the stage for the following more detailed
lectures. The overall main parameters and their constraints will be introduced
via the important issue of the achievable luminosity (the luminosity scaling
laws). Once these primary parameters have been introduced, the basic methods of
achieving them will be discussed using an overview of the various sub-systems
of a linear collider. In addition to the specific LC material, the lecture will
also contain a review of the necessary basic accelerator physics and
terminology that will be required for the remainder of the course, including:
concept of transverse phase space; transverse emittance; emittance
preservation; effect of acceleration on transverse emittance; the b function and betatron oscillations;
longitudinal emittance.
Summary:
- Why an LC and not super-LEP
(cost optimization)
- The LC Luminosity Issue:
- luminosity scaling with
center-of-mass energy
- collider luminosity expression
- derivation of scaling laws for
LC luminosity
- identification of primary free
parameters – total RF power;
vertical emittance; bunch length; beamstrahlung etc.
- constraints on free parameters
and realistic parameter sets
- Overview of LC sub-systems (the
‘Generic’ LC)
- sources (e+ and e-)
- damping rings
- bunch compression
- main linac
- final focus
Unit 2: Beam-Beam and other Interaction Region (IR) effects
The
intense beam-beam interaction in a linear collider constrains the available luminosity
through energy loss (beamstrahlung) and associated beam-beam induced
backgrounds. This lecture will focus quantitatively on the particle dynamics of
the beam-beam interaction (the ‘classical’ effects), while dealing somewhat
more qualitatively with the subject of beamstrahlung and pair production (the
quantum effects).
Summary:
- electric field from a very flat
beam
- equation of motion of an
electron in the field of the opposing beam
- the linear solution and the
Disruption Parameter Dx,y
- oscillation number
- the beam-beam kick and its
importance for collision control (feedback)
- beam-beam instabilities
- single-bunch instability
- multi-bunch kink instability,
crossing-angles and crab-crossing
- quantum effects
- beamstrahlung and the ˇ parameter
- pair production and
backgrounds
Unit 3 & 4: The Main Linac
The main
linacs and their associated technology are at the heart of the linear collider.
The following two lectures will specialize on the challenging problems of peak
RF power generation and the design of
the accelerator structures and the beam dynamics of acceleration.
Unit 3: Accelerating Structures
and Linac Beam Dynamics
The
linacs are constructed from many thousands of accelerating structures. An
accelerating structure is a cavity or wave guide used to accelerate the beam.
Apart from the primary goal of producing longitudinal acceleration, the
structures are required to reduce other non-desirable effects such as
wakefields and so-called higher-order modes (HOMs). Efficiency of energy
transfer from RF to beam is also a primary concern and forms the principle
design criterion for optimization. The
beam dynamics of the particles bunches within the structures is of pivotal
concern in maintaining the required small emittance of the beam (both
transverse and longitudinal).
Summary:
- Overview: The main linac as a repetitive array of
acceleration and focusing elements
- Making a usable accelerating
wave (introduction to modes & wave guides)
- electromagnetic waves in free
space and why they are useless for acceleration
o
EM
waves in regular cylindrical wave guide and why they are equally useless for
acceleration
o
Single-cell
accelerating cavities (in which we develop the basic formalism and “rules of
the road” for…
- multi-celled accelerating cavities
(aka disc-loaded wave guides)
- Travelling- and Standing-Wave
Accelerator Structures
Unit 4a: Accelerating Structures and Linac Beam
Dynamics (2)
Summary:
- Limits to Acceleration Devices
- multibunch beam loading and
compensation
- single-bunch (very transient)
beam loading
- self-loading
- multibunch transverse
wakefields and BBU, and cures for same
- single-bunch transverse
wakefields, and cures for same
- Limits to Accelerating
Gradient
- Special Issues in
Superconducting Linacs
- Optimization Issues
Unit 4b: Production of High-Powered Microwaves
The
generation of short bursts of high-powered
microwaves required to accelerate the beam is a major challenge to the designs of the
linear collider. Much R&D is currently being invested in the production of
the necessary components (modulators, klystrons, high-power wave guides). Of
particular importance are ‘pulse compression’ techniques, which can be used to
generate the required short-pulse high peak-power from a longer, lower
peak-power one.
Summary:
- qualitative description of a
modulator
- semi-quantitative description
of a klystron
- How (qualitatively) a klystron works
- Explain perveance
quantitatively
- Frequency/power limitations in
high-powered klystrons
- transportation and compression
of microwaves
- wave guides and modes
- wave guide losses and their
mitigation, over-moded guides
- manipulation: hybrids, combiners, splitters
- pulse compression concepts:
(a) SLED
(b) SLED-II
(c) Binary Pulse Compression
(d) DLDS - two-beam acceleration as a substitute
for klystrons at high frequencies
Unit 5: Damping Rings
Generation
of extremely small vertical emittance beams using a damping ring is of fundamental importance to achieving the
required high luminosity. Damping rings are storage rings, and have a great
deal in common with other electron storage rings (such as modern light
sources). However, damping rings differ significantly from their contemporary
counterparts in the need for very much smaller emittances, faster damping, and
high injection efficiency. The extreme requirements push the present day
storage ring technology well beyond what has been achieved. This lecture will
deal with the fundamental design issues of a damping ring with special emphasis
on the challenges these important sub-systems present.
Summary:
- principles of radiation damping
- synchrotron and betatron
oscillations in a storage ring
- classical radiation:
longitudinal and transverse damping
- quantum excitation and
equilibrium emittance
- lattice design for low
emittance storage rings
- adjusting damping times
using wigglers
- choice of design parameters
- acceptance issues
- chromaticity and chromatic
correction
- dynamic aperture
- RF acceptance
- alignment and stability
- betatron coupling
- vertical dispersion
- closed orbit distortion
- sensitivity indicators
- alignment, correction and
tuning techniques
- introduction to collective
effects
- impedance effects
- Touschek effect and intra-beam
scattering
- space-charge
- electron cloud
- ion effects
Unit 6: Bunch Compression
Bunch
compression is required to reduce the long bunch coming from the damping ring
(~millimeters) to
the bunch lengths compatible with the linac RF wavelength and the beam-beam
interaction (luminosity). This lecture will review the standard method of bunch
compression using RF cavities to introduce a longitudinal energy correlation
along the bunch, followed by a non-isochronous magnetic system. These systems
general require careful balancing of non-linear terms arising from the
non-linearity of both the RF and the magnetic fields. As well as classical
effects, quantum effects (both incoherent and coherent synchrotron radiation)
must also be considered.
Summary:
- the basic approach (linear
optics)
- longitudinal phase space
transformations
- choice of RF and magnet
parameters
- T566 compensation (non-linear optics)
- phase space distortion from
non-linear effects
- use of RF nonlinearities to
compensate magnet nonlinearities
- design constraints
- emittance growth from
synchrotron radiation
- effects of coherent
synchrotron radiation
- twostage systems
- why use a two-stage bunch
compressor?
- telescopes in longitudinal
phase space
Unit 7: Beam Delivery Systems
The Beam
Delivery System (BDS) is the term used for the high-energy transport system
from the exit of the linac to the interaction point (IP). It serves several
functions, the most important of which is the strong demagnification of the
beam at the IP. The magnetic optics design requires special attention to
high-order aberrations arising from the required correction of the strong
chromaticity of such systems. Synchrotron radiation effects must also be
considered, and ultimately set the limits on the achievable beam sizes at the
IP. The BDS systems also contain the halo collimation systems which are
necessary to shield the physics detector from the beam ‘halo’. The design and
constraints on the BDS are some of the most challenging in the linear collider.
Summary:
- Fundamental
issues affecting BDS design
- Beam phase
volume
- Chromaticity
(estimate FD chromaticity)
- Synchrotron
radiation
- Methods to compensate
chromaticity (estimate geometric terms and introduce pairs of sextupoles)
- Example of FFTB
(also discuss IP beam size instrumentation)
- SR in bends
(estimate SR in bends and needed length)
- Local
chromaticity correction in FD (estimate 2nd order dispersion
term)
- Comparison of
New/Old FF performance in illustrations
- Methods of
design
- Errors in FF and
their influence (estimate tolerances for some examples)
- Tuning FF with
knobs
- Beam halo and IR
background
- Collimators and
design constraints (wakes, survivability)
- Nonlinear
collimation
Unit 8: Stability Issues in Linear Colliders
Colliding
nanometer beams at the IP places unprecedented requirements on the stability of
the accelerator components. Many man-years have been invested in the study and
modeling of ground motion effects (‘fast’ vibration and long-term drift) on the
performance of a linear collider. The extremely tight tolerances on alignment
(ranging from hundreds of microns to a few nanometers) mean that continuous
correction algorithms (feedback) are mandatory. In the following lecture, the issues of
ground motion and beam-based feedback
correction will be introduced.
Summary:
- Tolerances—example
of FD
- Disturbing
effects—ground motion
- Scale of the
phenomena
- Methods of description—power
spectrum
- Importance of
correlation in space and time
- Two dimensional
spectrum
- Fast and slow
motion and IP feedback
- Waves, ATL,
systematic motion
- Methods of
evaluation of ground motion effects
- Example of simple
beamline and uncorrelated motion
- Spectral
response function
- Technical
noises—illustration
- IP feedbacks
- Beam-beam and
deflection curve
- Beam-beam
feedback gain function
- train-to-train
and intratrain cases
- Illustration—integrated
simulations of LC stability
Unit 9: The SLC and the Alternatives
The SLAC
Linear Collider (SLC) operated between 1988 and 1998, and is often quoted as a
proof of principle of a linear collider. The SLC differed in many respects from
a ‘true’ LC, not least in the fact that it used the same linac to accelerate both electrons and
positrons (with looped ‘arcs’ at the end to bend the beams in collision). To
conclude this series of lectures, we will review the original design
specifications of the SLC in the light of what we have learnt. We will then
discuss both the final achieved SLC parameters, and the various proposals for
the next linear collider. This lecture will differ somewhat from the previous
ones, in that it will form a more open ‘workshop’ discussion session of the various issues.
Summary:
- The original SLC parameter set
- The ‘final’ SLC parameter set
- The current LC proposals:
- NLC/JLC X-band (11.4 GHz) warm RF machine
- TESLA L-band (1.3 GHz) superconducting machine
- CLIC X-band (30 GHz) two-beam accelerator
Tutorials and Simulations
sessions
These
sessions are intended to provide real and useful practical experience on both
design of a linear collider subsystem
and operating the collider (at least in a simulated world). You will feel and
enjoy how you can improve (or otherwise?) the luminosity by adjusting klystron
phases to optimize BNS damping, applying beam-based correction (even invented
or improved by you), or by tuning the final focus. You will play with the same
tools that linear collider designers use (and maybe you will suggest how these
tools can be improved!).
Tutorial Session Summaries
To be
completed – stay tuned!