Post on 16-Dec-2015
How Accelerators Work• Electric fields accelerate particles
• Magnetic and electric fields focus and deflect particles
LULI
"I can see no escape from the conclusion that [cathode rays] are charges of negative electricity carried by particles of matter." But, he continued, "What are these particles? are they atoms, or molecules, or matter in a still finer state of subdivision?"
Accelerator-based energy frontier physics
From Katsouleas and Joshi, Physics Today
Troubles….how to get to the next collider?The problem was realized in 1982, but the solution is elusive.
Always look for newapplications
Particle energy exchange with fields
Design your own accelerator---BUT obey the LAWS:
Maxwell Equations & Lorentz force
Watch out for limits: wakefields, breakdown, phase space and beam brightness
Does anyone want your beam?
Practical issues are important!
Free-Space Acceleration in plane waves
So why not shine laser on electron to get TeV? Diffraction?, dephasing?, depletion? A generic problem?
How well can you do for linear acceleration?
No net linear acceleration by far free space fields
Assume accelerating field is:Free space: no material or particles nearby to create near fields
Net: Energy gain (or loss) outside of finite interaction region
• No other non-acceleration fields (wigglers, bend magnets, …)– Relativistic particle (v ~c) (can be relaxed)– Particle motion in straight line in absence of accelerating field
• Then no linear acceleration is possible (Lawson-Woodward Theorem).
Interaction
Try to ‘get around’ the theorem:
Smooth Waveguide:No diffraction: for long interaction lengthAxial E field (TM-modes)
Bad---quickly dephases.
copper
copper
RF
BEAM
Near field for acceleration- want small bWhat are limits to this?
Self-destructive properties of particle beams, beam qualityThere is always too much of a good thing
“In accelerator jargon,we say that this concept starts to look
iffy.”D. Whittum (Varian)
http://www.slac.stanford.edu/grp/arb/tn/arbvol2/ARDB107.pdf
We will look for better ways to avoid the hypothesis for the theorem
Bend fieldControl the phase velocityLimit the interaction length
Slow wave structuresStanding wave linacsDielectric accelerating structuresPlasma-based accelerators
Dave has nice FEL notes too.
Linear acceleration with surface waves
E-field component in direction of particleSlow phase velocity
Linear acceleration with dielectrics
copper
Dielectric
Dielectric
copper
Slac pub 8666 (Whittum) also many other groups (this is not a review)
Linear acceleration with dielectrics vph=c
Transverse focusing---Ex is linearSurface field/accelerating field—breakdownNeeds vph=c
From waveguide to….
slow-wave structure
Field has phase velocity ~c and longitudinal component
Closed cavity
http://cas.web.cern.ch/cas/http://uspas.fnal.gov/course-materials/index.shtml
Novel surface-wave accelerator driven by a high-power CO2 laser
x
z
•Supports w= kc mode can accelerate relativistic particles
•Near field (small gap) attractive ratio Ez/Ex
•Acceleration by surface phonon polaritons (SPP)
SiC/vacuum SPP’s are excitable by a CO2 laser
SiC ε < 0
SiC ε < 0
3 μm
Coupling blues: (a) how do you couple 10.6 μm radiation into a 3 μm hole?? (b) SPP’s group velocity is very small how will they get to the other end??
Shvets and Kalmykov, AAC Conf. Proc. 2004
Vacuum laser acceleration[LEAP E163 SLAC, Colby Talk]
high damage thresholdmaximum couplingsmall beam disruptions(symmetry, smoothness)
Damage ~few J/cm^2, so energy gain~20KeV
Energy exchange between radiation and particles
Particle viewpoint (near field)
Energy balance viewpoint (far field): geometry of interaction must allow for spontaneous emission: the interference of this emission with applied field balances the energy change of particle
Overall energy must be conserved---but how???
The total field energy must be decreased. This can only through the emission of a particleField to inter with the accelerating field.
Only Accelerating fieldParticle moves with constant velocityIs there Linear Acceleration (?)
The energy conservation perspective
Closed Surface A
Field Perspective: Energy Balance
Key points in the derivation
Split the field into various parts
Integrate over time with limits before and after fields have left the interaction region
Fourier Transform the fields
Huang et al 2005; Xie 2004; Zolotorev et al unpublished 2001; Palmer 1995
No Acceleration without Spontaneous Emission
• IFEL• ICARM
Inverse cerenkov• What about plasma-wakefield (Cerenkov in
plasma)—charge must be able to emit a plasma wave if it is accelerated by a wave.
Motion in a plane wave: Nonlinear Acceleration
For particle initially at rest with slow field changes:
Collective Acceleration
• Maxwell Equations (w/w/o sources) for applied fields.• Idea—use the moving fields of a beam or plasma• The concept predates plasma acceleration with lasers,
although beatwave and wakefield accelerators are collective accelerators
• The old ideas have some conceptual similarity with plasma-based ion acceleration
• The old ideas did not work too well. See Sessler’s reviews (http://adsabs.harvard.edu/abs/1982cfac.conf...27S; http://adsabs.harvard.edu/abs/1982AIPC...87..919S).
An old idea: The ERA
• Electron ring accelerator
Ring is unstable—breaks into azimuthal clumps: Negative MassInstabilty
A Stable Ring
Ring is stable for same reason electronring was unstable (Maxwell)
Plasma-based Electron Linac
Broken down medium tolerate(for fs-ps timescales) high E-field
U. Nebraska
Channel has phase velocity ~c, but not so easy to create!
Creation of Accelerating Structures in Plasmas: Femtosecond Engineering
Laser Beatwave
Laser Wakefield
Beatwave
Plasma
Wakefield
Laser or ebeam
Accelerating field
Can also think about counter propagating case
SRS (self-modulated)Timescales set by plasma frequencyLength scale set by skin depth
The realization of collective acceleration:Ion Acceleration in Plasmas
after ?
Main Target (Al)
Contaminants (H2O)
--
--
-
+-
-
- ++
+++
Gitomer et al, 1986
-- --
-
--
-
-
H+
H+
H+
H+
H+
H+
H+
Snavely et al. (2000)Fuchs et al 2005, Allen et al (2005)
Laser:P>10TW
1018 W/cm2
ApplicationsSpallation sourceplasma probefast ignitortherapyhigh energy density physics
Proper heating of target can accelerate heavy ionsSentoku et al. (2003)Hegelich et al. (2002)
Ion and Proton Acceleration
Target properties (curvature, surface treatment, thickness, material)Laser pulse shaping Technology and physics open a new realm of proton/ion acceleration
Target normal sheath accelerationBreakout afterburnerRadiation PressureBasic mechanisms are being explored and invented.
Mechanisms:
Flippo et al
Target shaping
Stochastic Acceleration
• Fields randomly kick particles • Diffusive energy gain (under appropriate
assumptions)• Fermi theory for cosmic rays (complicated and
active area of research). Reflections off of shocks and moving magnetic turning points.
36
“I must confess that one reason we have undertaken this biological work is that we thereby have been able to get financial support for all of the work in the laboratory. As you know, it is much easier to get funds for medical research.”
—Lawrence to Niels Bohr, 1935
Who wants my beam?
Varian Medical Accelerartor (electron, x-ray)PSI Compact Proton Accelerator
37
Who wants your beam? FEL Example
Emittance and energy spread constraints
Six dimensional beam brightness:Conserved (ignoring scattering, nonlinearity/mismatch, CSR, wakefields…)
n
Transverse and longitudinal constraints
Go to board
Electron-positron Colliders
Principle is to understand your applicationrequirements
+constraints on beamstrahlung & background
Slac pub 9914
Concluding Thoughts • Net linear acceleration with a pure vacuum field in free space in the absence of other fields is not possible.
This guides you in inventing new concepts, and in understanding why specific concepts fail or work.• There is no acceleration without spontaneous radiation—a charged particle cannot experience net
acceleration in a system where it will not be able to spontaneously emit photons.• Global energy balance is maintained by the interference of a spontaneously emitted field with an accelerating
field. • Even for a single particle spontaneous emission can set an energy limit. The mass dependence of spontaneous
emission is why high-energy electron colliders are linear, and why a TeV muon collider could be circular. • At high charge collective losses from the accelerated bunch itself must be included. These effects can destroy
the utility of a bunch well before dominating acceleration (e.g., coherent synchrotron radiation in FEL sources).----------------------------• Know the needs of potential users, but do not be a slave to existing technology in envisioning what might be in
the future. Lasers are a great example of technological advances unlocking new physics regimes. • Postdiction is easier than prediction---codes are important but be aware of how you are using them and how
well they perform.• Do not let a theorist ‘prove by intimidation.” Do not immediately believe it when an experimentalist says
something is impossible, has been tried, or is really dumb. Understand why. • Learn to recognize and not engage in hype.• Be careful not to dismiss out of hand someone else’s idea without properly understanding it. You will likely find
yourself right and wrong at times.• Our field uses great technology, is undergoing rapid evolution, and is filled with fun topics—e.g., nonlinear
dynamics, collective motion and instability, fundamentals of radiation emission, particle and nuclear physics, understanding parts of astrophysics, and applications in particle physics, X-ray sources, medicine…
Muon Collider cf. Neutrino Factory
STEVE GEER Accelerator Seminar SLAC 24 March, 2011 40
NEUTRINOFACTORY
MUONCOLLIDER
In present MC baseline design, Front End is same as for NF