lecture 3 - e. wilson - 22 oct 2014 –- slide 1 lecture 3 - magnets and transverse dynamics i...
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Lecture 3 - E. Wilson - 22 Oct 2014 –- Slide 1
Lecture 3 - Magnets and Transverse Dynamics I
ACCELERATOR PHYSICS
MT 2014
E. J. N. Wilson
Lecture 3 - E. Wilson - 22 Oct 2014 –- Slide 2
Slides for study before the lecture
Please study the slides on relativity and cyclotron focusing before the lecture and ask questions to clarify any points not understood
More on relativity in:http://cdsweb.cern.ch/record/1058076/files/p1.pdf
Lecture 3 - E. Wilson - 22 Oct 2014 –- Slide 3
Relativistic definitions
E
m0c2
1 2
p mv
m0v
1 2
m0c
1 2
1
1 v c 2
1
1 2
E
E0
E0 m0c2
Energy of a particle at rest
Total energy of a moving particle (definition of g)
E E0 m0c2
Another relativistic variable is defined:
Alternative axioms you may have learned
You can prove:
momentum c
energy
pc
E
v
c
pc E m0c2 ()
Lecture 3 - E. Wilson - 22 Oct 2014 –- Slide 4
Newton & Einstein
Almost all modern accelerators accelerate particles to speeds very close to that of light.
In the classical Newton regime the velocity of the particle increases with the square root of the kinetic energy.
As v approaches c it is as if the velocity of the particle "saturates" One can pour more and more energy into the particle, giving it a
shorter De Broglie wavelength so that it probes deeper into the sub-atomic world
Velocity increases very slowly and asymptotically to that of light
Lecture 3 - E. Wilson - 22 Oct 2014 –- Slide 5
dpdt
pddt
pdds
dsdt
p
dsdt
ev B edsdt
B
Magnetic rigidity
1
dds
B pe
pcec
Eec
E0
ec
m0ce
Fig.Brho 4.8
d d
GeV pc 3356.3
.T.m 1 smc
eVpcecpc
B
Lecture 3 - E. Wilson - 22 Oct 2014 –- Slide 6
Equation of motion in a cyclotron
Non relativistic
Cartesian
Cylindrical
xyz
zxy
yzx
ByBxqdt
zmddtmvd
BxBzqdt
ymddt
mvd
BzByqdt
xmddt
mvd
r
zr
z
BrrBqdt
zmd
BrBzqrmdtmrd
BzBrqmrdt
rmd
2
2
Bvv q
dtmd )(
Fv
dtmd
Lecture 3 - E. Wilson - 22 Oct 2014 –- Slide 7
For non-relativistic particles (m = m0) and with an axial field Bz = -B0
The solution is a closed circular trajectory which has radius
and an angular frequency
Take into account special relativity by
And increase B with g to stay synchronous!
Cyclotron orbit equation
20
0
0
2
0
z
z
m r r qr B
m r r qrB
m z
z
pR
qB
w q
m0
B0
0z
qB
m
000 E
Emmm
Lecture 3 - E. Wilson - 22 Oct 2014 –- Slide 8
Cyclotron focusing – small deviations
See earlier equation of motion
If all particles have the same velocity:
Change independent variable and substitute for small deviations
Substitute
To give
20
0 0z
mvd dm ev B
dt dt
0v z
2 0z
d mrmr q r B zB
dt
000 - x, , BBBdsd
vdtd
zz
00 mvp
0 20 0 0 0
1 10zBd dx x
pmv ds ds B
Lecture 3 - E. Wilson - 22 Oct 2014 –- Slide 9
From previous slide
Taylor expansion of field about orbit
Define field index (focusing gradient)
To give horizontal focusing
Cyclotron focusing – field gradient
0 20 0 0 0
1 10zBd dx x
pmv ds ds B
........!2
1 22
2
0 xxB
xx
BBB zz
z
xB
Bk z
00
1
011
200
xk
ds
dxp
ds
d
p
Lecture 3 - E. Wilson - 22 Oct 2014 –- Slide 10
Cyclotron focusing – betatron oscillations
From previous slide - horizontal focusing:
Now Maxwell’s
Determines
hence In vertical plane
Simple harmonic motion with a number of oscillations per turn:
These are “betatron” frequencies
Note vertical plane is unstable if
011
200
xk
dsdx
pdsd
p
xB
zB zx
0
0 B
kzBBx 00
01
00
kz
dsdz
pdsd
p
kQkQ zx ,1
2
yx QQ ,
2
1
k
Lecture 3 - E. Wilson - 22 Oct 2014 –- Slide 11
Lecture 3 – Magnets - Contents
Magnet types Multipole field expansion Taylor series expansion Dipole bending magnet Magnetic rigidity Diamond quadrupole Fields and force in a quadrupole Transverse coordinates Weak focussing in a synchrotron Gutter Transverse ellipse Alternating gradients Equation of motion in transverse coordinates
Lecture 3 - E. Wilson - 22 Oct 2014 –- Slide 12
Dipoles bend the beam
Quadrupoles focus it
Sextupoles correct chromaticity
Magnet types
x
By
x
By
0
100
-20 0 20
Lecture 3 - E. Wilson - 22 Oct 2014 –- Slide 13
Multipole field expansion
(r,)Scalar potential obeys Laplace
whose solution is
Example of an octupole whose potentialoscillates like sin 4around the circle
2x2
2y2 0 or
1r2
2 2
1r
r
rr
0
nrn sin n
n1
Lecture 3 - E. Wilson - 22 Oct 2014 –- Slide 14
Taylor series expansion
n
n1
rnsin n
Field in polar coordinates:
Br
r
, B 1
r
Br nnrn 1 sinn , B nnrn 1 cosn
Bz Br sin B cos
nnrn 1 cos cosn sin sin n
nnrn 1 cos n 1 nnxn 1 (when y0)
To get vertical field
Taylor series of multipoles
Fig. cas 1.2c
Octupole Sext Quad Dip.
...!3
12
!11
......4.3.2.
33
32
2
2
0
34
2320
xxB
xxB
xx
BB
xxxB
zzz
z
Lecture 3 - E. Wilson - 22 Oct 2014 –- Slide 16
Bending Magnet
Effect of a uniform bending (dipole) field
If then
Sagitta
/ 2
BB
222sin
BB
2
1616
2cos12
2
Lecture 3 - E. Wilson - 22 Oct 2014 –- Slide 17
dp
dt p
ddt
pdds
ds
dt
p
ds
dt
ev B edsdt
B
Magnetic rigidity
1
dds
B T .m pc eV
c m.s 1 3.3356 pc GeV
B pe
pcec
Eec
E0
ecm0c
e
Fig.Brho 4.8
d
Lecture 3 - E. Wilson - 22 Oct 2014 –- Slide 19
Fields and force in a quadrupole
No field on the axis
Field strongest here
B x(hence is linear)
Force restores
Gradient
Normalised:
POWER OF LENS
Defocuses invertical plane
Fig. cas 10.8
1
. yBk
B x f
1
. yBk
B x
x
By
Lecture 3 - E. Wilson - 22 Oct 2014 –- Slide 21
Weak focussing in a synchrotron
Vertical focussing comes from the curvature of the field lines when the field falls off with radius ( positive n-value)
Horizontal focussing from the curvature of the path The negative field gradient defocuses horizontally and
must not be so strong as to cancel the path curvature effect
The Cosmotron magnet
Lecture 3 - E. Wilson - 22 Oct 2014 –- Slide 25
Equation of motion in transverse co-ordinates
Hill’s equation (linear-periodic coefficients)
where at quadrupoles
like restoring constant in harmonic motionSolution (e.g. Horizontal plane)
Condition
Property of machineProperty of the particle (beam) ePhysical meaning (H or V planes)
EnvelopeMaximum excursions
k 1
B dBz
dx
s ds
s
y s ˆ y / s
s
y s sin s 0
d 2 yds 2 + k s y 0
Lecture 3 - E. Wilson - 22 Oct 2014 –- Slide 26
Summary
Magnet types Multipole field expansion Taylor series expansion Dipole bending magnet Magnetic rigidity Diamond quadrupole Fields and force in a quadrupole Transverse coordinates Weak focussing in a synchrotron Gutter Transverse ellipse Alternating gradients Equation of motion in transverse coordinates