Transcript to high power beam

High power proton driver
Alessandra Lombardi
AB/ABP
CERN
Accelerator2
Nufact05 School , Capri, 15 june 2005
1
Contents

Motivation and introduction

Radio frequency cavities and magnets

Components of proton driver : some basics accelerator
physics.

Challenges of a proton driver : technological limits and
cost optimisation

Conclusions
2
Why high power?
pion production vs. incoming proton beam energy
(30 cm long mercury target)
3
Power
Space
charge,
beam
loading
Economics
RF power;
cooling,
activation
Power = current *energy*pulse length*repetition
Powerful
source
Powerful and
efficient
accelerators
rate
High duty cycle
4
Accelerator dynamics
RF CAVITY
MAGNET
dp
dx


 q  E   B
dt
dt


In order to increase the energy of a beam of particles while keeping them
confined in space, we need to provide a longitudinal field for
ACCELERATION and a transverse force for FOCUSING.
5
RF cavity
Building block for transferring energy to the
beam
dp
dx


 q  E   B
dt
dt


dW dx dp

dt
dt dt
6
principle of acceleration
RF power supply
1)
RF power source:
generator of electromagnetic wave of a
specified frequency
2)
Cavity : space enclosed in a
metallic boundary which resonates with
the frequency of the wave and tailors the
field pattern to the
3)
Wave guide
Power coupler
Beam : flux of particles that pass
through the cavity when the field is
maximized for acceleration
Cavity
7
designing an accelerator

cavity design : 1) control the field pattern inside
the cavity; 2) minimise the ohmic losses on the
walls/maximise the stored energy.

beam dynamics design : 1) control the timing
between the field and the particle, 2) insure that
the beam is kept in the smallest possible volume
during acceleration
8
electric field in a cavity

assume that the solution of the wave equation in a bounded
medium can be written as
E( x, y, z, t )  E ( x, y, z )  F (t )
function of
space
function of time
oscillating
between -1 and 1
9
cavity parameters-0

average electric field ( E0 measured in V/m) is the space
average of the electric field along the direction of
propagation of the beam in a given moment in time when
F(t) is maximum.
L
1
E0   Ez ( x  0, y  0, z )dz
L0


physically it gives a measure how much field is available
for acceleration
it depends on the cavity shape, on the resonating mode
and on the frequency
10
cavity parameters-1

Shunt impedance ( Z measured in Ω/m) is defined as the ratio of
the average electric field squared (E0 ) to the power per unit
length dissipated on the wall surface.
Z

E
2
0
L

P
Z
2
0
E
dL

dP
Physically it is a measure of well we concentrate the RF power in
the useful region . NOTICE that it is independent of the field level
and cavity lenght, it depends on the cavity mode and geometry.
11
cavity parameters-2

Quality factor ( Q dimention-less) is defined as the ratio
between the stored energy and the power lost on the wall
in one RF cycle
2   f
Q
U
P

Q is a function of the geometry and of the surface
resistance of the material

superconducting : Q= 1010
normal conducting : Q=104

example at 700MHz
12
cavity parameters-3


transit time factor ( T, dimensionless) is defined as the maximum
energy gain of a particles traversing a cavity over the average
field of the cavity.
Write the field as
Ez( x, y, z, t )  Ez ( x, y, z )e i (t )

The energy gain of a particle entering the cavity on axis at phase
φ is
L

W   qEz (o, o, z )e i (t  ) dz
0
13
cavity parameters-3

assume constant velocity through the cavity
(APPROXIMATION!!) we can relate position
and time via
z  v  t  ct

we can write the energy gain as
W  qE0 LT cos 

and define transit time factor as
L
 E  z e

 j


z
c
z
T 
0
L
 E  z dz
z
0




dz
T depends on the
particle velocity and on
the gap length. IT
DOESN”T depend on
the field
14
cavity parameters-3

NB : Transit time factor depends on x,y (the
distance from the axis in cylindrical symmetry).
By default it is meant the transit ime factor on
axis
 Exercise!!! If Ez= E0 then
 L 
L=gap lenght
β=relativistic parametre
λ=RF wavelenght
sin 

 

T
 L 


  
15
cavity parameter-3
ttf for 100 keV protons, 200 MHz., parabolic distribution
1
0.8
0.6
0.4
0.2
0
0
2
4
6
8
10
12
-0.2
lfield (cm)
if we don’t get the length right we can end
up decelerating!!!
16
effective shunt impedance

It is more practical, for accelerator designers
to define cavity parameters taking into
account the effect on the beam

Effective shunt impedance ZTT
Z
E
2
0
L

P
measure if the structure
design is optimized

E T
ZTT 
2
0
L

P
measure if the structure is optimized
and adapted to the velocity of the
particle to be accelerated
17
Magnets
Elements that focus the beam (keep confined
around the direction of propagation) and/or guide
the beam along a circular path. Magnetic field
doesn’t change the tot energy of the beam
dp
dx


 q  E   B
dt
dt


18
Focusing
MAGNETIC FOCUSING
(dependent on particle velocity)


 
F  qv  B
ELECTRIC FOCUSING
(independent of particle velocity)



F  qE
19
Solenoid
F
B
Beam
B
F
v
I
Input : B = B
Beam transverse rotation :
B
F  v·B
F
v  v·B ·r
v
Middle : B = Bl
v
F
v
x<0
B
F
B
F  v ·B  v·B2 ·r
Beam linear focusing
x>0
20
Magnetic quadrupole

B
Magnetic field
Magnetic force
 Bx  G  y

B y  G  x
 Fx  q  v  G  x

 Fy  q  v  G  y
Focusing in one plan, defocusing in the other
y envelope
x envelope
sequence of focusing and defocusing
quadrupoles
21
FODO


periodic focusing channel : the beam 4D phase space is
identical after each period
Equation of motion in a periodic channel (Hill’s equation)
has periodic solution :
xz    0  z   cos z 
emittance
beta function ,
has the
periodicity of the
focusing period
 z  l    z 
transverse phase
advance
z
dz
 z   
 z 
0
CAS review N. Pichoff
22
course
Bending magnet
Magnetic field ┴ to the direction
of propagation
Particle move on a curved
trajectory related to its
magnetic rigidity
The angle of deflection depends
on the integrated field in the
magnet
mo c momentum
B 

q
ch arg e
BL

B
23
Dispersion
1>0
2>0
dx
var iation _ transv _ pos
Dx 

dp / p var iation _ momentum
24
Single pass vs. multipass

To accelerate the beam in a controlled way we
need a system of RF cavities interlaced with
quadrupoles

To get to the final energy :


Sequence of fundamental blocks in a straight line
Sequence of fundamental blocks on a circle with the
beam passing several times through the same
cavities and magnets
25
Q+
R
F
Q-
Q+
R
F
Q-
Q+
R
F
Q-
Q+
R
F
Q-
Reference
trajectory
26
Q
+
R
F
Q-
R
F
R
F
Q-
Q
+
Q
+
Q-
Q+
R
F
27
Q-
Q
+
R
F
Q-
Q-
Q
+
Q
+
Q28
Q+
Q-
High Energy
 Linear
accelerator if the final energy is
some GeV
 (Linear
+ ) Circular accelerators if the final
energy is above 10 GeV
29
DRIVERS
We have finished introducing the building
blocks of an accelerator, now let’s look at
what type of accelerator we need for a
driver of a neutrino source
30
Neutrino sources

there are two conceptually different way to generate
neutrinos :
1) the “parents” are in un-controlled optical condition
CNGS
SUPERBEAM
2) the parents are in controlled optical condition
BETABEAM
NEUTRINO FACTORY
31
SUPERBEAM -neutrinos
3.5 GeV





the total number of neutrinos produced depends on the power
on target : min 4 MW
the divergence of the pions/muons/neutrinos beam depends on
the driver energy and the collection system
need accumulation to enhance the signal w.r.t. the atmospheric
neutrinos
select ν or anti-v by the collection system (horn)
the driver energy must be matched to the decay tunnel length,
and distance to the detector.
32
Neutrino Factory neutrinos





the total number of neutrinos produced depends on the power on target :
min 4 MW
the divergence of the pions/muons/neutrinos beam depends on the driver
energy and the collection system
repetition rate matched to the muon lifetime
macro-time structure must be matched to smallest of the muons rings
micro-time structure should (but not necessary in all NF scheme) be of the
order of few ns (less is not important as 1 ns is the time jitter of pions decay)
: need compressor ring
33
proton driver beam on target time
structure
Lring/c
 lifetime
time
1 ns
34
High power

Existing circular machine can provide beam power of
0.1-0.2 MW at energies of several GeV

There aren’t existing linac that deliver beam of several
MW at some GeV (the closest, but not enough, is SNS 1
GeV, 1MW)

Upgrade existing machine vs. designing new (space
charge limits, radiation limits and magnet cycling limits)

High energy vs fast repetition rate
35
power on target comparison
Average power on target
ISOLDE
3 kW (2 μA*1.4 GeV)
10 kW (upgrade)
CNGS
200 kW
EURISOL for betabeam
200 kW
SUPERBEAM
4000 kW
SOLID TARGET
LIQUID TARGET
NUFACT
4000 kW
EURISOL converter
spallation
5000 kW
36
Proton Drivers R&D Needs
 upgrade
of existing machines:
• demonstrate short bunches (order 1 nsec)
• faster cycling
 new
machines (spallation neutron source
drivers with short bunches)



high space charge (halo control for hands-on
maintenance)
fast rising chopper
low beta SC cavities development
37
SWITCH TO
DESIGNING A PROTON DRIVER
38
PROTON DRIVER COMPONENTS

low energy end (0-few MeV) : source, radio frequency
quadrupole . Max freq 400MHz.
CHOPPING

medium energy section (few –few hundred MeV) normal
conducting accelerating stucture, following the velocity
profile of the beam

High energy section (few hundred MeV- few GeV) can
be made superconducting . It can be made MODULAR
after 1 GeV (beta=0.87)
H- TO PROTON CONVERSION

Synchrotron accelerator(s) to the final energy
39
Low energy-1-source
 Magnetron
 Penning
 Filament
 ECR
40
H- Ion Sources - Magnetron - Status
BNL Magnetron - Circular aperture
J Alessi, BNL
e-
~1 mm
Mo
Cathode (-)
Cs
M Stockli,
R Welton, SNS
H
H+H2+
H-
eAnode (+)
e-
B
41
H- Ion Sources
100%
H- Ion Sources For Accelerators
(including development sources)
90%
Duty Factor (%)
80%
70%
60%
50%
40%
30%
20%
SPL
10%
SNS
0%
0
20
40
60
80
100
120
140
160
180
H- Current (mA)
42
Low energy-2-RFQ
CERN RFQ1
520 keV
protons
43
RFQ represented the “missing link”
to high power beam
•High current and small emittance (powerful source)
•High energy (powerful and efficient accelerators)
POWERFUL
SOURCE :
200 mA proton
beam
Emittance 1 pi
mm mrad
POWERFUL
ACCELERATOR
50%
90%
44
Link between source and efficient
accelerator
The Radio Frequency Quadrupole is a linear
accelerator which
• focuses
• bunches
• accelerates
a continuos beam of charged particles with high
efficiency and preserving the emittance
Both the focusing as well as the bunching and
acceleration are performed by the RF field
45
transverse field in an RFQ
+
-
alternating gradient
focussing structure with
period length 
(in half RF period the
particles have travelled a
length /2 )
+
-
-
+
+
-
46
acceleration in RFQ
longitudinal modulation on the electrodes creates a longitudinal
component in the TE mode
47
Longitudinal plane-bunching
RF signal
time
Smootly change the
velocity profile of the
beam without changing
its average energy
continous beam
bunched beam
S  90deg
48
Why is the RFQ so efficient in
bunching a beam

Discrete bunching

Vs adiabatic bunching : movie
49
Longitudinal plane-acceleration
late part.
synchr. part.
use the rising part of the RF :
receive less acceleration,
late particles more (PHASE FOCUSING)
early part.
90deg  S  0
50
Why don’t we accelerate to the final
energy by using only RFQs ?
Accelerating effciency
0.5
0.4
Max
accelerating
efficiency is
limited by
geometry
0.3
0.2
0.1
0
0
CERN RFQ2
50
100
150
200
z (cm)
51
Medium Energy-Drift Tube Linac
52
Drift Tube Linac
mode is TM010
53
DTL
The DTL operates in 0 mode
for protons and heavy ions in the
range =0.04-0.5 (750 keV - 150 MeV)
1.5
E
1.5
1
1
0.5
0.5
0
Synchronism condition (0 mode):
0
0
20
40
60
0
-0.5
-0.5
-1
-1
-1.5
-1.5
80
20
100
40
120
60
140
80
l=
100
120
z
140
l
c
f
 
The beam is inside the “drift tubes” when the
electric field is decelerating
The fields of the 0-mode are such that if we
eliminate the walls between cells the fields are
not affected, but we have less RF currents
and higher shunt impedance
54
Drift Tube Linac
1. There is space to insert
quadrupoles in the drift
tubes to provide the strong
transverse focusing needed
at low energy or high intensity
2. The cell length ()
can increase to
account for the
increase in beta
 the DTL is the ideal
structure for the
low  - low W range
55
Focusing in the DTL vs RFQ
56
RFQ vs. DTL
DTL can't accept low velocity particles, there
is a minimum injection energy in a DTL due to
mechanical constraints
57
Side Coupled Linac
58
The Side Coupled Linac
multi-cell Standing Wave
structure in /2 mode
frequency 800 - 3000 MHz
for protons (=0.5 - 1)
Rationale: high beta  cells are longer  advantage for high frequencies
• at high f, high power (> 1 MW) klystrons available  long chains (many cells)
• long chains  high sensitivity to perturbations  operation in /2 mode
Side Coupled Structure:
- from the wave point of view, /2 mode
- from the beam point of view,  mode
59
Room Temperature SW structure:
The LEP1 cavity
5-cell Standing Wave
structure in  mode
frequency 352 MHz
for electrons (=1)
To increase shunt impedance :
1. “noses” concentrate E-field in “gaps”
2. curved walls reduce the path for RF currents
“noses”
BUT: to close the hole between
cells would “flatten” the dispersion
curve  introduce coupling slots to
provide magnetic coupling
60
example of a mixed structure : the
cavity coupled drift tube linac
linac with a reasonable shunt impedance in the range of 0.2 <  < 0.5, i. e.
at energies which are between an optimum use of a DTL and an SCL
accelerator
61
Various types of cavity : Coupled cavity
CCDTL (medium energy ~5-100 MeV)
CCL (high energy ~80 MeV-2 GeV)
62
Example of use of effective shunt impedance ZT2
19 MeV 45 MeV 85 MeV
234 MeV
375 MeV
The effective shunt impedance of the structures has been chosen to set the
transition energy between sections for TRISPAL project (C. Bourra, Thomson).
63
overview
take with
CAUTION!
Ideal range of beta
frequency
RFQ
Low!!! - 0.05
40-400 MHz
DTL
0.04-0.5
100-400 MHz
CCDTL
0.2-0.6
200-400 MHz
SCL
Ideal Beta=1
But as low as beta 0.5
800 - 3000 MHz
64
After 200 MeV : SC structure
Elliptical (high energy ~100MeV- 2 GeV)
65
Modern trends in linacs
Superconductivity is now bridging the gap between electron and
ion linacs.
The 9-cell TESLA SC cavities at 1.3 GHz
for electron linear colliders, are now proposed for High
Power Proton Accelerators…
66
SC
 Modular
: advantage for construction cost
… disadvantage for Beta<1
 Low
RF losses : all the power goes to the
beam
67
Linacs made of superconducting
cavities
Need to standardise construction of cavities:
only few different types of cavities are made for some ’s
more cavities are grouped in cryostats
Example:
CERN design, SC linac 120 - 2200 MeV
68
phase slippage
Lcavity = βgλ/2
particle enters the cavity with βs< βg. It is accelerated
the particle has not left the cavity when the field has changed sign : it is
also a bit decelerated
the particle arrives at the second cavity with a “delay”
........and so on and so on
we have to optimize the initial phase for minimum phase slippage
for a given velocity there is a maximum number of cavity we can accept in a
tank
69
Phase slippage
In each section, the cell length (/2,  mode!) is correct only for one beta (energy):
at all other betas the phase of the beam will differ from the design phase
Example of phase slippage:
CERN design for a 352 MHz
SC linac
Four sections:
 = 0.52 (120 - 240 MeV)
 = 0.7 (240 - 400 MeV)
 = 0.8 (400 MeV - 1 GeV)
 = 1 (1 - 2.2 GeV)
Phase at the first and last
cell of each 4-cell cavity
(5-cell at =0.8)
70
limit to the field in a cavity
 normal


heating
sparking
 super


conducting :
conducting :
magnetic field on the surface
quenching
71
Limit to the final energy of a LINAC
 NC
linac : power that it takes to run the
facility. Tipically stop at few hundreds
MeV. 1 GeV is the max and at low duty
cycle.
 SC
linac : 15 MV/m real estate gradient
After beta=1 one needs some 60-70 m for
each additional GeV
72
After few GeV….
 Synchrotron
:
 How does it work: ramp the magnets to
keep the beam on the same traj, tune rf
freq to keep synchro beam and the
accelerating field.
 Closed orbit
 Fodo and resonances
 Chromaticity
73
acceleration
Q
+
Focusing :
tune
Q-
Q-
Q
+
Q
+
Q-
Injection
Keep on
curved
trajectory;
Dispersion;
Closed
orbit
74
Q+
Q-
Periodic focusing FODO
In synchrotron, the tune is the phase advance over one turn.
Resonance :
Q
n x  Qx  n y  Q y  n
Resonance’s order :
nx  n y
1
2
ds
Circ  s 
Int(Q y)+1
Qy
Avoid resonances :
find the best working point
in tune diagram
Int(Q y)
Int(Q x)
Qx
Int(Q x)+1
75
Tune spread induced by chromaticity
Chromaticity :
Cu 
dQu

Int(Qy)+1
Generaly :
Higher energy
Qy
 higher rigidity
 lower Q
High energy particles
 Cu < 0
Int(Qy)
Int(Qx)
Low energy particles
Qx
Int(Qx)+1
Compensation with sextupoles but non linearity
76
Chromatic closed orbit
Off-momentum particles are not oscillating around the design orbit,
but around a chromatic closed orbit, whose distance from the design
orbit depends linearly from .
x   s   D p s   
Dp is the periodic dispersion function
Design orbit
Design orbit
Chromatic close orbit
On-momentum
particle trajectory
Off-momentum
particle trajectory
77
chopping

“longitudinal matching” from a linac to a ring with
the purpose of controlling the losses



rise time of the injection kickers/length of the
machine.
Shave the linac beam to match the RF bucket of the
ring
Perform the chopping at low enough energy but
when the beam has already imprinted the RF
structure, i.e. after the first stage of acceleration.
78
Chopping-example
LOSSES
2.84 ns
Beam from a 352 MHz linac
injected in a 40 MHz ring
2.84 ns
Injected in the stable area
of the bucket : no losses
79
H- injection through a foil
Proton 5 turn injection.
Need to populate different
area of phase space.
80
Summary

Building blocks of any accelerator : Rf cavities
and magnets

Specific of a neutrino driver : high power, short
bunches

Travelled through the components of a “generic”
proton driver

Closer look at two tricky issues (chopping and
injection in a synchrotron)
81