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High power proton driver

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Components of proton driver : some basics accelerator physics. ... H- Ion Sources - Magnetron - Status. J Alessi, BNL. BNL Magnetron - Circular aperture ... – PowerPoint PPT presentation

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Title: High power proton driver


1
High power proton driver
  • Alessandra Lombardi
  • AB/ABP
  • CERN

Accelerator2 Nufact05 School , Capri, 15 june
2005
2
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

3
Why high power?
  • pion production vs. incoming proton beam energy
  • (30 cm long mercury target)

4
Power
Space charge, beam loading
  • Power current energypulse lengthrepetition
    rate

RF power cooling, activation
Economics
Powerful source
Powerful and efficient accelerators
High duty cycle
5
Accelerator dynamics
  • 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.

6
RF cavity
  • Building block for transferring energy to the beam

7
principle of acceleration
  • RF power source generator of electromagnetic
    wave of a specified frequency
  • Cavity space enclosed in a metallic boundary
    which resonates with the frequency of the wave
    and tailors the field pattern to the
  • Beam flux of particles that pass through the
    cavity when the field is maximized for
    acceleration

8
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

9
electric field in a cavity
  • assume that the solution of the wave equation in
    a bounded medium can be written as

function of space
function of time oscillating between -1 and 1
10
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.
  • 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

11
cavity parameters-1
  • Shunt impedance ( Z measured in O/m) is defined
    as the ratio of the average electric field
    squared (E0 ) to the power per unit length
    dissipated on the wall surface.
  • 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.

12
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
  • Q is a function of the geometry and of the
    surface resistance of the material
  • superconducting Q 1010
  • normal conducting Q104

example at 700MHz
13
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
  • The energy gain of a particle entering the cavity
    on axis at phase f is

14
cavity parameters-3
  • assume constant velocity through the cavity
    (APPROXIMATION!!) we can relate position and time
    via
  • we can write the energy gain as
  • and define transit time factor as

T depends on the particle velocity and on the gap
length. IT DOESNT depend on the field
15
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

Lgap lenght ßrelativistic parametre ?RF
wavelenght
16
cavity parameter-3
if we dont get the length right we can end up
decelerating!!!
17
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

measure if the structure is optimized and adapted
to the velocity of the particle to be accelerated
measure if the structure design is optimized
18
Magnets
  • Elements that focus the beam (keep confined
    around the direction of propagation) and/or guide
    the beam along a circular path. Magnetic field
    doesnt change the tot energy of the beam

19
Focusing
  • MAGNETIC FOCUSING
  • (dependent on particle velocity)
  • ELECTRIC FOCUSING
  • (independent of particle velocity)

20
Solenoid
F
B
B
Beam
v
F
I
Input B B?
Beam transverse rotation
F ? vB
v? ? vB r
Middle B Bl
v?
F
F ? v? B ? vB2 r
Beam linear focusing
v?
F
B
B
x lt 0
x gt 0
21
Magnetic quadrupole
Magnetic field
Magnetic force
Focusing in one plan, defocusing in the other
y envelope
x envelope
sequence of focusing and defocusing quadrupoles
22
FODO
  • periodic focusing channel the beam 4D phase
    space is identical after each period
  • Equation of motion in a periodic channel (Hills
    equation) has periodic solution

transverse phase advance
emittance
beta function , has the periodicity of the
focusing period
CAS review N. Pichoff course
23
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

24
Dispersion
25
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

26
Reference trajectory
27
(No Transcript)
28
Q-
Q
Q
Q-
Q
Q-
Q
Q-
Q
Q-
Q-
Q
Q-
Q
29
High Energy
  • Linear accelerator if the final energy is some
    GeV
  • (Linear ) Circular accelerators if the final
    energy is above 10 GeV

30
DRIVERS
  • We have finished introducing the building blocks
    of an accelerator, now lets look at what type of
    accelerator we need for a driver of a neutrino
    source

31
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

32
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 ? or anti-v by the collection system
    (horn)
  • the driver energy must be matched to the decay
    tunnel length, and distance to the detector.

33
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

34
proton driver beam on target time structure
35
High power
  • Existing circular machine can provide beam power
    of 0.1-0.2 MW at energies of several GeV
  • There arent 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

36
power on target comparison
37
Proton Drivers RD 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

38
  • SWITCH TO
  • DESIGNING A PROTON DRIVER

39
PROTON DRIVER COMPONENTS
  • low energy end (0-few MeV) source, radio
    frequency quadrupole . Max freq 400MHz.
  • 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 (beta0.87)
  • Synchrotron accelerator(s) to the final energy

CHOPPING
LINEAR
H- TO PROTON CONVERSION
40
Low energy-1-source
  • Magnetron
  • Penning
  • Filament
  • ECR

41
H- Ion Sources - Magnetron - Status
BNL Magnetron - Circular aperture
J Alessi, BNL
Mo
Cathode (-)
Cs
e-
M Stockli, R Welton, SNS
H
e-
H
H2
1 mm
B
Anode ()
H-
e-
42
H- Ion Sources
H- Ion Sources For Accelerators (including
development sources)
43
Low energy-2-RFQ
CERN RFQ1 520 keV protons
44
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
45
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

46
transverse field in an RFQ
alternating gradient focussing structure with
period length ?? (in half RF period the particles
have travelled a length ??/2 )
47
acceleration in RFQ
longitudinal modulation on the electrodes creates
a longitudinal component in the TE mode
48
Longitudinal plane-bunching
Smootly change the velocity profile of the beam
without changing its average energy
49
Why is the RFQ so efficient in bunching a beam
  • Discrete bunching
  • Vs adiabatic bunching movie

50
Longitudinal plane-acceleration
use the rising part of the RF receive less
acceleration, late particles more (PHASE
FOCUSING)
51
Why dont we accelerate to the final energy by
using only RFQs ?
Max accelerating efficiency is limited by
geometry
CERN RFQ2
52
Medium Energy-Drift Tube Linac
53
Drift Tube Linac
mode is TM010
54
DTL
The DTL operates in 0 mode for protons and heavy
ions in the range b0.04-0.5 (750 keV - 150 MeV)
Synchronism condition (0 mode)
E
z
lbl
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
55
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 (bl) can
increase to account for the increase in
beta
? the DTL is the ideal structure for the
low b - low W range
56
Focusing in the DTL vs RFQ
57
RFQ vs. DTL
DTL can't accept low velocity particles, there is
a minimum injection energy in a DTL due to
mechanical constraints
58
Side Coupled Linac
59
The Side Coupled Linac
multi-cell Standing Wave structure in p/2
mode frequency 800 - 3000 MHz for protons (b0.5
- 1)
  • Rationale high beta ? cells are longer ?
    advantage for high frequencies
  • at high f, high power (gt 1 MW) klystrons
    available ? long chains (many cells)
  • long chains ? high sensitivity to perturbations
    ? operation in p/2 mode

Side Coupled Structure - from the wave point of
view, p/2 mode - from the beam point of view, p
mode
60
Room Temperature SW structure The LEP1 cavity
5-cell Standing Wave structure in p
mode frequency 352 MHz for electrons (b1)
To increase shunt impedance 1. noses
concentrate E-field in gaps 2. curved walls
reduce the path for RF currents
BUT to close the hole between cells would
flatten the dispersion curve ? introduce
coupling slots to provide magnetic coupling
noses
61
example of a mixed structure the cavity coupled
drift tube linac
linac with a reasonable shunt impedance in the
range of 0.2 lt ? lt 0.5, i. e. at energies which
are between an optimum use of a DTL and an SCL
accelerator
62
Various types of cavity Coupled cavity
63
Example of use of effective shunt impedance ZT2
375 MeV
85 MeV
45 MeV
19 MeV
234 MeV
The effective shunt impedance of the structures
has been chosen to set the transition energy
between sections for TRISPAL project (C. Bourra,
Thomson).
64
overview
65
After 200 MeV SC structure
66
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…
67
SC
  • Modular advantage for construction cost …
    disadvantage for Betalt1
  • Low RF losses all the power goes to the beam

68
Linacs made of superconducting cavities
Need to standardise construction of
cavities only few different types of cavities
are made for some bs more cavities are grouped
in cryostats
Example CERN design, SC linac 120 - 2200 MeV
69
phase slippage
Lcavity ßg?/2 particle enters the cavity with
ßslt ßg. 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
70
Phase slippage
In each section, the cell length (bl/2, p 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 b 0.52 (120 - 240
MeV) b 0.7 (240 - 400 MeV) b 0.8 (400 MeV - 1
GeV) b 1 (1 - 2.2 GeV)
Phase at the first and last cell of each 4-cell
cavity (5-cell at b0.8)
71
limit to the field in a cavity
  • normal conducting
  • heating
  • sparking
  • super conducting
  • magnetic field on the surface
  • quenching

72
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
    beta1 one needs some 60-70 m for each additional
    GeV

73
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

74
acceleration
Focusing tune
Q
Q
Q-
Q-
Q
Keep on curved trajectory Dispersion Closed
orbit
Q-
Q
Q-
Injection
Q
Q-
Q
Q-
Q-
Q
Q-
Q
75
Periodic focusing FODO
In synchrotron, the tune is the phase advance
over one turn.
Resonance
Resonances order
Avoid resonances find the best working point in
tune diagram
76
Tune spread induced by chromaticity
Chromaticity
Chromaticity
  • Generaly
  • Higher energy
  • ? higher rigidity
  • lower Q
  • Cu lt 0

Compensation with sextupoles but non linearity
77
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 d.
Dp is the periodic dispersion function
78
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.

79
Chopping-example
LOSSES
Beam from a 352 MHz linac injected in a 40 MHz
ring
Injected in the stable area of the bucket no
losses
80
H- injection through a foil
Proton 5 turn injection. Need to populate
different area of phase space.
81
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)
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