High power proton driver Alessandra Lombardi ABABP CERN

High power proton driver Alessandra Lombardi AB/ABP CERN Accelerator 2 Nufact 05 School , Capri, 15 june 2005 1

Contents Ø Motivation and introduction Ø Radio frequency cavities and magnets Ø Components of proton driver : some basics accelerator physics. Ø Challenges of a proton driver : technological limits and cost optimisation Ø 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 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 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 Ø cavity design : 1) control the field pattern inside the cavity; 2) minimise the ohmic losses on the walls/maximise the stored energy. Ø 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 Ø 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 9

cavity parameters-0 Ø average electric field ( E 0 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 Ø it depends on the cavity shape, on the resonating mode and on the frequency Ø 10

length dissipated on the wall surface. cavity parameters-1 Ø 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 Ø 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 Ø Q is a function of the geometry and of the surface resistance of the material superconducting : Q= 1010 Ø normal conducting : Q=104 Ø example at 700 MHz 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. Ø Write the field as Ø Ø The energy gain of a particle entering the cavity on axis at phase φ is Ø 13

cavity parameters-3 Ø assume constant velocity through the cavity (APPROXIMATION!!) we can relate position and time via Ø we can write the energy gain as Ø T depends and define transit time factor as 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 Ø Exercise!!! If Ez= E 0 then Ø L=gap lenght β=relativistic parametre λ=RF wavelenght 15

cavity parameter-3 if we don’t get the length right we can end up decelerating!!! 16

effective shunt impedance Ø It is more practical, for accelerator designers to define cavity parameters taking into account the effect on the beam Ø Effective shunt impedance ZTT measure if the structure design is optimized 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 18

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

Solenoid F B Beam B F v I Input : B = B F v·B Beam transverse rotation : B F v v·B ·r v Middle : B = Bl v F v x<0 B F B x>0 F v ·B v·B 2 ·r Beam linear focusing 20

Magnetic quadrupole Magnetic field Magnetic force 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 4 D phase space is identical after each period Ø Equation of motion in a periodic channel (Hill’s equation) has periodic solution : Ø emittance beta function , has the periodicity of the focusing period transverse phase advance 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 23

Dispersion e 1>0 e 2>0 24

Single pass vs. multipass Ø To accelerate the beam in a controlled way we need a system of RF cavities interlaced with quadrupoles Ø To get to the final energy : l l 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- Reference trajectory 26

R F Q- Q + + Q - F Q R Q- R F 27 Q+ R F + - Q Q F F R R + Q QR F Q + Q- Q +

Q Q + R F Q+ Q Q + Q - Q- Q + Q- + Q Q - Q- Q+ Q- Q + 28

High Energy Ø Linear accelerator if the final energy is some Ge. V Ø (Linear + ) Circular accelerators if the final energy is above 10 Ge. V 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 Ø 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 Ge. V Ø Ø Ø 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 Ø Ø Ø 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 1 ns 34

High power Ø Existing circular machine can provide beam power of 0. 1 -0. 2 MW at energies of several Ge. V Ø There aren’t existing linac that deliver beam of several MW at some Ge. V (the closest, but not enough, is SNS 1 Ge. V, 1 MW) Ø Upgrade existing machine vs. designing new (space charge limits, radiation limits and magnet cycling limits) Ø High energy vs fast repetition rate 35

power on target comparison Average power on target ISOLDE 3 k. W (2 μA*1. 4 Ge. V) 10 k. W (upgrade) CNGS 200 k. W EURISOL for betabeam 200 k. W SUPERBEAM 4000 k. W NUFACT 4000 k. W EURISOL converter spallation 5000 k. W SOLID TARGET LIQUID TARGET 36

Proton Drivers R&D Needs Ø upgrade of existing machines: • demonstrate short bunches (order 1 nsec) • faster cycling Ø new machines (spallation neutron source drivers with short bunches) l l l 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 Ø low energy end (0 -few Me. V) : source, radio frequency quadrupole. Max freq 400 MHz. CHOPPING R A Ø medium energy section. E (few –few hundred Me. V) normal N I stucture, following the velocity conducting accelerating L profile of the beam Ø High energy section (few hundred Me. V- few Ge. V) can be made superconducting. It can be made MODULAR after 1 Ge. V (beta=0. 87) H- TO PROTON CONVERSION Ø Synchrotron accelerator(s) to the final energy 39

Low energy-1 -source Ø Magnetron Ø Penning Ø Filament Ø ECR 40

H- Ion Sources - Magnetron - Status BNL Magnetron - Circular aperture J Alessi, BNL e~1 mm Mo Cathode (-) Cs M Stockli, H H+H 2+ H- e. Anode (+) e- R Welton, SNS B 41

H- Ion Sources For Accelerators (including development sources) 42

Low energy-2 -RFQ CERN RFQ 1 520 ke. V 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 m. A 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 Smootly change the velocity profile of the beam without changing its average energy 48

Why is the RFQ so efficient in bunching a beam Ø Discrete bunching Ø Vs adiabatic bunching : movie 49

Longitudinal plane-acceleration use the rising part of the RF : receive less acceleration, late particles more (PHASE FOCUSING) 50

Why don’t we accelerate to the final energy by using only RFQs ? Max accelerating efficiency is limited by geometry CERN RFQ 2 51

Medium Energy-Drift Tube Linac 52

Drift Tube Linac mode is TM 010 53

DTL The DTL operates in 0 mode for protons and heavy ions in the range =0. 04 -0. 5 (750 ke. V - 150 Me. V) E z l=bl Synchronism condition (0 mode): 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 b - low W range 55

Focusing in the DTL vs RFQ 56

RFQ vs. 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 p/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 p/2 mode Side Coupled Structure: - from the wave point of view, p/2 mode - from the beam point of view, p mode 59

Room Temperature SW structure: The LEP 1 cavity 5 -cell Standing Wave structure in p 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 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 Me. V) CCL (high energy ~80 Me. V-2 Ge. V) 62

Example of use of effective shunt impedance ZT 2 19 Me. V 45 Me. V 85 Me. V 234 Me. V 375 Me. V 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 Me. V : SC structure Elliptical (high energy ~100 Me. V- 2 Ge. V) 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 Ø Modular : advantage for construction cost … disadvantage for Beta<1 Ø 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 Me. V 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 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, 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 Me. V) b = 0. 7 (240 - 400 Me. V) b = 0. 8 (400 Me. V - 1 Ge. V) b = 1 (1 - 2. 2 Ge. V) 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 Ø normal conducting : l l heating sparking Ø super conducting : l l magnetic field on the surface quenching 71

Limit to the final energy of a LINAC Ø NC linac : power that it takes to run the facility. Tipically stop at few hundreds Me. V. 1 Ge. V is the max and at low duty cycle. Ø SC linac : 15 MV/m real estate gradient After beta=1 one needs some 60 -70 m for each additional Ge. V 72

After few Ge. V…. Ø Synchrotron : Ø 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. Ø Closed orbit Ø Fodo and resonances Ø Chromaticity 73

Q + Q+ Focusing : tune Q F R Q - acceleration Q + Q - Q- Q + Q- Injection Keep on curved trajectory; Dispersion; Closed orbit + Q Q - Q- Q+ Q- Q + 74

Periodic focusing FODO In synchrotron, the tune is the phase advance over one turn. Resonance : Resonance’s order : Avoid resonances : find the best working point in tune diagram 75

Tune spread induced by chromaticity Chromaticity : 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 d. Dp is the periodic dispersion function Design orbit Chromatic close orbit On-momentum particle trajectory Off-momentum particle trajectory 77

chopping Ø “longitudinal matching” from a linac to a ring with the purpose of controlling the losses l l Ø 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 Ø Building blocks of any accelerator : Rf cavities and magnets Ø Specific of a neutrino driver : high power, short bunches Ø Travelled through the components of a “generic” proton driver Ø Closer look at two tricky issues (chopping and injection in a synchrotron) 81