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Acceleration. RF. RF Systems. C. Ohmori KEK. Contents. RF Acceleration Direc tvoltage and RF accelerations Electrostatic accelerators: Cockcroft&Walton , Van de Graaf RF accelerations: Linac , synchrotron(+FFAG) Electron and Proton/ions b =1 vs. b<1

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RF Systems

C. Ohmori



RF Acceleration

  • Directvoltageand RF accelerations

    • Electrostatic accelerators: Cockcroft&Walton, Van de Graaf

    • RF accelerations: Linac, synchrotron(+FFAG)

  • Electron and Proton/ions

    • b=1 vs. b<1

    • Cavities (high-Q, ferrite, MA)

  • Beams

    • Synchrotron motion

    • Phase stability & transition

    • Dual harmonic acceleration

    • beam loadings

      RF Systems

  • Cavities

    • Designing of MA Cavities

  • LLRF

  • FFAG cavities

FFAG [email protected]

1 1 direct voltage and rf accelerators
1.1 Direct voltage and RF accelerators

Electrostatic accelerator

  • Voltage is limited to 10MVbecause of corona discharge.


FFAG [email protected]

Question: If static acceleration is appliedto a circular accelerator, what happens ?

FFAG [email protected]

Rf acceleration
RF acceleration accelerators.

  • Direction of RF voltage changes regularly.

  • RF field can be confined in RF structure (cavity).

  • 1928 Wideroe K+, N+

  • 1931 Sloan, Lawrence Hg+ (10 MHz RF)

  • 1937-45 Hansen, Varian Klystron

  • 1945- Alvarez 200 MHz Linac(DTL)

  • 1947- disk-loaded linac 2856 MHz

  • 1965 Wilson superconducting linac

⇒Long tubes

⇒Higher Energy

FFAG [email protected]

Circular machines
Circular machines accelerators.

  • RF cavity (RF is confined in it).

  • frf=h frev

    h:Harmonic number

  • Beam forms a bunch structure.

FFAG [email protected]

Rf parameter of accelerators
RF parameter of accelerators accelerators.

J-PARC : proton machines

Others: electron machines

FFAG [email protected]

1 2 electron and proton accelerations
1.2 Electron and proton accelerations accelerators.

  • Mass

    • Electron: 0.511 MeV

    • Proton: 938 MeV

  • In electron machines (E>10 MeV), b~1

  • For proton, E>20 GeV to be b~1

  • For many electron linac, fixed cavity structure.

  • For many proton linac, need to change structure according to acceleration.

  • For many electron rings, fixed rf frequency.

  • For many proton rings, need to change rf frequency.

FFAG [email protected]

R. accelerators.Wideroe-type linac

Below 10 MHz, in a glass


  • Proton linac needs to change length of drift tube according to acceleration.

Over 100 MHz RF high power sources become available after WWII. High energy acceleration became possible.

Alvarez linac (DTL) in KEK PS

About 200 MHz, cavity structure

TM(Transverse Magnetic)010 mode


FFAG [email protected]

TM01 accelerators.

  • Electron linac uses a fixed structure and higher frequency.

    • Alvarez type does not fit as b=1 and drift tube becomes long.

  • TM01 mode is used.

  • For multi-cell structure, traveling wave type cavity is used.

    • Standing wave type has higher shunt impedance, but difficult for multi-cell.


m: circumferential

n: radial


TM010:Only longitudinal E-field

TM01 : Simplest mode in waveguide

Putting discs for slow-wave structure.

Phase velocity of RF matches velocity of electron

FFAG [email protected]

Acceleration mode
Acceleration Mode accelerators.

2p/3 mode has a large shut impedance.

FFAG [email protected]

Cavity for electron ring
Cavity for Electron ring accelerators.

Fixed frequency

same structure as linac cavity


Low voltage because of high duty

Standing wave type cavity for high shunt impedance

  • Pill box cavity

  • TM010 mode is excited.

  • Resonant frequency is

    independent on d.

  • To resonate at 500 MHz, a = 0.2295 m

  • Toresonate at 18.1 MHz, a =6.347 m

FFAG [email protected]

d accelerators.


  • Transit time factor

    E field which particle sees

(a)Pill box

(b) With nose cone

large G

(c) Curved surface

High shunt impedance

KEK-PF cavity

Original design

FFAG [email protected]

Cavity for proton ion ring
Cavity for accelerators.Proton/Ion ring

  • Proton synchrotron needs to change RF frequency according to acceleration.

  • l/4resonator

  • Ferrite cavity needs bias circuit to tune cavity.

FFAG [email protected]

Magnetic alloy cavity
Magnetic Alloy Cavity accelerators.

  • Stable mQf at high RF field

  • Low Q (frequency sweep w/o bias circuit)

  • High Curie temperature

mQf and Brf

are proportional to

shunt impedance and RF voltage

FFAG [email protected]

Ma cavities
MA cavities accelerators.


  • Low Q (~0.6)

    CERN LEIR cavity

  • Hybrid cavity

    • With external inductor Q~2

      J-PARC RCS

  • Cut core cavity

    • High Q

      J-PARC MR


  • Air

    PRISM cavity, (EMMA MA cavity)

  • Forced air

    PS booster cavity, many MA cavities for medical use

  • Indirect water cooling

    Gunma U. medical acc.

  • Direct water cooling

    J-PARC RCS&MR, LEIR cavities

  • Oil/Fluorinate cooling

    KEK PS 2nd H cavity

    KEK induction cavity

FFAG [email protected]

Impact on rcs design by high field gradient
Impact on RCS design by accelerators. High Field Gradient

Left: J-PARC RCS has about 350 m circumference. Black line means the space for RF (44 m, including Q-magnets).

Middle: 3 GeV Ring using ferrite cavity. Red line means additional space by low gradient. Blue is by the extended circumference.

Right: 3 GeV ring using ferrite cavity with 2nd H system (orange).

Length for RF in Ring

Ferrite cavity

With 2ndHarmonics

Ferrite cavity

PRISM [email protected] College

Rf acceleration in ring
RF acceleration in Ring accelerators.

Large p ⇒large r

When a-1/g2=0, it is “transition”.

transition gamma

Electron ring does not have it as g is much larger than transition gamma.

In proton rings,

KEK PS : gt=6.76

AGS : gt=8.3


It is a source of beam loss.

J-PARC has no transition.

Momentum compaction factor

Slippage factor

FFAG [email protected]

Momentum spread (Full width) is given by accelerators.

In RCS, h=-0.69, frev=0.469 MHz at injection.

Bunch is lengthen by 500 ns in 300 turns.

Momentum spread (full width) is 0.11 %

  • Slippage factor

Contour plot shows

de-bunching of beam in RCS.

FFAG [email protected]

Synchrotron motion accelerators.

  • Synchrotron motion

    • Synchronous particle

    • Non-synchronous particle

  • Equations


For small amplitude,

Synchrotron oscillation

FFAG [email protected]

Stable rf phase
Stable RF phase accelerators.

for stability.

Below transition, (h<0)



Above transition, (h>0)



At transition crossing,

RF phase fs will move top-fs

FFAG [email protected]

Rf bucket

Bucket height accelerators., Wm and Acceptance, A

for moving bucket

RF Bucket

Particles are oscillating in the bucket.

  • When acceleration starts, fs is increasing, then Wm and A reduced.

  • Large RF voltage is necessary to accelerate high intensity beam.

Bucket height and acceptance for stationary bucket

FFAG [email protected]

Rf voltage pattern for proton ring
RF voltage pattern for proton ring accelerators.

  • In proton synchrotrons, bunch length is gradually shortened by adiabatic damping. Momentum spread is increasing and emittance is constant.

  • In electron ring, synchrotron oscillation is damped by synchrotron radiation. But, the radiation also excites the particles. Emittance is given by two effects.

  • In proton ring, emittance matching is important as no synchrotron radiation.

(a): dB/dt = const

(b): RCS

  • A computer code, RAMA, is suitable to estimate the bunch size and energy spread during acceleration from magnet pattern.

    • Developed by Rick Baartman, TRIUMF.

  • The emittance given by a booster accelerator can be used for another accelerator (Main Ring).

  • It also gives longitudinal beam impedance and instability effects.

  • Hoffman-Pedersen distribution is used to estimate the longitudinal space charge.

FFAG [email protected]

Motion of particles
Motion of particles accelerators.

If bunch is matched to

rfucket, dipole and quadrupole oscillations will not be excited .

If it is not matched, filamentation and emittance growth occur.

FFAG [email protected]

Way to adjust rf frequency to initial energy
Way to adjust RF frequency to initial energy accelerators.

When bea was injected to RCS, dipole oscillation was observed. It suggests, rf frequency is high. The frequency was reduced to 938175Hz from 939471Hz to match.

FFAG [email protected]

Dual harmonic rf
Dual Harmonic RF accelerators.

Dual harmonic AVC

As tuning-circuit is not used for cavity, MA cavity is assumed a passive circuit.

But, impedance at injection is much smaller than that at extraction. AVC gain is changed during acceleration.

In case of ferrite cavity, two different RF system have to be handle.

Dual harmonic RF voltage (red)

FFAG [email protected]

Space charge effects
Space charge effects accelerators.

  • Dual Harmonic is a powerful tool to reduce the space charge effects.

FFAG [email protected]

High intensity beam
High intensity beam accelerators.

  • To accelerate high intensity beam,

    • Beam loading on RF sys.

    • Space charge effects

    • Instability

    • others

      should be care.

Example of beam loading on RCS cavity.

300 kW beam is accelerated.

FFAG [email protected]

Beam loading
Beam loading accelerators.

  • Assuming cavity is parallel circuit of LCR,

  • Phase of beam is 90 deg late from RF phase.

High Q system: a phaser of RF frequency

Low Q system : phaser diagrams for higher components

FFAG [email protected]

  • Feed forward

In case of ferrite system, tuning loopmakes the system complicated.

FFAG [email protected]

10 cavities for acceleration,

1 for measuring wake voltage

Left FF OFF, Right FF ON

Gap voltage

Right acceleration, Left Extraction

Upper w/o beam

Middle with 300kW beam FF OFF

Lower with 300 kW beam FF ON

Gap voltage for each RF component

Upper FF OFF, Lower FF ON

FFAG [email protected]

After 2 ms, h=4 and 6 show oscillation.

It is quadrupole (or higher).

Beam bunch becomes narrow

(emittance growth was reduced)

FFAG [email protected]

Rf systems
RF Systems accelerators.

FFAG [email protected]

Proton rings ferrite cavities
Proton Rings accelerators.Ferrite cavities

  • In hadron accelerators, velocity of particle changes (not b=1 as electron accelerators). When synchrotron was developed ferrite cavity was the most possible scheme to sweep the rf frequency.

Beam pipe

Ferrite ring

FFAG [email protected]

Ferrite cavity
Ferrite cavity accelerators.

HIMAC Ferrite Cavity & AMP

Courtesy of M. Kanazawa

FFAG [email protected]

Ferrite cavities
Ferrite Cavities accelerators.


Single gap


Single tube


FFAG [email protected]

Magnetic alloy cavity1
Magnetic Alloy cavity accelerators.

  • MIMAS Cavity

  • Probably, this is the first MA cavity.

Courtesy of A. Schnase

FFAG [email protected]

Production of ma
Production of MA accelerators.

Production of Ferrite Core

Ferrite powder with glue

Forming a ring core shape

High pressure press


Polishing surface

FFAG [email protected]

Comparison accelerators.

FFAG [email protected]

Designing of ma cavity
Designing of MA Cavity accelerators.

  • Keys

    • Bandwidth

    • RF voltage

    • Beam current

FFAG [email protected]

Leir rf system ring section

Free space is used for H=2 RF system, now.

FFAG [email protected]

Magnetic Alloy cores accelerators.

(water-proof coating)

FFAG [email protected]

Bandwidth accelerators.

  • Wideband uncut core cavity

  • Bandwidth (upper) is limited by floating and tube capacitances.

FFAG [email protected]

Leir rf system cavity model half cavity z 1 core
LEIR RF SYSTEM accelerators.Cavity Model: Half Cavity Z (1 Core )

Additional C:





Response similar to that of an RC circuit.

Above cutoff the slope is mainly dependent on C value.

Structure capacitance is ~12pF.

M. Paoluzzi

FFAG [email protected]

Uncut inductor for medium bandwidth
Uncut + inductor accelerators.for medium bandwidth

FFAG [email protected]

Control of q value w o cutting
Control of Q-value w/o cutting accelerators.

CAVITY DESIGN for High intensity

  • Parallel inductance reduces inductance of resonant circuit. Additional capacitor

  • Preferable Q-value for RCS, 2, for BL and dual H.

  • It is too high for uncut core and too low for cut core.

FFAG [email protected]

Cut core configuration for higher q
Cut Core accelerators.Configuration for higher Q

FFAG [email protected]

Cut surface
Cut Surface accelerators.

Bad polishing

Good Polishing

FFAG [email protected]

Cut core cavity
Cut Core cavity accelerators.

  • Cut core cavity is used for

    • Accelerator with transient beam loading + very short beam …………J-PARC MR

    • Accelerator with high resonant frequency and narrow band ……….RIKEN beam chopper

  • High voltage and high power use requires a good cutting scheme to avoid destruction of ribbon insulation. Destruction of insulation of 2mm will cause local heating of cut surface.

FFAG [email protected]

Materials accelerators.

A large MA core using new material, FT3L,

Is under developing at J-PARC

FFAG [email protected]

Rf system
RF System accelerators.

MA Cavity

Ferrite loaded cavity

Ferrite Cavity

Bias PS for tuning cavity

Final stage AMP

Narrow band

Driver AMP

About 1 kW

Low Level RF

Single H

ALC, phase, orbit, tuning loop, beam loading

  • MA cavity

    • Passive LCR circuit

  • Final stage AMP

    • Wideband circuit, multi-H

    • CG: all pass network

  • Driver AMP

    • Few kW

  • Low Level RF

    • Multi-H

    • ALC, (phase, orbit), beam loading

FFAG [email protected]

LLRF accelerators.

FFAG [email protected]

  • Frequency tuning during acceleration (RCS) accelerators.

  • B-field becomes stable after few days from restart.

  • Digital LLRF + MA cavity give a very stable RF system. (MA cavity is passive system)

  • Usually, orbit feedback is necessary for high intensity ring.

  • In RCS, orbit remain stable whole cycle after tuning.

Stability of orbit during high power test (300 kW). The time jitter is only 1.7 ns.

FFAG [email protected]

  • Phase feedback damps dipole oscillation. accelerators.

  • Upto 100 kW beam, still phase feedback is not necessary for operation.

  • Above 300 kW, beam loading compensation using feed forward technique is necessary.

FFAG [email protected]

Ffag cavities
FFAG Cavities accelerators.

  • MA cavities

    • PoP, 150 MeV

    • PRISM

    • EMMA MA

  • EMMA cavities

FFAG [email protected]

Ffag cavities1
FFAG cavities accelerators.

FFAG [email protected]

Cavity assembly 150 mev ffag
Cavity assembly accelerators.150 MeV FFAG

Indirect cooling

  • Number of cores 2~4

  • Outer size 1.7m x 1m

  • Inner size 1m x 0.23m

  • RF frequency 1.5 - 4.6 MHz

  • RF voltage 9 kV

  • RF output 55 kW

  • Power density 1 W/cm^3

  • Cooling water 70 L/min

FFAG [email protected]

Air cooling accelerators.

FFAG [email protected]

Cavity design parameters
Cavity Design Parameters accelerators.

# Includes Distribution losses and LLRF Control

FFAG [email protected]

1st September 2008

FFAG08 Carl Beard

Beam Acceleration in FFAG accelerators.

Beam acceleration in Synchrotron

  • Pattern of frf is given by B field.

  • Required rf voltage is given by dB/dt and fs

Beam acceleration in FFAG

  • B-field is constant.

    Scaling FFAG

  • Pattern of frf is given by energy gain; rf voltage and fs

Very fast acceleration in Non scaling FFAG

  • Constant rf frequency.

  • RF voltage is given by top and bottom energy

FFAG [email protected]

Summary accelerators.

In this lecture,

  • RF Acceleration

  • RF System

FFAG [email protected]