FFAG Concepts and Studies

David Neuffer

Fermilab

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Outline Introduction

Feasibility studies use Linacs and recirculating linacs Need to develop “cheaper” acceleration

FFAG Acceleration ?? Fixed-field permits fast-acceleration ~10 turns possible ….

FFAG Lattice styles (DIMAD simulations): “scaling” FFAG – Machida & Mori “non-scaling” FFAG – Johnstone FMC-like –Dejan Trbojevic

Longitudinal Motion Constraints simulations

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Study 2 Costs …. Study I, II -Factory – feasible but too expensive Biggest cost item: acceleration (~600M$)

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FFAG Acceleration? Linacs/RLA’s require a lot of rf RLA multiple-pass transports + spreader/recombiners

complicated and expensive Muons decay too quickly for fast-cycling magnets

Need: Fixed-field lattice that can accept beam over large energy spread (620 GeV ?) for multipass return transports: FFAG lattice ?

With same transport for all turns can accelerate over more turns: less rf.

FFAG lattice can have large momentum acceptance; large transverse acceptance; need less cooling

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POP-first Proton FFAG

First Proton FFAG built and operated

All systems verified

(magnet, rf, injection)

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FFAG magnet- 150 MeV FFAG Figure shows yoke-free FFAG

triplet used for 150 MeV proton FFAG

150 MeV FFAG is under construction (magnets done)

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PRISM – low-energy muon ring Low-energy muon source

e experiments …

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Japan Neutrino Factory Scenario

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JNF Scenario Use 50 GeV p-bunch to

produce pions Capture beam in 20-T 5-T

transport channel Short decay line; inject beam

directly into low-energy FFAG Capture beam in low-frequency

rf bucket Accelerate up chain of FFAGs

to 20GeV Inject into 20GeV storage ring

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“Scaling” FFAGS Lattice

Advantages:•Naturally Zero chromaticity

Disadvantages:•Large negative bends (large circumference …)•Nonlinear fields (from rk expansion)•Not isochronous

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JNF- FFAGs lattice design Lattices are “scaling” radial-

sector FFAGs Triplet focusing with reverse-

bend D-quads Low to high energy orbit width

is ~0.5m

0.3 1.0 GeV, 1 3.0 GeV 3.010 GeV 10 20 GeV FFAGs

Lattices have been generated using SAD, DIMAD

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Parameters for JNF FFAG lattices

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Acceleration and Decay Acceleration must avoid

muon decay

Need ~1MV/m to avoid decay (2 MV/m gradient in cavities)

rf

2

rf

2

VeL

cm

rf0

0VeL

cm

0

0 sVeE

E

E

E

N

N

N

L

1

ds

dN

rfVeds

dE

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Acceleration Parameters For acceleration, use superconducting (smaller-radius) FFAGs At 1MV/m, ~ 10 turns acceleration / FFAG Assume harmonic h = 1 on lowest-energy FFAG; keep frequency

constant h = 1 4.75 MHz rf (???) Initial beam from decay

300150MeV/c; 10ns

 

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Scenario requires ~2MV/m rf Harmonic=1 (for lowest energy

FFAG) implies 4.75 MHz;

Harmonic=2 implies 9.5 MHz; works OK in 1-D simulation

Experience indicates 26MHz cavity is more realistic (Iwashita)

Use 26 MHz + 3rd harmonic ?

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Longitudinal Motion in FFAG Equations of motion:

Motion is not very isochronous

h = 1 and h = 2 accelerations are OK;

)sin(eVEE nrf1nn

1P

Ph2 s

s1nn

1k1

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Injection and Extraction Requires fast, large-aperture

kickers; particularly for low-energy FFAGs

Risetime for 1GeV FFAG must be less than 200ns; 20 GeV FFAG can be > 1s

Example: 150MeV FFAG will need 500g, 0.6m, 150ns

1GeV FFAG needs ~0.53 T-m

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“Non-scaling” FFAG (Johnstone) 620 GeV lattice; C=2100m

Basic lattice unit is FODO cell ~7m long;

Primary bending magnet is “D”;

Lattice is more isochronous (transition at ~13 GeV)

B = constant (tune varies with energy)

Good linear behavior …(but large chromaticity …)

F D

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Dejan Trbojevic Lattice 1020 GeV 270m circumference Strong focusing to very

small dispersion: 6cm

No negative bends at central energy

Chromaticity corrected: Tune (E) more nearly constant (but does cross integers)

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Summary FFAG accelerators have been built and operated at KEK

POP FFAG 150 MeV FFAG (under construction) PRISM (proposed) RIKEN radioactive ion FFAG

FFAG accelerators could be used in neutrino factory Need magnet, rf , injection/extraction R&D Muon production and cooling options to be explored

Comparisons with other design approaches are not completed FFAG or RLA or linac or ??? Cost estimates ???

Integration into complete neutrino factory scenario is also required