Options for a 50Hz, 10 MW, Short Pulse Spallation Neutron Source

G H Rees, ASTeC, CCLRC, RAL, UK

Premises

Kinetic energy for the 10 MW, proton beam (GeV) ≤ 3.2

Total proton pulse duration each 50 Hz pulse (s) ≤ 2.2

The number of proton bunches in each 50 Hz pulse ≤ 8

Some Potential ISIS RCS Upgrades

ISIS injecting into a 50 Hz, 3.2 GeV RCS, for a 1 MW source

400 MeV Hˉ linac with the 3.2 GeV RCS, for a 2 MW source

800 MeV Hˉ linac with the 3.2 GeV RCS, for a 5 MW source

(The ESS linac-compressor(s) appears better option at ≥ 2 MW)

Limit for a single, 3.2 GeV RCS appears to be 5 MW (2 1014 ppp)

10 MW, 50 Hz Ring Options

A 3.2 GeV Hˉ linac feeding two, 3.2 GeV, 5 MW compressors: it is probably feasible, but is considered to be too difficult

A 0.8 GeV Hˉ linac feeding two, 3.2 GeV, 5 MW RCS rings: this option needs a delay of ~ 1 ms for one of the RCS

A 1 GeV Hˉ linac & compressor, & two 3.2 GeV, 5 MW NFFAG: some bunch compression in compressor before extraction

Schematic Layout for 3.2 GeV, 5 MW RCS

800 MeV H ˉ H ˉ, H° beam

cavities collectors

R = 65 mn = h = 4N = 2 1014

triplet

triplet

dipoles

8° dipole

dipoles

extraction cavities

Choice of Lattice

ESS-type, 3-bend achromat, triplet lattice chosen Lattice is designed around the Hˉ injection system Dispersion at foil to simplify the injection painting Avoids need of injection septum unit and chicane Separated injection; all units between two triplets Four superperiods, with >100 m for RF systems Locations for momentum and betatron collimation Common gradient for all the triplet quadrupoles Five quad lengths but same lamination stamping Bending with 20.5° main & 8° secondary dipoles

Parameters for a 50 Hz, 0.8-3.2 GeV RCS

Number of superperiods 4 Number of cells/superperiod 4(straights) + 3(bends) Lengths of the cells 4(14.5004) + 3(14.7) m Free length of long straights 16 x 11.0 m Mean ring radius 65.0 m Betatron tunes (Qv, Qh) 6.38, 6.30 (Q ~ 0.2) Transition gamma 6.6202 Main dipole biased cosine fields 0.4208 to 1.1591 T Secondary dipole fields 0.1252 to 0.3448 T Triplet length/quad gradient 3.5 m / 2.2 to 6.2 T m-1

RCS Betatron and Dispersion Functions

RF Parameters for the 3.2 GeV RCS(Z/n = j 5 Ω, reduced g and ηsc < 0.3)

Number of protons per cycle 2 1014 (5.1 MW)

RF cavity straight sections 110 m

Frequency range for h = n = 4 2.4717 to 2.8597 MHz

Bunch area for h = n = 4 1.8 eV sec

Voltage & p/p @ 0.8 GeV 61.4 kV & ± 3.9 10ˉ3

Voltage & p/p @1.96 GeV 717 kV & ± 4.6 10ˉ3

Voltage & p/p @ 3.2 GeV 470 kV & ± 5.3 10ˉ3

FFAG Ring Types

Non-linear, scaling, non-isochronous FFAG Linear, non-scaling, near isochronous -FFAG Non-linear, non-scaling, isochronous IFFAG Non-linear, non-scaling, non-isochronous NFFAG

Radial, scaling, FFAG rings have BF(+) and BD(-) magnetsNon-scaling, -FFAG rings have BF(-) and BD(+) magnets IFFAG & NFFAG rings have bd(-), BF(+) & BD(+) magnets

Here, only bd-BF-BD-BF-bd cells for NFFAGs are considered Though of zero chromaticity, the tunes do vary with amplitude

1.0 GeV Compressor Ring

Needed as NFFAG cells are unsuitable for Hˉ injection

Use a similar lattice to that for the 3.2 GeV, RCS rings

Replace the 8°dipoles by (2°, 4° and 2°) dipole sets

Optimise for Stark states 5, 6 with B(for 4°) = 0.1123 T

Separate injection fillings are required for each NFFAG

Some bunch compression is needed before extraction

High & low foils may be needed for lower temperatures

.

1 GeV, 50 ma (35 av), Hˉ beam 5 mm

H°

H° Protons

Protons

Cooled copper graphite block

Foil support 545 keV, 2 x 1.7 kW, eˉ beam

Stripping Foil ρ = 26.4 mm, B = 0.1123 T

Foil lattice parameters : βv = 5.9 m, βh = 8.4 m, Dh = 5.4 m, Dh /√ βh = 1.87 m½

Hˉ parameters at stripping foil ; βv = 2.0 m, βh = 2.0 m, Dh = 0.0 m, Dh' = 0.0

NFFAG Collector Electron Containment

Pumplet Cell for the 3.2 GeV NFFAG Ring

bd(-) BF(+) BD(+) BF(+) bd(-)

2.32 0.65 1.00 1.40 (m) 1.00 0.65 2.32 –3.2086° 6.6043° 3.2086° 6.6043° –3.2086° Lengths and angles for the 36 cells of the 3.2 GeV closed orbit

NFFAG Non-linear Lattice Code

A linear lattice code is modified for estimates to be made of the non-linear fields in a group of FFAG magnets. Bending radii are found from average field gradients between adjacent orbits & derived dispersion values, D. D is a weighted, averaged, normalized dispersion of a new orbit relative to an old, and the latter to the former. A first, homing routine obtains specified betatron tunes. A second routine is for exact closure of reference orbits A final, limited-range, orbit-closure routine homes for -t. Accurate estimates are made for reference orbit lengths. Full analysis needs processing the lattice output data & ray tracing in 6-D simulation programs such as Zgoubi.

Non-linear Fields and Reference Orbits

Low ampl. Twiss parameters are set for a max. energy cell.

Successive, adjacent, lower energy reference orbits are then found, assuming linear, local changes of the field gradients.

Estimates are repeated, varying the field gradients for the required tunes, until self-consistent values are obtained for:

the bending angle for each magnet of the cell the magnet bending radii throughout the cell the beam entry & exit angle for each magnet the orbit lengths for all the cell elements, and the local values of the magnet field gradients

3.2 GeV Betatron & Dispersion Functions

0.6 m

0.0 m

NFFAG Combined Function Magnet Data

bd bend field range(-) 1.0490 to 1.1583 T bd gradient range 0.2546 to 0.0134 T m-1

BF bend field range 0.1945 to 1.5497 T BF gradient range(-) 2.1936 to 4.9487 T m-1

BD bend field range 1.4004 to 0.5378 T BD gradient range 2.0690 to 5.7518 T m-1

BF units approximate four poles of a sextupole

Reduction of Non-linear Effects

Cells Qh Qv 3rd Order Higher Order

4 0.25 0.25 zero nQh=nQv & 4th order 5 0.20 0.20 zero nQh=nQv & 5th order 9 0.222 0.222 zero nQh=nQv & 9th order

13 4/13 3/13 zero to 13th, except 3Qh=4Qv

Use (13 x 3 ) - 1 = 38 such cells for the NFFAG (36) Betatron tune variations with amplitude still remain Gamma-t = 14.02 (j) at 1.0 GeV & 12.43 at 3.2 GeV

RF Parameters for the 3.2 GeV NFFAG(Z/n = j 5 Ω, reduced g and ηsc < 0.3)

Number of protons per cycle 2 1014 (5.1 MW)

RF cavity straight sections 110 m

Frequency range for h = n = 4 2.5717 to 2.8597 MHz

Bunch area for h = n = 4 1.8 eV sec

Voltage & p/p @ 1.0 GeV 99.5 kV & ± 4.1 10ˉ3

Voltage & p/p @1.96 GeV 290 kV & ± 3.3 10ˉ3

Voltage & p/p @ 3.2 GeV 258 kV & ± 3.9 10ˉ3

Compare to the 3.2 GeV RCS 717 kV & ± 5.3 10ˉ3

Vertical Loss Collection in an FFAGLoss collectors Y

X

1.0 GeV proton beam 3.2 GeV proton beam

Coupling may limit the horizontal beam growth

ΔP loss collection requires beam in gap kickers

3.2 GeV: NFFAG versus RCS

Pros: Volts per turn for acceleration is less than half No need for a biased ac magnet power supply No need for an ac design for the ring magnets No need for a ceramic chamber with rf shields Gives more flexibility for the holding of bunches

Cons: Requires a larger (~ 0.27 m) radial aperture Needs an electron model to confirm viability Needs a 1.0 GeV, Hˉ injection compressor ring

Conclusions re a 50 Hz, 10 MW Source

A 3.2 GeV Hˉ linac & two compressors looks a difficult option

A 0.8-3.2 GeV RCS option needs 2 rings & large, ~3 MHz, rf

A 3.2 GeV NFFAG needs a 1 GeV compressor and two rings

NFFAGs offer the potential of greater reliability, but R and D is

needed on electron models & new space charge tracking codes .