100 MeV- 1 GeV Proton Synchrotron for Indian Spallation Neutron Source Gurnam Singh Beam Dynamics...

transcript

100 MeV- 1 GeV Proton Synchrotron for

Indian Spallation Neutron Source

Gurnam SinghBeam Dynamics Section

CAT, Indore

CAT-KEK-Sokendai School on Spallation Neutron Sources, 2004

BASIC LAYOUT OF INDIAN SPALLATION NEUTRON

SOURCE100

MeV Linac

Linac to PS Transfer Line

PS to TargetTransfer Line

100 MeV – 1 GeVPROTON

SYNCHROTRON (PS)

Target

H-Source

RFQ4.5

Outline:

1.Preliminary design aspects of Rapid Cycling Proton Synchrotron

2.Linac to Synchrotron Transfer Line

Preliminary design aspects

OfRapid Cycling

Proton Synchrotron

Key parameter in a spallation sourceBeam Power

Pbeam = q.Np.E .R = I.E

• Pbeam:Beam power (W) at target• q :Charge on proton (C)• Np :No. of protons in ring• E :Final proton energy (eV)• R :Repetition rate (Hz)• I :Average current at target (A)

To increase the beam power

Two Ways

Increase beam energyLarge machine, big cost

Increase beam CurrentSevere space charge Collective beam instabilities

Choose optimum energy & current

Accelerator choice

Full Energy Linac&

Accumulator Ring

Linac&

•High power achievable but high cost•High injection energy means very tight beam loss control at injection•High injection energy, so more heating of injection foil

•Low injection energy, thus more space charge problem•Rapid acceleration, means powerful RF systems•Ceramic chamber

Indian Spallation Neutron Source

•100 MeV Linac & RCS based

•Beam power :100 kW

•Final energy of the beam : 1 GeV

•Average current : 100 A

[@ 25 Hz Repetition Rate]

2.41013 protons in synchrotron

Design Considerations

1. Injection energy 100 MeV The first estimation of current in thesynchrotron is made by space charge tune

shift.

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=> For the required N, the tune should not cross any dangerous resonances. Thus tune should have sufficient room for movement. In our design, allowable tune shift taken as 0.2.

For decreasing the tune shift (for enhancing the average current handling capability of the synchrotron)

* Increase the injection energy => Increase the cost of Linac.* Decrease the N and increase the repetition rate, so that average current remains same => Constraints from technology and frame overlap in time of flight type experiments.*Increase the bunching factor at injection => Deciding factor of RF programme of the machine.

2. Beam loss control Beam loss control is of major concern in the high intensity machines. 1W/m is the allowable limit of uncontrolled loss for hands on maintenance.=> @ injection, average beam power 10 kWUniform loss on whole length of ring gives the upper most limit: 2% allowed uncontrolled loss.

=> Thus for controlled loss, betatron and momentum collimators needed.

3. Sufficient space Large dispersion free straight sections are needed for 1) RF systems. 2) Betatron collimators3) Injection systems 4) Extraction system

Apart from these, other systems which should be accommodated in the ring are diagnostic devices, vacuum pumps, correctors etc.

4. High tune for working well below the transition

Options for the lattices

Many lattice configurations can fulfill these requirements:

For making an arc with achromatic conditions1. FODO with Missing dipole scheme (IPNS, KEK-JAERI etc.)2. Achromat design (eg. SNS)

Obtaining the long straight dispersion free sections1. FODO2. Doublet/ Triplet structures

Lattice for the ISNS

FODO structure:Simple, smooth variation of beta function

means less prone to errors.

Missing dipole for the dispersion matching

Four superperiodsThe four long straight sections will be used for

the injection system, collimators (beam collimatoss), RF- system and extraction system respectively.

Four superperiods have better stability for structure resonance than the three period structure.

HALF UNIT CELL

One period

Qf2 Qd2 Qf1 Qd1 Qf1 Qd1 Qf3

ARC SECTION

•Half-cell length of FODO: 4.425 m•Total cells in arc: 4 (one period)•Total straight section cells: 2 (one period)•Quadrupole families: 5 (3f & 2d)•Length of the period: 53.1 m•Length of long straight: 43.875=15.5 m

Choice of tune

90 phase advance per cell requires a tune of 6.0, so the tune of the machine is kept near 6.0. In horizontal plane, it is higher than the 6.0 and in vertical plane it is on the lower side. But it has wide tunability range and it can be operated at split and un-split working points.

Structure & half integer resonance diagram( upto 4th order)

Shaded region is the space for different tune options

5 6 7 84

X+2 y=20

X+2 y=16

2X+2 y=24

2X- y=83X- y=12

3X+y=28

X+3 y=24

(6.88, 5.88)

X- y=1

X- y=0

•• (7.3, 6.3)(6.3, 6.3)

* Further selection depends on imperfection resonance

The lattice can have various tune points in these regions.Primarily selected tune is 6.88 and 5.88 [other options are 6.3, 6.3 and 7.3, 6.3 (higher tunes)].Tune is far away from resonance up to 3rd order.Tune drift of –0.2 (due to space charge) does not hit any resonance up to 3rd order.

Lattice parameters

Preliminary tracking results with sextupoles (without error)

Horizontal phase space, 5000 turns Vertical phase space, 5000 turns

Initial co-ordinates are chosen corresponding to maximum displaced particle in both the planes with 1% p/p.

Further optimization needed in sextupoles for vertical plane.

H - Injection

500 s ( 300 turns ) pulse length of H- ions from 100 MeV linac to be injected through a stripping foil.

Constraints imposed by Liouville’s theorem

on conventional multi-turn injection do not apply in this case.

possible to inject a large number of turns.

Goals Of Injection

To fill transverse acceptances ( x = y = 300 mm

mrad) in K-V distribution

uniform filling

avoid excessive space charge forces

referred as injection painting

Injection Paintings

Horizontal Phase Space : Variable Bump by four bump magnets located in a long straight section

Angle of Injection

Peak of the bump at the stripping foil

Minimum number of traversal of beam

through the foil

Partially stripped particles H0 do not pass through high magnetic field ( centre of QD )

Sripped H- (Magnetic field) unwanted halo formation around circulating particles

Layout of the injection system of ISNS

Time Dependence of Four Kick Bump Angles

Orbit Bump and its Slope at the Location of Stripping Foil (Injection Point) vs Injection Turn Number

Bending Angle with Injection Turn Number

0 50 100 150 200 250 300 3500

Y - Amplitude

X - Amplitude

scilla

Injected Turn Number (N)

Amplitude of Betatron Oscillations of Injected Particles with Turn Number During Injection

Painting in vertical normalized phase space

Spatial distribution of nearly 350 injected turns

Striping Foil

• Thickness of the foil:(High stripping efficiency ) At 100MeV 60g cm-2

is adequate

• Foil materials: Polyparaxylene,carbon

or Aluminium oxide

Beam loss and Collimators

The lattice should accommodate the collimators (betatron and momentum) for controlled loss. At injection only 2% particle loss is allowed (if distributed uniformly all over the length) in the ring.

Key parameter in collimator design: Phase advance between primary and secondary

collimators and their apertures

Collimators remove the Halo from the beam at the predefined locations.

The first collimator scatters the halo particles, with low impact parameter. Due to scattering, the amplitude increases and these are collected at secondary collimator, which is placed at a proper phase advance.

Proper phase and critical kick is given by

n1 and n2 are the apertures of primary and secondary in terms of beam size.The critical kick is

22 nnK c

Phase difference between primary and secondary collimatorX – Plane: 158 and n2/n1=1.08Y – Plane: 144 and n2/n1=1.20

Material choice in collimators

Two Effects:

When a proton traverse through a primary collimator, it loses energy. If this loss is high, particle may be out of bucket or longitudinal acceptance. (Acceptance of ring 1%)

The primary collimator has to give a large kick, so protons hit the secondary collimator with large impact parameter. This kick is largely imparted through the multiple Coulomb scattering.

The first effect demands a very thin collimator, which does not cause the much energy loss.

The second effect demands a high Z material.Thus choices are among Pt, W etc.

Other requirements are good thermal conductivity, high melting point, good polishing capability, radiation damage.

As high Z has the shower effects, which is drawback. Therefore, for proper choice of material and optimization of its thickness, simulation studies are essential.

Tentative locations of betatron collimatorsIn next period to injection.

44 46 48 50 52 54 56 58 60 62

Bx Bz Eta Fx Fz

y-plane

x-plane

Phase difference between primary and secondary collimatorX – Plane: 158 and n2/n1=1.08, beam sizes at the collimators: 4.2cm, 3.8 cm, 3.2 cmY – Plane: 144 and n2/n1=1.20, beam sizes at the collimators: 3.8cm, 5.6 cm, 3.6 cm

Tentative locations of momentum collimators

Phase difference between primary and secondary collimatorX – Plane: 150 and n2/n1=1.15

In arc next to injection system.

Preliminary beam diagnostic requirements

48 Beam position monitors ( one @ each quadrupole). Beam loss monitors distributed all over the ring. Beam profile monitors. Current monitors (Average and Pulse).

Parameter ValueBeam power 100 kW

Energy 0.1 – 1.0 GeV

Repetition Rate 25 Hz

Circumference (m) 212.4

Periodicity 4

No. of bending magnets 24

Bending angle 15 per magnet

Bending Magnet Field 0.207 - 0.789 T

Bending radius (m) 7.1626

No. of quadrupoles 48

Maximum gradient (T/m) 4.5

Nominal tune point 6.88, 5.88

x,max, y,max (m) 16.4, 16.4

No. of sextupoles 16 (two families 8F and 8D)

Parameters of Synchrotron

Parameter ValueDmax (m) 2.4896

Chromaticity -8.954, -7.640

Momentum compaction 0.031989

-transition 5.591

Dispersion free straights 43.874=15.5 m / period

Straight with dispersion 23.875=7.75 m / period

RF (MHz) 1.21 – 2.47 for h = 2

Revolution Time 1.65 – 0.81 µs

Peak energy gain per turn 60 keV

Beam size (max) 9.6 cm @ 1% p/p @ Qf1

Emittance after painting 300 mm.mrad after injection in both planes

Peak RF voltage 120 kV

• Parameters of Linac (injector)

Parameter Value

Energy 100 MeV

Pulse length 500 s

Pulse current 25 mA

Energy spread 0.3 %

Emittance (normalized) 0.23 mm mrad

Magnet apertures

Magnet Max (m)

Strength

Good field

radius

Qf1 16.4 2.6 -0.67 120

Qd1 16.1 2.5 0.56 100

Qf2 16.1 0.0 -0.64 100

Qd2 16.4 0.0 0.61 100

Qf3 13.2 2.5 -0.67 120

BM ~8 ~2 0.8 T 100, 100

Linac to SynchrotronTransfer Line

• Design Philosophy

• To match the beam parameters from the linac output to synchrotron injection point.

• To provide the adequate space for installation of various components, as

1.RF cavity for energy jitter correction.2.Diagnostic elements (Profile monitors,

Current monitors, Beam position monitors and Beam loss monitors).

3.Dump line.4.Bumpers for injection painting.

• To install collimators for control of beam loss.

-10 0 10 20 30 40 50 60 70-15

Path length (m)

Optics parameters of Transfer Line

Matching section 4 Quads

2 FODO Achromat 1 FODO Matching section 4 Quads

-10 0 10 20 30 40 50 60 70-15

Path length (m)Primary collimatorSecondary collimator XSecondary collimator Y

RF Cavity

Parameter Value

Length 62.95 m

No. of quadrupoles 21 (11 F & 10 D)

Maximum strength (m-2) 6.1

No. of dipoles 2

Bending field (T) 0.65

x,max, y,max (m) 27.6, 15.8

x,inj, y,inj, Dinj (m) 0.99, 1.95, 0.00

x,ext, y,ext, Dext (m) 13.0, 2.5, 0.0

Conclusions

Only preliminary linear studies have been carried out.

• Studies to be carried out

1. Non-linear behavior and sextupole scheme2. Detailed studies of Longitudinal dynamics with space charge and deciding the RF program3. Space charge issues and beam loss control4. Detailed studies of injection and extraction5. Design of transfer lines5. Transverse and Longitudinal instabilities

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Exact formulation of Tune Shift(including the image terms)