high gradient superconducting cavity development for ffags

High gradient Superconducting Cavity Development for FFAGs

Sergey V. Kutsaev, Zachary A. Conway, Peter N. OstroumovC. Johnstone and R. Ford

Cyclotrons’13September 23, 2013

Vancouver, BC, Canada

2FFAG13 TRIUMF

FFAG is typically configured to accelerate a large momentum admittance within a modest magnet aperture.

1 GeV CW FFAG for C TherapyC. Johnstone – Trinity College, 2011

3 to 10 GeV muon double beam FFAGT. Planche - Nufact09

They are brilliant designs, as long as you don’t want too much beam current…P. McIntyre, FFAG13

Confusion about the new breed of nonlinear nsFFAGs with constant tune (above and PAMELA, for example) and EMMA (swept tune)

H

A

BC

DE

F G

HH

cA cB

cC

cD

cE cFcG cH

A 1. ,4.88616 B2.674 ,4.20703 C3.03267 ,3.85867 D3.43662 ,3.43662 E1. ,3.2672 F 2.00856 ,2.8764 G 2.2306 ,2.67393 H2.45023 ,2.45023 cA 1. ,5.37477 cB3.25798 ,5.37477 cC 3.31087 ,4.2696 cD 3.79024 ,3.79024 cE1. ,2.58876 cF 1.86471 ,2.58876 cG 2.03113 ,2.37929 cH2.20521 ,2.20521

0.5 1.0 1.5 2.0 2.5 3.0 3.5

2.53.03.54.04.55.0

2m

80 0 100 0 12 00 14 00 160 0P , MeV c1 .0

1 .5

2 .0

2 .5

3 .0v

100 0 120 0 140 0 160 0P , MeV c

4 .0

4 .5

5 .0

Rav g

Parameter 250 MeV 585 MeV 1000 MeVAvg. Radius (m) 3.419 4.307 5.030

Cell x / y (2 rad)Ring.

0.380/0.2371.520/0.948

0.400/0.1491.600/0.596

0.383/0.2421.532/0.968

Field F/D (T) 1.62/-0.14 2.06/-0.31 2.35/-0.42

Magnet Size F/D Inj 1.17/0.38 1.59/0.79 1.94/1.14

• Comments and further work– Tracking results indicate ~50-100 mm-mr; relatively insensitive to errors– Low losses

General Parameters of an initial 0. 250 – 1 GeV non-scaling, near-isochronous FFAG lattice design

Clockwise: Matematica: Ring tune, deviation from isochronous orbit (%), and radius vs. momentum

1000 1200 1 400 1 600P , MeV c0 .01

0 .02

0 .03

0 .04

0 .05

0 .06

0 .07

D Rad

Advanced design and simulation of anIsochronous 250-1000 MeV Nonscaling FFAG

May 13, 2013

FFAG cavity

4

5FFAG13 TRIUMF

F-D quads control betatron motion

We can lock x, y to any desired operating point.

BTC quads are tuned in 2 x 5 families.

Sextupole correctors at exit of each BTC are tuned in 2 x 6 families.

First 2 turns each have dedicated families so that they can be tuned first for rational commissioning.

0.0 0.5 1.0 1.5 2.0 2.5

5.5

5.0

4.5

4.0

Uniform gradient in each channel: excellent linear dynamics.

P. McIntyre, FFAG13, “ Strong-focusing cyclotron”

6FFAG13 TRIUMF

Now look at effects of synchrobetatron and space charge with 10 mA at extraction:

1/3 order integer effect 1/5 order integer effect

1/5-order islands stay clumped, 1/3-order islands are being driven. Likely driving term is edge fields of sectors (6-fold sector geometry). We are evaluating use of sextupoles at sector edges to suppress growth.

Move tunes near integer fraction resonances to observe growth of islands

After two turns … bunch is lost by 20 MeV

7FFAG13 TRIUMF

Synchrobetatron/space charge in longitudinal phase space:

Injection extraction

Phase width grows x5 at extraction

Tunes again moved to approach resonances, but retaining transmission through lattice

Simulations of a proton FFAG with space charge

FFAG’12, Osaka, Japan

14/11/2012 Dr. Suzie Sheehy, ASTeC/STFC 8

Dr. Suzie Sheehy, ASTeC/STFC

9

Overview Motivation 4-cell 1GeV FFAG

– Single energy simulations– Serpentine acceleration with SC

6-cell 1GeV FFAG simulations– Single energy simulations– Serpentine acceleration with SC

Two discussion points:– Cost to build an FFAG vs linac– Terminology

Summary

14/11/2012


10

4-cell Lattice design

F

D

C. Johnstone et al, AIP conference proceedings 1299, 1, 682-687 (2010)

Total tune variation = 0.12 (H) / 0.56 (V)

14/11/2012


11

Serpentine channel acceleration

Using Carol’s 4-cell 1 GeV FFAG ring 330MeV-1GeV Serpentine acceleration is possible due to 3%

time of flight variation10 MV/turn

From IPAC’11, S. L. Sheehy, WEPS088

12 MV/turn

15 MV/turn

Using single particle tracking:

14/11/2012


12

Setting up the simulation in OPAL

Beam matching at 300 MeV Single energy tracking at realistic

intensities shows no emittance growth.

At very high intensities emittance grows, as expected.– (See HB2012 MOP258)

Single bunch10 π mm mrad in horiz/vertical

Length ~ 4% of ring circumference50 turns at 300 MeV with no acceleration

Current is average of full turn14/11/2012


13

Setting up the simulation

Cavity phase (deg)

Ener

gy (M

eV)

• Harmonic number = 1 (for now)• Space charge ‘off’ – track bunch of 2000 particles with parabolic profile

Issue: OPAL outputs after set num of tracking steps NOT azimuthal position ie. not a full ‘turn’ if not perfectly on phase! In the newer version of OPAL (1.1.9) I can use ‘probe’ element to get ‘screen-like’ physical position readout (in process of updating on SCARF).

For now – the following simulations were run overnight on my quad core desktop!

14/11/2012


14

Turn-by-turn orbit with bunch• Try a short & small 1 π mm mrad beam to see what happens• Using ‘probe’ element to get turn-by-turn at 0 degree position, 200 particles

X Position [mm]

Mom

entu

m [β

γ]

Acceleration! With VRF/turn= 22 MV

14/11/2012


15

With space charge ‘on’

• 1 pi mm mrad bunch• Tracked for 30 turns in

serpentine channel• Vary (average) beam current

up to 10 mA • 10 MW beam at 1 GeV

• NOTE: VERY SHORT BEAM!• Total beam length approx.

1cm so peak current = 22A!• Lots of space charge!!

Acceleration with 3 cavitiesVRF/turn= 22 MV

14/11/2012


16

Longitudinal evolution

1 2 5 10

15 20 25 28

Δt [ns]

ΔE [M

eV]

14/11/2012


17

Horizontal pos vs time

1 2 5 10

15 20 25 26

Δt [ns]

Δx [m

m]

14/11/2012


18

Vertical pos vs time

1 2 5 10

15 20 25 26

Δt [ns]

Δz [m

m]

14/11/2012


19

Emittance evolution

horizontal

vertical

longitudinal

300 MeV – no acceleration300 MeV – 30 turn serp. Accel.NOTE DIFFERENT SCALES!!

(NB as discussed before these are per ‘turn’ wrt time so not ‘real’ turns)

Question: why does vertical emit grow even with no space charge?

14/11/2012


20

6-cell 1GeV Ring Initially I had issues modeling this ring as short magnets were not well-

reproduced with the interpolation in OPAL I now have a much finer field map! (thanks to K. Makino) The main driver behind this design was to achieve better isochronicity but

the vertical tune variation also improved.

• Total tune variation = 0.42 (H) / 0.234 (V)• Lower B field (cf 2.35T in 4-cell design)• Larger radius

14/11/2012

Field profiles from C. Johnstone, IPAC’12, THPPR063


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Single energy with space charge

• No acceleration, 10 pi beam• 50 turns with space charge• Short beam (0.04% ring)

• No acceleration, 10 pi beam• 50 turns with space charge• Longer beam (4% ring)

14/11/2012


22

Serpentine channel

Much smaller TOF variation than 4-cell version

Required RF frequency & energy gain per turn to open up the serpentine channel (δE) can be estimated by

total energy gain required is ΔE and ω =2πFRF

In this case, we expect δE ~ 3.7 MV/turn.(A few quick simulations confirm this)

If we make it ‘isochronous enough’, perhaps we can use the serpentine channel without superconducting cavities?

4MV/turn !

14/11/2012


23

Acceleration & space charge

Use ‘reasonable’ 8MV/turn (equiv. 4 PSI-type cavities) Beam matched at 330 MeV Same emittance & length beam as in 6-cell simulations

14/11/2012

Outline Motivation and Background

• Next generation ultra-compact, high-energy fixed field accelerators• Medical, security, energy applications• CW FFAGs ; i.e. strong-focusing cyclotrons

– Relativistic energies: ~200 MeV – 1 GeV – Ultra-compact– Constant machine tunes (optimized gradients)– High mA currents (low losses)

These machines require high gradient acceleration; SCRF required for

• Compactness• Low extraction losses

– Large horizontal aperture of the FFAG, like the cyclotron, is a challenging problem for SCRF design

September 16, 2013

FFAG cavity

24

NEXT-generation CW high-energy Fixed-field Compact Accelerators

Reverse gradient required for vertical envelope Isochronous or CW (serpentine channel relaxes tolerances) Stable tune, large energy range The footprint of CW FFAG accelerators is decreasing rapidly

80 0 10 00 12 0 0 1 400 16 0 0P , MeV c1 .0

1 .5

2 .0

2 .5

3 .0v

Machine tunes: r ~1.4 z ~0.8 – factor of ~4 > than compact cyclotron

September 16, 2013

26September 16, 2013

MAGNETS and modeling Parameter Units Value

Number of magnets 6

Number of SC coils 12

Peak magnetic field on coils T 7

Magnet Beam Pipe gap mm 50

Superconductor type NbTi

Operating Temperature K 4.0

Superconducting cable Rutherford

Coil ampere-turns MA 3.0

Magnet system height M ~1

Total Weight tons ~10

One straight section occupied by RF cavities and injection/extraction in the other

80 mm x 293 mrH = 23,440π mm-mr

norm = 16,059π mm-mr Tracking: Horizontal – 1 cm steps, Vertical – 1 mm steps

cyclotron cyclotron

10 mm x 9 mr

V = 90π mm-mrnorm =62π mm-mr

FFAG Horizontal Stable beam area @1000 MeV vs. DA of 800 MeV Daealus cyclotron*

FFAG Horizontal / Vertical Stable beam area @200 MeV

Tracked: 130 mm x 165 mr = 21450π mm-mr

norm = 38820π mm-mr F. Meot, et. al., Proc. IPAC2012

*FFAG vert. stable area at aperture limits.

< 3

m

< 5 m

Acceleration Gradient required for low-loss extraction

Kinetic Energy(MeV)

Acc Gradientper turn

(MV)

RSRadius @center of

straight(m)

r

(cm)

800

1.1955

785

15 1.1879 0.76

775

25 1.1816 1.39

765

35 1.1751 2.04

Reference radius in center of straight for the energy orbits preceding extraction. For an accelerating gradient of ~20 MV/m orbits are sufficiently separated for a “clean” (beam size: 1.14 cm; =10 mm-mr normalized) or low-loss extraction through a septum magnet.

For 20 MV/turn, and a 2m straight section, we require 10 MV/m – implies a SCRF cryomodule – in order to achieve extraction with manageable shielding, radiation levels, and activation. This requirement drove the design of the high-energy stage.

Design specifications Large horizontal beam aperture of 50 cm Cavity should operate at 150 or 200 MHz (harmonic of the

revolution frequency) Should provide at least 5 MV for proton beam with energies

200 – 900 MeV Peak magnetic field should be no more than 160 mT

(preferably, 120 mT or less) Peak electric field should be minimized Cavity dimensions should be minimized

FFAG cavity

28September 16, 2013

Cavity options Half-wave resonator H-Resonators

May 13, 2013

FFAG cavity

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Beamtrajectories

HWR is very dependent on particle velocityCan’t be used efficiently for such a wide range of particle energies

Dimensions are very large as is peak magnetic field on the electrode edge

Rectangular Cavity Rectangular cavity operating at H101 mode has electric field

concentrated in the center of the wall To concentrate electric field at beam aperture, we introduced

tapers To reduce peak magnetic field the blending was introduced

May 13, 2013

FFAG cavity

30

W

L

H

Beamdirection

Gap and Frequency Optimization The voltage at 160 mT maximum field dependence on gap

length was calculated for cavities with different frequencies and lengths

May 13, 2013

FFAG cavity

31

150 MHz 1.5 m structure has a potentially higher possible voltage or lower peak magnetic field at 5 MV200 MHz structure is more compact

Beam Energy = 200 MeV

Voltage in the center of the aperture

Peak magnetic field = 160 mT

Cavity shape optimization A taper was introduced to distribute the

magnetic field over a larger volume keeping the electric field concentrated around the beam aperture

Such a cavity design has smaller dimensions for the same volume

All edges were rounded and improved reentrant nose shape reduced the peak magnetic field by more than 15% and the transverse dimensions by more than 10 cm

Final study was an elliptical cell shape where the magnetic field varies along the cavity wall such that there are no stable electron trajectories and multipacting is inhibited

May 13, 2013

FFAG cavity

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Comparison of different cavity geometries

Rectangular cavity is better than elliptical by all parameters except multipactor resistivity

May 13, 2013

FFAG cavity

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The RF performance degrades at higher frequencies.

ParameterRectangular

topRectangular

middleEllipticalbottom

Frequency, MHz 200 200 200Length, cm 100 100 120Height, cm 104.5 92.9 142

Voltage (β=0.56, edge), MV 4.67 4.66 4.68Voltage (β=0.78, center) , MV 6.72 6.71 6.89Voltage (β=0.86, edge) , MV 5.00 5.00 5.00R/Q (β=0.86, edge), Ohms 82.8 89.7 75.0

G, Ohms 147.9 150.2 134.2Peak magnetic field, mT 92.1 72.7 77.2

Peak electric field, MV/m 55.2 47.0 48.1

Parameters of the different cavity designs.

RF input coupler design As 1 mA beam is accelerated by 4 cavities from 200 to 900

MeV, each cavity requires about 175 kW of power One of the options is to attach 2 100kW couplers to the

cavity

May 13, 2013

FFAG cavity

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ANSYS estimations show no significant overheating

80K

4K126K

133K300K

Heat Flows:To 4K = 9.8WTo 60K = 92.0WFrom 300K = 18.8W

Magnetic power coupling and mechanical design External Q-factor should be ~ 1.9*106

Preliminary results predict ~1.1mm Nb and ~0.6mm SS deformation at magnetic field area

FFAG cavity

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The complete mechanical design: 1 – niobium shell, 2 – RF ports, 3- extra ports, 4 – frequency tuning, 5 – steel jacket, 6 – rails

Summary Two options of 200 MHz cavities were studied: "rectangular"

and " elliptical". The "elliptical" option has been introduced to avoid multipacting (MP) problem. The latter can be problem for rectangular cavity

However, the rectangular option provides better parameters (peak fields, dimensions etc)

100 kW RF coupler has been designed. More studies are needed to minimize cryogenic load.

Initial mechanical design was created and structural analysis has been performed. Both cavities can be made structurally stable.

May 13, 2013

FFAG cavity

36

high gradient superconducting cavity development for ffags

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