High gradient Superconducting Cavity Development for FFAGs
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Transcript of 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
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FFAG cavity
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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
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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
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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
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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)
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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:
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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
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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!
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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
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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
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Longitudinal evolution
1 2 5 10
15 20 25 28
Δt [ns]
ΔE [M
eV]
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Horizontal pos vs time
1 2 5 10
15 20 25 26
Δt [ns]
Δx [m
m]
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Vertical pos vs time
1 2 5 10
15 20 25 26
Δt [ns]
Δz [m
m]
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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?
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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
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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)
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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 !
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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
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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
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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
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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
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Cavity options Half-wave resonator H-Resonators
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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
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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
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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
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Comparison of different cavity geometries
Rectangular cavity is better than elliptical by all parameters except multipactor resistivity
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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
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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
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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.
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