SC magnets, SC RF and other key technologies for Future Circular Colliders
CERN Academic Training Lecture – FCC 8
5 February 2016
Grateful thanks to E. Jensen, R. Kersevan, P. Chiggiato
Particle accelerators
The first cyclotron (E.O. Lawrence, 1930)
The cyclotron magnet (LBNL, 1945)
Cathodic tube, an electron linac(J.J. Thomson, 1897)
The LHC (CERN, 2008)
Beyond the LHC: the FCC’s
LHC27 km, 8.33 T14 TeV (c.o.m.)1300 tons NbTi0.2 tons HTS
FCC-hh
80 km, 20 T100 TeV (c.o.m.)9000 tons LTS2000 tons HTS
FCC-hh
100 km, 16 T100 TeV (c.o.m.)6000 tons Nb3Sn3000 tons Nb-Ti
HE-LHC
27 km, 20 T33 TeV (c.o.m.)3000 tons LTS700 tons HTS
Geneva
PS
SPS
LHC
Technology
• Generate a charged particle beam (sources)
• Accelerate it and “bunch” it (radio frequency)
• Steer it along the desired trajectory (magnets, power converters)
• In a confined space with no obstacles (vacuum)
• Assisted by several “services” (e.g. cryogenics)
• Observe it and control it (diagnostics)
Technology has a prime role in modern accelerators, and will be the key of the success of the FCC
Overview
• Some basics on accelerator technology
• Vacuum technology and main challenges
• SC accelerator magnet technology and main challenges
• SC RF technology and main challenges
Apologies, excuses and references
A modern circular collider
Bending magnet
RF cavities
Beam pipe
Detector Detector
Vacuum
J.M. Jimenez (CERN)
Beam lifetimeEmittance growth
Instabilities
Levels of vacuumY. Li, X. Liu (Cornell University) USPAS
Sourc
es
Hig
h inte
nsi
ty
ion a
ccele
rato
rs
Cry
ogenic
in
sula
tion
Fabrica
tion,
SEM
, Lin
acs
Sto
rage r
ings
LHC solution: beam screen
5…20 K
1.9 K
5…20 K
Gases are “cryo-pumped” on the surface of the beam pipe (magnet bore)
Synchrotron radiation continuously desorbs molecules
Cu-coating (50 mm), high conductivity (RRR ≈ 100)
Low impedance (high conductance surface, smooth geometry) to reduce beam instabilities, image current heating, sparking
“Electronic weather forecast”
The photomultiplier SEY (Secondary Electron Yield)
depends on material, geometry, surface state…
“electron cloud”
FCC beam vacuum system challenges
• Synchrotron radiation• LHC ≈ 0.2 W/m
• FCC ≈ 30 W/m (Ph. Lebrun CERN-ACC-2014-0220)
• Impedance Z• Magneto-resistivity (x 2)
• Higher temperature (x 4)
• Smaller bore (x ?)
• Operating temperature
(W.A. Barletta (MIT) USPAS)
Increase Z
Minimum heat load
Maximum vacuum quality
YBCO much better than Cu up to 10 GHz
SR accumulates in the slits
FCC beam vacuum systemC. Garion, R. Kersevan, Ph. Lebrun (CERN)
40K…60 K: new cryogenic cycles
SR slits
Large cooling pipes
Thermal shunt
SC coating ?
Other cures to SR power
• Dedicated warm photon stops for efficient cooling between dipoles as developed by FNAL for VLHC
• Open midplane magnets
http://inspirehep.net/record/628096/files/fermilab-conf-03-244.pdfAlso P. Bauer et al., "Report on the First Cryogenic Photon Stop
Experiment," FNAL TD-03-021, May 2003
R. Gupta (BNL)
Magnets
Magnets: bending (dipole)
I
IB
BvqFL
Lorentz force
The particle trajectory is a circle only in ideal conditions
B
Need focusing !
Magnets: focusing (quadrupole)
Particles experience a force proportional to the distance
from the field axis
FL = kx
A quadrupole that is focusing in one plane is de-focusing in the other plane (div(B)=0)
focusing
de-focusing
Alternating gradients (FODO cells)
Accelerator magnet design primer
• Dipoles
– Design for the largest feasible and economic B to reduce the accelerator radius
• Quadrupoles
– Design for the largest feasible integrated gradient to reduce the magnet bore size
E[GeV]= 0.3´B[T]´r[m]
G q[T ] =
2E[GeV ]
0.3L[m]b[m] » 3.4L[m]
Beam energy
Dipole field
Bending radius
Beam sizeEmittance
Beta function
FODO cell
length
Integrated quadrupole
gradient
Lorentz factor
Superconducting magnets !
Highest “dipole” fields
Magnets with bore
LBNL HD1
Record fields for SC magnets in “dipole” configuration
CERN RMC
1. Magnetic field
• NC: magneto motive force, reluctance and pole shapes
• SC: Biot-Savart law and coil shapes
B ≈ m0 NI / g
B g
g =100 mmNI =100 kAturnB =1.25 T
Hopkinson's law
+I-I
+I-I +I-I
B
Biot-Savart law
B ≈ m0 NI / r
r
r =45 mmNI =1 MAturnB =8.84 T
J.C. Maxwell, J.B. Biot, F. Savart
Design of an ideal dipole magnet
I=I0 cos() Intersecting circles
Intersecting ellipses
B1=-m0 I0/2 r B1=-m0 J d/2
+J-J
d
B1=-m0 J d b/(a+b)
r
+J-J
da
b Several solutions are possible and can be extended to higher order multi-pole magnets
None of them is practical
Magnetic design - sector coils• Dipole coil • Quadrupole coil
B=-2m0/ J (Rout - Rin) sin(j)
This is getting much more practical !
G=-2m0/ J ln(Rout/Rin) sin(2j)
RinRout
+J-J
j
RinRout
+J
-J
j+J
-J
The field is proportional to the current density J and the coil width (Rout-Rin)
Evolution of coil cross sections
• Coil cross sections (to scale) of the four superconducting colliders
• Increased coil complexity (nested layers, wedges and coil blocks) to achieve higher efficiency and improved field homogeneity
Tevatron HERA RHIC LHC
B=4.3 T B=5 T B=3.5 T B=8.3 T
t=15 mm t=20 mm t=30 mmt=10 mm
Nb-Ti the workhorse
3000 A/mm2
1 m
m
Rutherford cables for the LHC
LHC inner cable
7500 km of superconducting cables with tightly controlled properties (state-of-the-art production)
Best of Superconductors JE
Graphics by courtesy of Applied Superconductivity Center at NHMFL
400 A/mm2
useful JE
LTS’s have reached maturity
Data by courtesy of J. Parrell (OST)
US-CDP
ITER wires HL-LHC
wires
A 16 T dipole (with two bores)
Camouflaged record magnets
D20: cos
HD2: block
13.5 T
13.5 T
Van Oort, Scanlan, 1994
McIntyre, 2005
Todesco, 2013
D20 and HD2 “maquillage” by E. Todesco (CERN)
J. van Nugteren, 2013
FCC-hh dipole optionsCos-theta (grading) Blocks (no grading)
S. Farinon, P. Fabbricatore (INFN) C. Lorin, M. Durante (CEA)
Ideas for 20 T A 20 T HE-LHC dipole E. Todesco, L. Rossi (CERN)
HTS
Nb3Sn
Nb-Ti
Cost optimized, graded winding
All options are based on an LTS winding (outsert), and an HTS
field booster (insert)
A 24 T LHC Tripler P. McIntyre (TAMU)
Stress managed winding
HTS for 20 T
5 T HTS (YBCO) stand-alone dipole for test in FReSCa2 (40 mm bore)
First HTS coil Feather0
… and ideas for the future…
Bend-able dipole
Spare and ribs for a two layer dipole
CCT concept: two current layers, the solenoid contribution cancels
Ribs support for the conductor (e.g. fragile HTS)
Modest stress range (80 MPa for 18 T)
D.I. Meyer and R. Flasck, Nucl. Instr. Meth., 80, 339, 1970
By courtesy of S. Caspi
(LBNL)
2. Forces
• An electric charged particle q moving with a velocity v in a field B experiences a force FL
called electromagnetic (Lorentz) force (N):
• A conductor carrying current density J(A/mm2) experiences a (Laplace) force density fL (N/m3):
BvqFL
BJfL
(O. Heaviside) E.A. Lorentz, P.S. Laplace
Electromagnetic forces: dipole
• The electromagnetic forces in a dipole magnet tend to push the coil:
– Vertically, towards the mid plane (Fy < 0)
– Horizontally, outwards (Fx > 0)
Tevatron dipole
Fy
Fx
Field Force
Graphics by courtesy of P. Ferracin, S. Prestemon, E. Todesco
Electromagnetic forces - ends
• In the coil ends the Lorentz forces tend to push the coil:
– Outwards in the longitudinal direction (Fz > 0), and, similar to solenoids, the coil straight section is in tension
Fz
Graphics by courtesy of P. Ferracin, S. Prestemon, E. Todesco
The real challenge of very high fields
• Force increases with the square of the bore field– Requires massive
structures (high-strength materials, volume, weight)
– The stress limit is usually in the superconducting coil (superconductor and insulation, mitigated by Je≈1/B)
• In practice the design of high field magnets is limited by mechanics
Force per coil quadrant in high-field dipoles built or designed for
accelerators applications and R&D
Stress and pre-stress - concepts
• The peak stress is where the force accumulate, i.e. in the mid-plane for a cos() winding
• The poles of the coil tend to unload
• The coil needs pre-loading to avoid displacements– Mechanical energy release (cause
quench and training)
– Deformation of the coil geometry (affect field quality)
B=0 T
B=8.33 T
max
dmax
LHC dipole
Graphics by courtesy of P. Ferracin, S. Prestemon, E. Todesco
Collaring operation
Pre-collared coil assembly under a press, load the coil to the desired pre-stress (in the range of 50…100 MPa)
Insert keys to “lock” the collars, unload the assembly
that is now self-supporting and provides the desired
pre-load to the coil
New concepts: QXF
Aperture (mm) 150
Gradient (T/m) 140
Current (A) 17500
Temperature (K) 1.9
Peak field (T) 12.1
Shell-based support structure (a.k.a. bladder-and-keys)
developed at LBNL for strain sensitive material
HQ image by courtesy of H. Felice (LBNL)
3. Protection• In spite of the complex
scaling (bore dimension, geometry), the energy stored in the magnetic field of accelerator dipoles has increased with the square of the bore field
• A large stored magnetic energy makes the magnet difficult to protect, and requires:
– Fast detection and dump
– High terminal voltage and operating current
Why is it a problem ?
• the magnetic energy stored in the field:
is converted to heat through Joule heating RI2. If this process happened uniformly in the winding pack:
• Cu melting temperature 1356 K
• corresponding Em=5.2 109 J/m3
limit would be Bmax 115 T: NO PROBLEM !
BUT
the process does not happen uniformly (as little as 1 % of mass can absorb total energy)
L
R
2
0
2
2
1
2LIdv
BE
V
m == ò m
This is why it is important !
Courtesy of A. Siemko, CERN
Detection, switch and dump
precursor
propagation
detection
detection threshold
trigger (t=0)
fire heaters
switch dump
dump
discharge ≈ detection + delay + switch + dump
Measurements by courtesy of M. Di Castro, CERN
Joule heatingdT
dtµRI 2Temperature increase
Quench heaters
• the quench is spread actively by firing heaters embedded in the winding pack, in close vicinity to the conductor
winding
heater
M3M3
Magnet strings
• magnet strings (e.g. accelerator magnets, fusion magnetic systems) have exceedingly large stored energy (10 GJ):
• energy dump takes very long time (10…100 s)
• the magnet string is subdivided and each magnet is by-passed by a diode (or thyristor)
• the diode acts as a shunt during the discharge
M1 M2 MN
Injection and Dump
• Huge energy (2x4.2 GJ, 8.5x LHC) to be extracted and dumped• Dump block has to deal with ~200kW average power..• Beam rigidity: 167 T.km => need a very long way to dilute the
beam, ~2.5km!
Beam dump system
47
F. Burkhart, B. Goddard (CERN), E. Fischer (GSI)
SC septumFly-by
quadrupoles
Very reliable kickers
Cavities
A piece of history
"On April 24 [1947], Langmuir and I [H. Pollock] were running the machine […]
Some intermittent sparking had occurred and we asked the technician to observe
with a mirror around the protective concrete wall. He immediately signaled to turn
off the synchrotron as "he saw an arc in the tube." The vacuum was still excellent,
so Langmuir and I came to the end of the wall and observed. At first we thought it
might be due to Cherenkov radiation, but it soon became clearer that we were
seeing Ivanenko and Pomeranchuk [Synchrotron] radiation.”
Energy loss per turn - reminder
• Synchrotron radiation
– The energy loss per turn grows dramatically with energy, and with the inverse of the particle mass.
dE[keV ] = 88.5E4[GeV ]
r[m]
1
m4
Beam energy
massBending radius
Energy loss per turn
Numerical examples
• Bending radius:
• Example : a 50 TeV (E=50,000 GeV) proton (q=1) is bent by a 16 T field on a radius r = 10416.7 m (L=65 km)
• Synchrotron radiation:
• Example : a proton (m = 1840) with 50 TeV energy (E=50,000 GeV) bent on r = 10416.7 m, looses a total of dE = 4632.6 keV per turn (4 MeV: 0.1 ppm/turn)
• Example : an electron (m = 1) with 120 GeV energy (E=120 GeV) bent on r = 10416.7 m, looses a total of dE = 1,761,724.9 keV per turn (1.8 GeV: 1 %/turn)
[ ][ ]
[ ]TqB
GeVEm
3.0=r
[ ][ ][ ] 4
4 15.88
mm
GeVEkeVE
rd =
FCC-hh
FCC-ee
RF design primer• The accelerating “kick” and the lost energy
are provided by an oscillating electric fieldV
d=V0
dcos wt( )
• Energy gain per pass
– Aim at the largest accelerating electric field (“gradient”)
dE » qV0TEnergy gain per pass
Charge Time factor
Jean Delayen (JLAB) USPAS
Efficiency of RF structure
• Part of the energy coupled to the cavity space is lost:– To the beam (as desired)
– To the wall (surface resistivity Rs)
– Coupling to the outside world
Jean Delayen (JLAB) USPAS
Energy stored in the cavity
Energy dissipated in the walls, per radian
Decrease the surface resistance
Superconducting RF cavities!
Surface resistance RsNormal conductors (Q0 ≈ 103…105)
• Skin depth proportional to w-
1/2
• Rs weakly temperature dependent, proportional to wn, with n ≈ 1/2…2/3
• Cu at 300K, 1 GHz, Rs≈8.3 mW
Superconductors (Q0 ≈ 1010…1011)
• Penetration depth independent of w
• Rs strongly dependent of temperature, proportional to w2
• Nb at 2 K, 1 GHz, Rs≈7 nW
LHC cavities
Jean Delayen (JLAB) USPAS
Surface resistance of Nb
2 KIdeally, it is convenient to
reduce the operating temperature down to 2 K
However, recall the C.O.P of the refrigerator decreases inversely with the cold-end temperature
FCC-ee RF challenges: SR power
• SR loss : 1.7 GeV/turn
• Beam current : 2 x 30 mA
• Total SR : 2 x 50 MW
• System dimensions are a major step:– LHC (400 MHz, 8 cavities)
• 2 MV / 250 kW RF per cavity
– FCC-ee (200…800 MHz, 600 cavities)• 20 MV / 180 kW RF per cavity
• Total of 12 GV / 100 MW to the beam
• Total of 2 x 80 kW to the cold end of
cryoplant (assuming Q0=3 x 109)
LHC cavities (400 MHz)
A. Butterworth, E. Jensen (CERN)
BNL3 cavity (704 MHz)
Cavity characteristics
• An ideal cavity has a constant Q0 till the upper critical field Hc2 of the superconductor
• For various reasons, intrinsic and extrinsic, real cavities cannot reach the upper field limit, and exhibit a Q-slope (reduced efficiency)
A. Yamamoto, K. Yokoya, RAST 7 (2014)
115-136
Nb cavity for TESLA
State of the art: Accelerating gradient Marc Ross: SRF2015
Maximum gradient (just before breakdown)
A. Grasselino, SRF2013 & M. Liepe SRF2015
State of the art in high Q0
N2 doped Nb
LCLS-II cavity
Nb3Sn coated cavitiesDaniel Hall, SRF 2015, Whistler, CDN
Best performance achieved with slow cool-down
This cavity exceeds the specification for LCLS-II !
Nb on Cu thin films
109
1010
1011
0 5 10 15 20 25
Q0 (
1.7
K)
Eacc
[MV/m]
conventional
sputtering
energetic
condensation
bulk Nb
S. Aull, S. Calatroni, A.-M. Valente, R. Validadeh
Pt
Nb
Cu2 μm
FIB-SEM showing a cross-section through the Nb/Cu coating: smooth interface & no porosity. Courtesy R. Validadeh (STFC)
• Nb-coating is cost-effective, and Cu is a good mechanical/thermal stabilizer
• Record Q0 only at low fields, the problem is the Q-slope
• Recent result on alternative deposition methods (ECR with energetic condensation, A.-M. Valente, JLAB) show decreased Rsand reduced Q-slope
Thin films with alternative materials?
Both materials exhibit very low surface resistance combined with high Tc.
G. Rosaz, K. Ilyina (CERN)
• Work starting at CERN on Nb3Sn and V3Si (A15 LTS), high Tc and low residual resistance– Nb3Sn: Tc ≈ 18 K
– V3Si: Tc ≈ 16 K
FCC-ee RF challenges: beam loading• The beam itself induces a voltage in the cavity that modifies
the accelerating voltage. At large beam current this becomes the dominating effect
• The quadrature component of this voltage is equivalent to a de-tuning of the cavity. Transients in the beam current cause time-variable de-tuning
• The RF system needs to cope with de-tuning, and maintain an optimum coupling (the cavity must act as a pure transformer)
• Higher Order Modes (HOM) are excited, that require strong damping for operation at large beam current
Erk Jensen (CERN)
Higher Order Modes
timebunch
bunch
bunch
Matthias Liepe, SRF-15 Tutorials, 10 September 2015
FCC RF challenges: system complexityE. Jensen (CERN)
ScaleOperationReliability
Availability
30 km
Focus of SRF R&D for FCC
• Optimize operating point for minimum power consumption– Maximum Q0 at the operating point(s)– Investigate new materials with higher Tc
• Optimize performance in the large parameter range requested– Large current (1.5 A at 45.5 GeV) and large voltage (10 GV at
175 GeV)– HOM damping and coupling for large beam currents (1.5 A)– Machine configuration change to cover the operation span
• System scale-ability to large dimension (continuous RF power of 100 MW)– Investigate new materials and minimize the use of costly raw
materials– Explore new design (coupler, cryomodule) and manufacturing
techniques (rapid forming, automated processing and assembly)
Erk Jensen (CERN)
SC RF “cryomodules”• The cavity is immersed in liquid
He (typically He-II at 2 K), in a Helium tank
• The He vessel is surrounded by thermal and magnetic shielding inside a vacuum vessel, forming a “cryomodule”
• A power coupler feeds RF power
• A tuner adjusts the resonance frequency squeezing the cavity
• Higher Order Mode (HOM) couplers damp unwanted modes.
RF Power Coupler
Bulk Niobium 5-cell cavity
Helium Tank
Tuner
HOM Coupler
Bi-phase helium tube
Magnetic shielding
Inter-cavity
support
Bellows
Double walled tube
Example: SPL/ESS 704 MHz CM (partial view)
X-FEL Cryomodule @ DESY (eight 9-cell cavities 1.3 GHz)
Erk Jensen (CERN)
A new approach to rapid forming
S. Atieh
• Electro-hydraulic forming (EHF) at Bmax (France)
Erk Jensen (CERN)
Less spring-backBetter shape accuracySurface roughness as in the sheet
Will be tested for the fabrication of LHC spare modules
Conclusions – 1/2
A primitive smasher of matter
The origins of circular colliders
The state of the art
Conclusions – 2/2
• Technology is at the very heart of the largest experiments ever built by humanity
• It provides at the same time a push (towards new discoveries) and a pull (for other fields of application)
High societal impact !