SC magnets, SC RF and other key technologies for Future Circular Colliders

Luca.Bottura@cern.ch

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 !