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Sergey Belomestnykh (Fermilab)
Seminar at John Adams Institute for Accelerator Science
Oxford, UK, February 12, 2018
Radio frequency superconductivity for
particle accelerators:
Recent trends in physics and technology
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 2
Outline
Introduction
What is RF superconductivity for particle accelerators?
SRF basics: surface resistance, Q vs. E
Recent SRF science breakthroughs & active areas of research:
o Nitrogen doping
o Nitrogen infusion
o Frequency dependence of Rs
o SRF in quantum regime
SRF over the world
Summary
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 3
Introduction
Over the past several years, the field of radio frequency superconductivity (SRF)
for particle accelerators is going through a period of Renaissance.
5 years ago, most of the community thought that the science and technology
reached maturity (even though we lacked understanding of some basic physics)
and one can achieve only incremental gains in the niobium cavity performance.
The field tended to be mostly technological with only few researchers trying to
study fundamental issues of SRF in niobium. Big improvement steps were thought
to be possible only with developing alternative materials (e.g. Nb3Sn).
Recent discoveries of nitrogen doping and infusion, magnetic flux expulsion,
opened new horizons and revived interest to studies of SRF basics, both
experimental and theoretical. More unexpected and intriguing results have been
obtained.
In this talk I will try to shed light upon some exciting recent results, show new
trends (Fermilab-centric view) and hopefully inspire young generation to turn their
attention to this field of research.
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 4
What is RF superconductivity
for particle accelerators?
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 5
Discovery of superconductivity: April 8th of 1911 Discovered in 1911 by Heike Kamerlingh Onnes and Gilles Holst after Onnes was
able to liquefy helium in 1908 (Nobel Prize in 1913).
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 6
Superconducting elements
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 7
Superconducting state The superconducting state is characterized by the critical temperature Tc and field Hc
The external field is expelled from a superconductor if Hext < Hc for Type I superconductors.
For Type II superconductors the external field can partially penetrate for Hext < Hc1 and will
completely penetrate at Hc2.
2
10c
ccT
THTH
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 8
Theories explaining superconductivity
Early developments: two-fluid model and London equations.
Phenomenological Ginzburg-Landau (GL) theory (1950, Nobel Prize in 2003) generalized
London equation to nonlinear problems.
Microscopic theory of superconductivity was developed by Bardeen, Cooper and Schrieffer
(BCS) in 1957 (Nobel Prize in 1972).
What do we need to recollect?
Magnetic field does not stop abruptly, but penetrates into the material with exponential
attenuation. The (London) penetration depth l is quite small, 20 – 50 nm.
According to BCS theory not single electrons, but (Cooper) pairs are carriers of the
supercurrent. However, the penetration depth remains unchanged.
The BCS ground state is characterized by the macroscopic wave function and a ground
state energy that is separated from the energy levels of unpaired electrons by an energy
gap. In order to break a pair an energy of 2D is needed:
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 9
Theories explaining superconductivity (2)
GL theory introduced coherence length x – a new scale of special variation of the superfluid
density and superconducting gap.
Also introduced is a dimensionless GL parameter k l / x, which is independent of
temperature.
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 10
What happens if AC field is applied?
At 0 < T < Tc not all electrons are bonded into Cooper pairs. The density of unpaired,
“normal” electrons is given by the Boltzman factor
Cooper pairs move without resistance, and thus dissipate no power. In DC case the lossless
Cooper pairs short out the field, hence the normal electrons are not accelerated and the SC
is lossless even for T > 0 K.
The Cooper pairs do nonetheless have an inertial mass, and thus they cannot follow an AC
electromagnetic fields instantly and do not shield it perfectly. A residual EM field remains
and acts on the unpaired electrons as well, therefore causing power dissipation.
D
Tkn
B
expnormal
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 11
What is RF superconductivity for accelerators? Radio frequency (RF) superconductivity for particle accelerators is a branch of accelerator
physics and engineering dealing with application of superconducting materials to
acceleration of charged particles in resonant RF cavities.
The science part of this field deals with investigating limitations of and developing methods
to improve the SRF cavity performance. In particular, how to reduce power dissipation in
SRF cavities and improve accelerating gradients.
Slowed down by factor of approximately 4x109 Input RF power at 1.3 GHz
~1 m
Niobium
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 12
RF superconductivity as a branch of accelerator
physics was born 57 years ago
“In a seminal paper published in June 1961 A. P. Banford and G. H. Stafford described how a future superconducting proton linear accelerator could run continuously, instead of at the 1% duty cycle of the 50 MeV proton accelerator that was operating at the time at the Rutherford High Energy Laboratory in the UK. The basic argument was that, because ohmic losses in the accelerating cavity walls increase as the square of the accelerating voltage, copper cavities become uneconomical when the demand for high continuous-wave (CW) voltage grows with particle energy. It is here that superconductivity comes to the rescue.”
(from Hasan Padamsee’s article “Advances in acceleration: the superconducting way,”
CERN Courier, November 2011)
Plasma Physics (Journal of Nuclear Energy Part C), 1961, Vol. 3, pp. 287 to 290.
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 13
Benefits of RF superconductivity The development of superconducting (SC) cavities for accelerators has enabled new
applications not previously possible with normal conducting (NC) structures.
SC cavities excel in applications requiring continuous wave (CW) or long-pulse accelerating
fields above a few MV/m (up to ~35 MV/m).
For NC cavities (usually made of copper) power dissipation in cavity walls is a huge constrain
in these cases cavity design is driven by this fact, optimized for lowest possible wall
dissipation small beam aperture.
The surface resistivity of SC cavities is 5-6 orders of
magnitude less than that of copper SC accelerating
system is more economical: less wall plug power, fewer
cavities required, …
Additional benefit: the cavity design decouples from the
dynamic losses (wall losses associated with RF fields)
free to adapt design to a specific application.
The presence of accelerating structures has a disruptive
effect on the beam and may cause various instabilities,
dilute beam emittance and produce other undesirable
effects. Fewer SC cavities less disruption. SC cavities
can trade off some of wall losses to a larger beam pipe
reduce disruption more.
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 14
SRF basics:
Rs, Q vs. Eacc
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 15
Surface resistance A convenient way to characterize power losses at radio frequency resonant cavities is to use
a so-called surface resistance. [ For normal conducting cavities Rs = 1/(sd), where s is the
specific conductivity and d is the skin depth. ] Then the power dissipation per unit area is
And the total power dissipation is obtained by integration
over the whole inner surface of the cavity.
Calculation of surface resistance must take into account numerous parameters. Mattis and
Bardeen developed theory based on BCS, which predicts
where A is the material constant
depends on the electron mean free path
20
2
1HRP sdiss
,2
T
T
TkBCS
c
cBeT
AR
D
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 16
Surface resistance (2)
While for low frequencies (≤ 500 MHz) it may be efficient to operate at 4.2 K (liquid
helium at atmospheric pressure), higher frequency structures favor lower
operating temperatures (typically superfluid LHe at 2 K, below the lambda point,
2.172 K).
Approximate expression for Nb:
]Ohm[1
1500
]MHz[102
67.172
4
T
BCS eT
fR
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 17
Surface resistance of cavities
The BCS surface resistance is described
by Mattis-Bardeen theory and comes from
thermally excited quasi-particles
The residual resistance can come from
different extrinsic contributions :
o Impurities/defects in the surface
o Hydrides precipitates
o Trapped magnetic flux
o ...
Residual resistance is significant for cavities
operating at 2 K.
𝑅𝑠 𝑇 = 𝑅𝐵𝐶𝑆 𝑇 + 𝑅𝑟𝑒𝑠
𝑅𝐵𝐶𝑆 𝑇 =𝐴𝜔2
𝑇𝑒−
∆𝑘𝐵𝑇
𝑅𝑟𝑒𝑠
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 18
Why Niobium?
Pure niobium has the highest critical temperature Tc among single elements, and Hc1 and Hc are both high.
Low Rs is needed for operation in superfluid helium at 2 to 4 K (typical accelerator operation domain).
High theoretical Meissner state breakup field (Hsh~240 mT) for an ideal surface, which scales with Hc.
Good formability is desirable for ease of cavity fabrication.
Pure intermetallic compounds, like Nb3Sn with a critical temperature of 18.1 K, look attractive for possible
4.2 K operation at first sight as they are “clean” superconductors. However, so far the gradients achieved in
Nb3Sn coated niobium cavities have been limited to below 19 MV/m, probably due to grain boundary effects
in the Nb3Sn layer. Residual resistance is also high and magnetic flux management is an issue.
Alloys are “dirty” superconductors due to their small mean free path and consequently have large BCS
surface resistivity and poor thermal conductivity.
High temperature superconductors have been tried in the past and showed very high surface resistances,
problems arise from very low coherence length = sensitivity to defects, gap anisotropy etc.
Type Tc Hc1 Hc Hc2 Fabrication
- K Oe Oe Oe -
Nb II 9.25 1700 2060 4000 bulk, film
Pb I 7.20 - 803 - electroplating
Nb3Sn* II 18.1 380 5200 240000 film
MgB2 II 39.0 300 4290 film
Hg I 4.15 - 411/339 - -
Ta I 4.47 - 829 - -
In I 3.41 - 281.5 - -
*) Other compounds with the same b-tungsten or A15 structure are under investigation as well.
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 19
Q(E) curve It is conventional to evaluate an SRF cavity performance using a Q(E) curve
L is the cavity length, Rs is the average surface resistance
R/Q is the cavity impedance (determined only by cavity shape)
G is the cavity geometry constant
Intr
insi
c q
ual
ity
fact
or
Q =
G/R
s
Increase max Eacc decrease accelerator length
Increase Q decrease required power
Accelerating gradient Eacc = Energy gain/cavity length
𝑃𝑑𝑖𝑠𝑠 =𝐸𝑎𝑐𝑐𝐿
2𝑅𝑠
𝐺∙𝑅 𝑄 =
𝐸𝑎𝑐𝑐𝐿2
𝑄∙𝑅 𝑄
Typical ILC-prepared cavity at T = 2 K
“Ideal” performance?
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 20
Q(E) evolution driven by science
0 5 10 15 20 25 30 35 4010
9
1010
1011
Q0
Eacc
(MV/m)
Elliptical Shape
1.3 GHz, 2 K
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 21
Q(E) evolution driven by science
1.3 GHz, 2 K
0 5 10 15 20 25 30 35 4010
9
1010
1011
Q0
Eacc
(MV/m)
Bulk RRR > 300
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 22
Q(E) evolution driven by science
1.3 GHz, 2 K
0 5 10 15 20 25 30 35 4010
9
1010
1011
Q0
Eacc
(MV/m)
High Pressure Water Rinse
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 23
Q(E) evolution driven by science
1.3 GHz, 2 K
0 5 10 15 20 25 30 35 4010
9
1010
1011
Q0
Eacc
(MV/m)
120◦C bake
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 24
Q(E) evolution driven by science 1.3 GHz, 2 K
0 5 10 15 20 25 30 35 4010
9
1010
1011
Q0
Eacc
(MV/m)
EXFEL
Typical ILC-recipe prepared cavity
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 25
Only thin surface layer matters Inner surface nanostructure within ~100 nm completely determines RF losses in the cavity
RF fields
Helium cooling
RF currents <100 nm
Niobium ~3 mm
RF fields <0.1% of thickness
Final treatment is crucial to performance
Nb2O5
Image from linearcollider.org
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 26
State of the art Q(E) curve ~5 years ago
Increase max Eacc decrease accelerator length
Increase Q decrease required power
Typical ILC-prepared cavity at T = 2 K State of the art until ~5 years ago
LFQS MFQS
HFQS
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 27
Recent SRF science breakthroughs
& active areas of research
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 28
Nitrogen doping
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 29
Nitrogen doping: a breakthrough in Q
Discovered while trying to investigate niobium nitride thin films on cavities.
Q-factor improvement after N-doping – up to 4 times higher Q than standard Nb cavities.
Typical Q vs Eacc curve obtained with
120 C bake (standard ILC treatment);
Avg Q with doping is 2-4 times state
of the art;
Example, for 1.3 GHz, 2 K, mid-field
Q ~ 1.5e10 versus > 3e10;
Systematically above Q obtained with
any other surface treatment.
Injection of small
nitrogen partial
pressure at the
end of 800 C
degassing
drastic increase
in Q.
0 5 10 15 20 25 30 35 4010
9
1010
1011
Q0
Eacc
(MV/m)
T= 2K
Anti-Q-slope
A. Grassellino et al., Supercond. Sci. Technol. 26, 102001 (2013) – Rapid Communications
MFQS
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 30
Doping Treatment: small variation from standard
protocol, large difference in performance
FNAL doping for LCLS-II (major steps):
o Bulk EP
o 800 C anneal for 3 hours in vacuum
o 2 minutes @ 800 C nitrogen diffusion
o 800 C for 6 minutes in vacuum
o Vacuum cooling
o 5 microns EP
Cavity after Equator Welding
EP 140 um
Ethanol Rinse
External 20 um BCP
Short HPR
800C HT Bake
RF Tuning
EP 40 um
Ethanol Rinse
Long HPR
Final Assembly
Long HPR
Helium Tank Welding
Procedure
VT Assembly
HPR
HOM Tuning
Ship to DESY
Leak Check
120C bake
XF
EL
X
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 31
Surface post nitrogen bake, pre-EP: poorly SC
nitride phases
Few Nb-nitride features
(Nb2N reflections) in Nb
near-surface.
Nitride “teeth” go ~0.2 μm
deep.
Flat Nb sample baked at 800C˚ for 2 min with N2 + 6 min annealing
Flat Nb sample baked at 800C˚ for 20 min with N2 + 30 min annealing
Bad (poorly SC) nitride phases that need to be removed
via EP correlate with poor performance (pre-EP) Q~1e7
Nb [113]+Nb2N [210]+? Pt layer
Y. Trenikhina, MOPB055, SRF15
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 32
Origin: “reversed” field dependence of RBCS
A. Grassellino et al, 2013 Supercond. Sci. Technol. 26 102001 (Rapid Communication) A. Romanenko and A. Grassellino, Appl. Phys. Lett. 102, 252603 (2013)
𝑅𝑠 𝑇 = 𝑅𝐵𝐶𝑆 𝑇 + 𝑅𝑟𝑒𝑠
Reverse field dependence of the BCS surface resistance component lowest RBCS .
Lower than typical residual resistances (seems to zero all contributions but trapped flux).
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 33
Physics – perceived BCS limit has been overcome
Anti-Q-slope emerges from the BCS
surface resistance decreasing with
field.
This was thought to be the lowest
possible BCS resistance.
N doping brings also lower than
typical residual resistance
< 2 nanoOhms (non trapped flux
related).
A. Grassellino et al, 2013 Supercond. Sci. Technol. 26 102001 (Rapid Communication) A. Romanenko and A. Grassellino, Appl. Phys. Lett. 102, 252603 (2013)
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 34
Nitrogen doping – from research to production Shortly after its discovery, the nitrogen doping was adopted by LCLS-II project.
After a short R&D period (Fermilab, JLab and Cornell), the recipe was successfully
transferred to industry.
Plot shows performance of SRF cavities from
two prototype LCLS-II cryomodules (Fermilab
and JLab): avg. Q = 3.6e10, avg. Eacc =
22.2 MV/m highest average Q ever
demonstrated in vertical tests of 1.3 GHz
nine-cell cavities at 2 K, 16 MV/m. Cavities
from vendors demonstrate similar
performance.
Higher Q would allow SLAC to use only one
cryoplant (of purchased two) to run the
machine and use the second cryoplant to
support the energy upgrade of LCLS-II
from 4.2 GeV to 8 GeV.
Two drawbacks of N-doping:
1. Achievable accelerating gradient is lower than that
of 120 C baked cavities (35-40 MV/m)
2. Nitrogen-doped cavities are more sensitive to
trapped flux losses.
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 35
Next step in the cavity performance 1.3 GHz, 2 K
0 5 10 15 20 25 30 35 4010
9
1010
1011
Q0
Eacc
(MV/m)
LCLS-II, PIP-II, PIP-III
Nitrogen doping
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 36
Do we understand how to choose best cavity treatment?
N-doping modify the mean free path → close to theoretical minimum of RBCS
N-doping seems to increase the reduced energy gap D/kBTc
Adding together all the 𝑅𝑆 contributions, it is possible to predict which treatments lowers Rs, taking into account also trapped flux
Best compromise is given by light N-doping treatments
𝑅𝑆 2 𝐾 = 𝑅𝐵𝐶𝑆 2 𝐾 + 𝑅𝑓𝑙 + 𝑅0
Residual resistance:
4 nW: 120 C baked cavities
2 nW: EP and optimally N-doped cavities
𝑅𝐹𝑙 = 𝐵𝑒𝑥𝑡 ∙ 𝜂 ∙ 𝑆 𝐵𝑒𝑥𝑡: external magnetic field 𝜂: flux trapping efficiency
M. Martinello et al, Appl. Phys. Lett. 109, 062601 (2016)
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 37
Nitrogen infusion
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 38
Nitrogen infusion: higher Q at higher gradients Composition and mean free path in first nanometers
of cavity surface have been shown to be crucial for
both Q and gradient performance.
N-doping at T > 800 C proven to manipulate mean
free path, but constantly throughout several
microns, giving high Q.
120 C bake known to manipulate mean free path at
very near surface on clean bulk, and produce the
highest gradients.
Therefore, it was decided to study how to better
“engineer” a dirty layer on top a clean bulk Nb,
using low T nitrogen treatments aim to create a
few to several nanometers of nitrogen-enriched
layer on top of clean EP bulk, to attempt to bring
together the benefit of the Q and gradient
Nitrogen enriched nanometric layer to be created in
the furnace post 800 C treatment – when no oxide
is present at the moment of injection of nitrogen at
low T.
Studies aim also at fundamental understanding of
HFQS and 120 C cure of high field Q-slope.
0 20 40 60 80 100 120 140 16010-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
Chamber pressure
Cavity temperature
Elapsed time (h)
Pre
ssu
re (
To
rr)
TE1AES015 & TE1PAV007 20161116 SKC
0
100
200
300
400
500
600
700
800
900
Tem
per
atu
re (°C
)
Heat treatment:
800 °C, 3 h in UHV
160 °C, 48 h with N2 at 25´10-3 Torr
160 °C, 96 h in UHV
0 10 20 30 40 50 60 7010-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
Chamber pressure
Cavity temperature
Elapsed time (h)P
ress
ure
(T
orr
)
TE1PAV010 20160106 SKC
0
100
200
300
400
500
600
700
800
900
Tem
per
atu
re (°C
)
Heat treatment:
800 °C, 3 h in UHV
120 °C, 48 h in UHV
0 10 20 30 40 50 60 7010-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
Chamber pressure
Cavity temperature
Elapsed time (h)
Pre
ssu
re (
To
rr)
TE1AES015 20160519 SKC
0
100
200
300
400
500
600
700
800
900
Tem
per
atu
re (°C
)
Heat treatment:
800 °C, 3 h in UHV
160 °C, 48 h with N2 at 25´10-3 Torr
0 10 20 30 40 50 60 7010-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
Chamber pressure
Cavity temperature
Elapsed time (h)
Pre
ssu
re (
To
rr)
TB9AES017 20160613 SKC
0
100
200
300
400
500
600
700
800
900
Tem
per
atu
re (°C
)
Heat treatment:
800 °C, 3 h in UHV
120 °C, 48 h with N2 at 25´10-3 Torr
o Bulk electro-polishing
o High T furnace:
• 800 C, 3 hours, high vacuum
• 120 C, 48 hours with N2
(25 mTorr)
o No chemistry post-furnace
o HPR, VT assembly
Slides on N-infusion are courtesy of A. Grasselino
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 39
Results: ILC recipe vs. nitrogen infusion
Same cavity, sequentially processed,
no EP in between
Achieved:
45.6 MV/m 194 mT, with
Q ~ 2e10!
Q ~ 2.3e10 at Eacc ~ 35 MV/m
Repeatable increase of Q by a
factor of two, increase of gradient
~15%
Potential application – ILC,
significant cost reduction of the
machine.
New potential breakthrough: very high Q at very high
gradients with low temperature (120C) nitrogen treatment
4/12/16Alexander Romanenko | FCC Week 2016 - Rome34
- Record Q at fields > 30 MV/m
- Preliminary data indicates potential 15% boost in achievable quench fields
- Can be game changer for ILC!
Slides on N-infusion are courtesy of A. Grasselino
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 40
Kubo and Checchin models on bi-layer potentially
increasing achievable accelerating gradients This idea is supported by Checchin (FNAL) and Kubo (KEK) models on bi-layer structure (e.g. dirty N-doped
layer on clean Nb) – claim that can enhance the achievable accelerating gradient.
Ideal Depth of this layer? Can this trick help push beyond the 200 mT or achieve 200 mT with higher yield?
We are investigating this empirically via low-T N-infusion (different T and durations)
TTC@Saclay 40
In addition to the BL barrier, we have the second barrier due to the S-S
boundary. The second barrier is also imperfect: easily weakened by defects.
However, we have a second chance to stop the vortex penetration.
The S-S bilayer
structure
defect
defect
defect
T. Kubo, TTC Meeting 2016
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 41
Exploring doping/infusion parameter space
This is still very active area
of research:
o Nitrogen infusion at
various temperatures /
exposure times
o Doping with other
materials
o Better understanding
surface properties to get
insight on how to ”nano-
engineer” niobium for
different applications
High Q0 (e.g. LCLS-II)
High Q0 & High Eacc (e.g. ILC)
Slides on N-infusion are courtesy of A. Grasselino
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 42
Large parameter space – T and duration being explored
Q ~ 6e10 at 15 MV/m!
Q > 3e10 at 31.5 MV/m!
Slides on N-infusion are courtesy of A. Grasselino
0 5 10 15 20 25 30 35 40 4510
9
1010
1011
Q0
Eacc
(MV/m)
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 43
Is the evolution nearly complete?
1.3 GHz, 2 K
Nitrogen infusion
ILC cost reduction
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 44
Frequency dependence of Rs
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 45
Frequency dependence of Rs and non-equilibrium SC
Mattis-Bardeen theory predicts quadratic frequency dependence of Rs. However, the theory is
valid only at “zero” fields. Does it need modifications when we consider field dependence?
Another active research area. The following cavities were studied so far:
0 5 10 15 20 25 30 35 40 45109
1010
1011
Q0
Eacc (MV/m)
N-doping
EP/BCP
120 C baking
1.3 GHz, 2 K
650 MHz 1.3 GHz 2.6 GHz 3.9 GHz
EP
BCP
120 C baking
2/6 N-doping Slides on frequency dependence are courtesy of M. Martinello
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 46
Normalized 𝑹𝑻 𝟐 𝑲 for 120 C Baking
*Some measurements were admin limited between 15-20 MV/m to avoid quench so, in order to compare the different curves, only data till ~20 MV/m are shown
Slides on frequency dependence are courtesy of M. Martinello
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 47
Normalized 𝑹𝑻 𝟐 𝑲 for 120 C Baking
Slides on frequency dependence are courtesy of M. Martinello
At low field RT follows the 2 trend suggested by the Mattis-Bardeen theory
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 48
Q-factor of 2.6 GHz at high field tends to the one at 1.3 GHz
120 C baked cavities
Q-factor of 2.6 GHz cavity converge to the one at 1.3 GHz at high gradients
T=2 K
Slides on frequency dependence are courtesy of M. Martinello
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 49
Normalized 𝑹𝑻 𝟐 𝑲 for N-doping
Higher frequency leads to stronger anti-Q-slope!
Higher frequency is
favorable for Q, and can
be also for higher
gradients.
Understanding the
reversal of RBCS with the
RF field:
o The non-equilibrium
quasiparticle distribution
driven by microwave
fields
o Need solid theoretical
basis and
measurements of some
Nb properties
M. Martinello et al, https://arxiv.org/abs/1707.07582
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 50
Comparison in terms of Q-factor at 2.0 K
Slides on frequency dependence are courtesy of M. Martinello
T = 2 K
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 51
Comparison in terms of Q-factor at 2.0 K
Slides on frequency dependence are courtesy of M. Martinello
T = 2 K
1.3 GHz wins over 650 MHz at ~10 MV/m
3.9 GHz wins over 2.6 GHz at ~13 MV/m
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 52
Unprecedented medium field Q0 at 3.9 GHz
Slides on frequency dependence are courtesy of M. Martinello
Q-factor of N-doped 3.9 GHz comparable to 120 C baked 1.3 GHz cavity at ~ 20 MV/m
T = 2 K
𝑄0~1.5 ∙ 1010
Further improvement with 900 C bake
0 20 40 60 80 100 120109
1010
1011
Q0
Eacc (MV/m)
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 53
Enabling future efficient HEP accelerators
Q > 2e10
At high field
Eacc > 100 MV/m
Q > 3e10
non-equilibrium SC?
new materials?
Nitrogen Infusion
Nitrogen infusion showed that it is possible to achieve both high Q and high
gradient at the same time.
We hope that further progress in SRF experiment and theory will allow us to
achieve much better performance and enable new particle accelerators.
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 54
SRF at low T & low field
(toward mK / single-photon scale):
from accelerators to quantum computers
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 55
Renewed (now practical) importance of the LFQS and low T
A. Romanenko et al, Appl. Phys. Lett. 105, 234103 (2014)
How will the best cavities we have behave at ultralow fields for
various applications? Quantum computing /
quantum memory Dark sector photons searches Gravitational effects search ….
These applications are
interested in high Q at very low fields
LFQS is present after all treatments
What is the cause of the low field Q slope and what happens with Q as we decrease the field further
LFQS
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 56
LFQS measurements toward quantum regime & TLS model
Q measured using a single-shot method (decay from PLL state)
Good news: LFQS stops below 0.1 MV/m with Q ~ 3x1010
Previous models: Halbritter,
Palmieri, Weingarten
From 2D resonator world: non-linear dissipation in two-level systems of an amorphous dielectric layer
Qu
alit
y F
acto
r
Eacc (MV/m)
0.001 0.01 0.1 1 10
1x1010
3x1010
5x1010
7x1010
9x1010
Saturation of the
Q decrease
Fit to TLS model
Ec = 0.1 MV/m
b = 0.19
CWSS RBW=10 kHzSS RBW=30 Hz
A. Romanenko and D. I. Schuster, Phys Rev Lett. 119, 264801 (2017)
121
2 =TTR
w
1tan -µ Ed
tan
d
E
T = 1.5 K
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 57
Effect of the oxide layer
Thicker oxide (anodization) has a drastic effect at low fields
Similar to how we characterize losses due to surface currents via the geometry factor, one can introduce a similar term, the surface participation ratio, characterizing dissipation due to surface dielectric losses
A. Romanenko and D. I. Schuster, Phys Rev Lett. 119, 264801 (2017)
Nb
Nb2O55 nm
Nb
Nb2O5100 nm
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 58
Next step: toward quantum regime
Demonstration of T = 10 mK and <N> ~ 1 photon high Q in 2018. Large dilution refrigerator allows exploring fundamental physics of residual resistance of
SRF cavities at very low temperature.
First SRF cavity is being mounted inside the dilution refrigerator at Fermilab
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 59
SRF accelerators around the world
RAONSoleil
Circular
Lightsrc
<10cavities
Linear
NPHEP
Produc’nOper’n10-100cavities
100-1000cavities >1000cavities
CEBAF
SNS
FRIBEICCLSISAC
LHCXFE
L
ESS
TLS
BESSY
ATLAS
FLASHCESR
C-ADS
LIPAcBEPC-IIHIE-ISOLDE cERL
SPIRAL2
ALPI
ALICEELBE
ANURIB
J-PARCSC-LINAC
SARAF
ANUUSP
PIP-II/IIIFCC
ILC
ISNSADS
MaRIELCLS-II CepC-SppC
SamPosen
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 60
SRF projects in progress / planned
EIC
SC-LINAC
SARAF
HIE-ISOLDE cERLJ-PARC
LIPAc
ANURIB
BEPC-II
TLS
ANUUSP
ALPI
BESSYELBE
FLASHSoleil
ALICE
LHC
ATLAS
CEBAF
SNS
CLS
CESR
ISACSPIRAL2
RAON
Circular
Planning
Light src
<10 cavities
Linear
NPHEP
Produc’nOper’n10-100 cavities
100-1000 cavities >1000 cavities
ESS
C-ADS
ISNSADS
MaRIE
PIP-II/IIIFCC
CepC-SppC
FRIBXFE
L
ILC
LCLS-II
Sam Posen
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 61
Major SRF Projects in Progress
Project No. of
Cryomodules
No. of
Cavities
Voltage
(MV)
LCLS-II 1.3 GHz 35 280 4000
LCLS-II 3.9 GHz 2 12 55
SCLF 75 600 8000
FRIB 46 328 200/nucleon
RISP 45 320 200/nucleon
SNS-upgrade 7 28 300 - 400
ESS 43 150 2000
LHC-HL 8 16
Total 250 1700 15 GV
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 62
Planned SRF Projects, Some ?
Project No. of
Cryomodules
No. of
Cavities
Voltage
(MV) LCLS-II-upgrade 20 160 4000
KEK-ERL 22 200 3000
euV ERL 9 72 800
FRIB-upgrade 46 328 200/nucleon
eRHIC ERL 18 72 1300
PIP-II 25 116 800
PIP-IIIa 18 110 2200
PIP-IIIb 22 176 5000
India SNS 27 126 1000
C-ADS 55 220 1500
Total 350 1900 25 GV
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 63
International Linear Collider
Overview of Future Colliders, H. ZhuInstitute of High Energy Physics
International Linear Collider (ILC)
• e+e- linear collider with Superconducting RF linac
• Baseline: √s = 500 GeV (31 km) → upgrade later to ~ √s= 1 TeV (50 km),
luminosity of 1.8 × 1034 cm-2 s-1 with optional upgrade, one interaction point
(IP) with two detectors: ILD and SiD with push-pull
4
Japanese Association of High Energy Physics (JAHEP) proposed the prompt construction in Japan of ILC as a Higgs Factory at 250 GeV. ICFA expressed its support for this ILC option.
The Linear Collider Board estimates cost of 250 GeV starting point will be 40% less than the cost of 500 GeV TDR cost.
A decision from the Japanese government is expected soon, in 2018? Continued SRF performance improvement R&D, cost reduction R&D and
optimization R&D will be very beneficial.
This will be the largest SRF project , >10 times E-XFEL
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 64
Summary SRF science and technology is going through a period of Renaissance.
There are several new research directions opened up in the last ~5 years, four
of each I reviewed in this presentation: nitrogen doping, nitrogen infusion,
frequency dependence of surface resistance, and SRF in quantum regime.
Other research areas, which I did not have time to discuss, include: nature of
losses due to trapped magnetic flux; study of flux expulsion; non-
equilibrium superconductivity and ultimate gradient limit; Nb3Sn SRF cavities
and other materials.
Nb-based SRF accelerator technology is mature and became the technology of
choice for many new SRF accelerators.
BUT: there are still many problems that need attention and careful
investigation.
These are exciting times and the field needs more young and energetic
researchers! There will always be ample opportunities for imagination, originality,
and common sense.
2/12/2018 S. Belomestnykh | SRF: Recent trends in science and technology | Seminar at JAI 65
Thank you!