Outlines Introduction to Superconducting RF - 上海光源ssrf.sinap.ac.cn/ocpaschools06/School Courses/2.7-SupercondutiongRF.pdf · Introduction to Superconducting RF Chaoen Wang

Introduction toSuperconducting RF

Chaoen WangNational Synchrotron Radiation Research Center

Part of figures and results in this lecture are not cited correctly. This will be updated soon.

Outlines• Fundamental

– Historical Remark – Electrical Resistivity and Thermal Conductivity– Basic of Superconducting RF – Fundamental of SRF Cavity for Accelerator– SRF Performance: Achievement and Challenge– Challenges on SRF Operations

• Applications– SRF Project at NSRRC: Project Review and Operational Experience– SRF Operational Challenges at TPS– Usage of passive SC Landau cavity for bunch lengthening

• Perspective and Further Study

Motivation:Behavior of Metallic Resistance near Absolute Zero

• Kelvin theorized that the resistivity of a metal would reach a minimum and then increase with decreasing temperature as the electrons froze to the atom.

• But Dewar claimed that the resistance would gradually vanish at cold because the thermal vibrations of atoms disappear…

Historical Remark


• C. Dorsman, G. J. Film, and G. Holst investigated metallic elements such as copper, gold and platinum, due to their excellent conducting properties at room temperature. They discovered that the resistivities of these materials leveled off at extremely low temperatures, this was attributed to impurities in the sample.

Historical Remark

Historical Remarkto Low Temperature Physics

Historical RemarkProgress of experimental science is led by development of instrumentation.

• The study of the electronic behavior of materials at extremely low temperatures was possible only after the successful liquefaction of helium. The Dutch physicist Heike Kamerlingh Onnes made liquid helium (and in a large amount) possible in 1908 at the Cryogenic Laboratory of the University Leiden. Onnes discovered the superconductivity in 1911!


• The mercury (Hg) was a material well known for processing a very high degree of purity. On cooling the mercury, Onnes found the resistance of the material dropped off abruptly at 4K. Onnesassigned a value of resistance to the mercury corresponding to the lowest level of sensitivity of his apparatus, as he simply could not believe that the resistance was zero. The DC resistance of a superconductor is lower than 10-20 Ω.

Historical Remark

Measuring Zero Resistance

Historical Remark

Superconductor vs. Normal Conductor

Historical Remark

1. Low temperature resistance of a typical normal metal (ex copper), showing a dc resistivity at low temperatures of the form ρ=ρ0+AT5

2. A superconductor showing zero resistivity below the superconducting transition temperature Tc.

3. Is superconductor a perfect conductor or more than a perfect conductor?

Normal Conductor w/ Zero Resistance?

Historical Remark Historical Remark

Superconducting Elements

• K. Shimizu et al., Superconductivity of Oxygen, Nature, 393 (6687), 767 (1998).

Historical Remark

Even Oxygen Becomes Superconducting at Very High Pressure

Ambient Pressure

High Pressure

Historical Remark

Superconducting Elements

Superconductivity is not a rare phenomenon. About half of the metallic elements have been found to become superconductor at low temperatures.

Historical Remark

Niobium is the best material for SRF application because of its highly workable for forming cavity in pure form with highest Tc of all pure elements.

(Courtesy of Brian Moeckly)

100-Year Journey Toward Absolute Zero

Historical Remark

100-Year Journey Toward Absolute Zero

Historical Remark

Source: Philippe Lebrun

DC Electrical Resistivity

• The simple metal model pictures a metal as a regular array (the crystal lattice) of positive ions surrounded by free (conduction) electrons. The electrical resistivity is then due to

– impurities and imperfections in the metallic crystal lattice;– Thermal oscillations of the positive ions.

• For T << θD, the electrical resistivity is dominated by phonon-electron interactions, electron mainly scattered by structure imperfections, i.e. impurity atoms, dislocations, and grain boundaries.

• For T > θD, same free electron motion mechanism for electric conduction and for heat conduction, the Wiedemann-Franz expression:

DC Electrical Resistivity

Cu

Electrical Resistivity and Thermal Conductivity

Residual Resistance Ratio

• RRR =

• High-purity metal has – high RRR values.– lower residual resistance.– higher thermal conductivity at cold.

• High-purity metal is softer– not good from mechanical rigidity point of view.

Question: How to measure the RRR of a superconductor?

Resistivity at 300K

Residual resistivity at low temperature (normal state)


Measurements of RRR


• The dc resistivity ρ=1/σ• If skin depth > > mean free path, the ac/rf

surface resistance is given by

• But at very low temp and/or high rf frequency, skin depth may be shorter than the mean free path.

– This makes the electron less effective to carry the rf current.

– This make the ac/rf surface resistanceis larger than that given by

DC and AC/RF Resistivity


For copper with RRR of 100, its dc conductivity increases by a factor of 100, but the ac/rf surface conductivity increases only by a factor of 6.

Measured Thermal Conductivity in Niobium Samples

Large RRR means high electrical and thermal conductivity at low temperature.Electrical Resistivity and Thermal Conductivity

What is the Superconductivity• A phase transition from the normal

conductivity state to superconducting state ….

– A phase transition is the transformation of a thermodynamic system from one phase to another. At the phase transition, there is a change of the degree of disorder.

– Example of phase transition in daily life: from gas to liquid; from liquid to solid etc. The molecules in a gas state do not interact with each other, forming a disordered state. As the temperature decreases, the degree of disorder decreases. The liquid is more ordered than the gas state, and the solid state is so ordered that we can know where its molecules are situated.

• The transition is reversible (quench). – Critical temperature– Critical magnetic field– Critical current

• It happens within 10-4 K (pure elements).

Basic of Superconductivity

Phase diagram of the solid - liquid - gas system.

Phase diagram of a type-I superconductor.

Quench of Superconductivity


Critical Temperature, Critical Magnetic Field, and Critical Current Density

Mag

netic

Fie

ld

Temperature

Highest Critical Temperature

Maximum Critical Magnetic Field

superconductor

Normal ConductorMagnetic Quench

Quench of Superconductivity:


Critical Temperature, Critical Magnetic Field, and Critical Current Density

Sketch of the critical surface of NbTi. Also indicated are the regions where pure niobium and pure titanium are superconducting. The critical surface has been truncated in the regime of very low temperatures and fields where only sparse data are available.

Superconductivity: Critical Magnetic Field


Notice the great difference in scale for the critical fields between type I and type II superconductors. Because of their high critical fields, Type II superconductors are of particular interest for superconducting magnet applications.

Superconductivity: Critical Magnetic Field - Temperature Dependence


Experimentally it is found that

( ){ }2coc TT1H)T(H −= ( ){ }2coc TT1H)T(H −=

Critical field

Critical Magnetic Field of Nb Superconductivity: Critical Current - Silsbee Hypothesis

• When the electric current flowing in a type-I superconductor produces a magnetic field at the surface of the material which equals or exceeds the critical field, the normal state is restored.

Features of Superconductivity

• Zero DC resistance (1911) in Meissner state

• Small AC/RF resistance• Meissner effect (1933)

– exclusion of magnetic fields (perfect diamagnet)• Quantum effects

– Magnetic flux quantization– Josephson junction (tunneling effect)

• Isotope effect (1950)• Phase transition


Zero DC Resistance




• Small AC/RF resistance• Meissner effect (1933)





RF Surface Resistance

• Normal conducting material (e.g. copper)

– anomalous skin depth (at cold)

• Superconducting material (e.g. niobium)– BCS resistance (RBCS)– residual resistance (Rres)– trapped magnetic flux (Rm)

Basic of Superconducting RF

Basic of Superconducting RF

Surface resistance of OFHC copper at 500MHz - comparing between anomalous and normal skin effect. (W. Weingarton)

Surface resistance of Niobium cavity (in superconducting sate). (W. Weingarton, 1999)

RF Surface ResistanceNormal conductor (Cu) Superconductor (Nb)

RBCS

BCS RF Resistance vs. Temp


BCS RF Resistance vs. Frequency

Basic of Superconductivity Basic of Superconducting RF

RF Surface Resistance

Why Does a Superconductor Have a Small RF Resistance?

• Recall two-fluid model:– n = nnormal +nsuperconducting– nnormal ≠ 0 at T < Tc– nnormal → 0 when T → 0

• While the Cooper pairs (superconducting electrons) move without friction, they do have inertia.- Because of their inertia, the Cooper pairs do not screen the applied field perfectly. - A time-varying electric field is present in the skin layer. - The time-varying electric field couples to the normal electrons to accelerate and - decelerate them, leading to dissipation…. Padamsee’s Textbook.



• Small RF resistance• Meissner effect (1933)





Meissner Effect (1933)

Screen current will exclude Bext penetration.


Bext

Bext

http://www.jesuitnola.org/upload/clark/labs/meissner.htm

Meissner Effect (1933)


Perfect Conductor vs. Superconductor

• A perfect conductor has zero resistance.– A perfect conductor (σ=∞) would be a perfect diamagnet.– Recall EM Theory: J=σE ⇒ E=0 ⇒ dBint/dt = 0

• A superconductor has zero resistance. • A superconductor is a perfect diamagnet,

– but there is more than this involved in the Meissner effect.– Bint = 0 (automatically dBint/dt = 0)


A Perfect Conductor is more than a superconductor.

Basic of SuperconductivityAssume a normal conductor can become a perfect conductor when the temp is lower than its critical temperature….

Perfect Conductor: ρ = 0 ⇒ dBint/dt = 0

dBint/dt = 0

dBint/dt = 0

Zero-field Cooled Field Cooled

A Superconductor is more than a Perfect Conductor!

Perfect Superconductor: Bint = 0


Bint = 0

Bint = 0

Zero-field Cooled Field Cooled

Type I & II Superconductor

• Type I Superconductor– lead

• Type II Superconductor– niobium


Type I Superconductor- Perfect Meissner Effect


Type II Superconductor- imperfect Meissner Effect









Vortex StateMixed State for Type II Superconductor


Vortex: Magnetic Flux QuantizationMixed State for Type II Superconductor


Vortex State

Vortex Observations


Direct Vortex Imaging Using Scanning Tunneling Microscope

Vortex Density

Higher Magnetic Field

Superconductor

(Meissner state)

Normal

Conductor

Vortex Density

Hc2Hc1 H0

-M

RF Residual Resistance Due to Trapped Flux

RF Residual Resistance Due to Trapped Flux Features of Superconductivity







Isotope Effect (1950)

• Tc changes for different isotopes of the same elements.

• Superconductivity is connected with the motion of nuclei.– Interaction between

free electron and lattice (phonon)

Superconducting Transition Temperature as a Function of Isotopic Mass








– Specific Heat or heat capacity (2nd order phase transition)– Thermal conductivity (1st order phase transition)


Specific Heat

• The specific heat or heat capacity C is the amount of energy required to heat one gram/mole of a substance one Celsius/K.

• For normal metals the specific heat heat exhibits a temperature dependence C = γT + βT3. – The first term (Ce = γT) arises from the electronic

contribution. – The second term (CL = βT3) arises from the lattice.

• As a superconducting metal is cooled below the transition temperature Tc, the electronic part of the specific heat of a superconductor, increases abruptly and then decays rapidly with decreasing temperature,, falling off to zero exponentially as T → 0. Such an exponential fall off in C(T) provided early evidence (1954) for an energy gap, Δ.

Specific Heat


The specific heat or heat capacity C is the amount of energy required

to heat one gram/mole of a substance one Celsius/K.

Specific Heat

Vanadium




Aluminum

Specific Heat


V3Ga



Thermal Conductivity of Normally Conducting and Superconducting Lead

Poor thermal conductivity

Theories of Superconductivity

• Two-fluid model (1934)• London equations (1935)• Ginzburg-Landau (GL) theory (1950)• BCS theory (1957)


• Assume– n = ns + nc

• ns: superconducting electrons

• nc: normal electrons

– Only ns(T) is capable of participating in a super current.

Two Fluid Model (1934)


Two Fluid Model (1934)



The London ModelAn important consequence of flux exclusion in superconductors is that

If magnetic flux density must remain zero in the bulk of a superconductor, then any currents flowing through the superconductor can flow only at the surface

However a current cannot flow entirely at the surface or the current density would be infinite

The concept of “penetration depth” must be introduced

In 1934 F and H London proposed a macroscopic phenomenological model of superconductivity based upon the two-fluid model

The London model introduced the concept of the (London) penetration depth and described the Meissner effect by considering superconducting electrodynamics

First and Second London Equations (1935)

• Introducing superconducting electrons

• Satisfying Meissner effect


London Equations (1935)



Niobium: 39 nm at 0K (temperature dependent)

The London Penetration Depth The London Surface Currents


We operate a 500 MHz sc cavity at 4.5K but a 1.5 GHz sc cavity at 2K.

GL Theory

• Strictly valid only at the temperature near Tc but not generally applicable at lower temperature.

• Distinguish type I and type II superconductor with GL parameter KGL, the ratio of London penetration depth to superconducting coherence length.– Niobium with KGL ≈ 1 (>0.707) is a type II superconductor– Lead with KGL≈ 0.45 (<0.707) is a type I superconductor


Ginzburg-Landau Parameter


BCS Theory & Cooper Pairs

• The isotope effect hints the superconductivity involves the lattice, not only the electrons.

How Does Superconductivity Occur?Below Tc, electrons form Cooper pairs and condense

into a quantum mechanical ground state

1. Electron #1 deforms the (positive ion) lattice when moving through it;

2. Electron #2 is attracted by the deformed lattice if the attraction is larger than the electron-electron repulsion, a pair is formed.

3. Pairs (called Cooper pairs) have distances up to few mm.

4. The binding energies of a “Cooper pair” are ~1 mV, only possible at low Temperature.

5. The attraction between paired electrons depends on the mass of the ion.

BCS Theory& Cooper Pairs

•• Paired electrons condense into coherent state Paired electrons condense into coherent state –– no DC resistance;no DC resistance;

•• Perfect diamagnetism Perfect diamagnetism -- electrons circulate to screen magnetic field electrons circulate to screen magnetic field (Meissner effect).(Meissner effect).


BCS Theory& Cooper Pairs


BCS Theory & Cooper Pairs

• Cooper pairs are bosons; as all bosons, Cooper pairs can all be in the same energetic state (Bose-Einstein condensation);

• In a Cooper pair, the electron spins (+1/2 or –1/2) cancel and the total spin is 0;

• All Cooper pairs are in the ground state;

• The Cooper pairs are described by one wave function;

• Collisions can only alter the whole systems of pairs, such a change requires a large amount of energy.


BCS Coherence Length

• A length related to the phase transition from the superconducting state to the normal state.

• The surface depth within few coherence lengths (38 nm for niobium) and London penetration lengths (39 nm for niobium) is important for the RF property of the SRF cavity. The others are of crucial for the heat transfer.


Energy Gap

• In a material at its normal state any change in the energy will cause the electrons to move into an excited or empty state.

• In a material at its superconducting state, the occupied state’s energy decreases due to the superconducting mechanism (Cooper paring), resulting in a gap in the excitation spectrum.

Empty states

Occupied states Occupied states

Empty statesEnergy

at T= 0 Kat B=0 T

Normal Conductor Superconductor


BCS Theory: Energy Gap & Critical Temperature


Phase Transition: Energy Gap


Free energy of aluminum in the normal and superconducting state as a function of T (after N.E. Phillips). The normal state is achieved by applying a magnetic field larger than Bc.

BCS Theory: Critical Magnetic Field & Specific Heat


BCS Theory vs. Two-fluid Model

• Two-fluid model: Normal conducting electrons and superconducting electrons.

• Density of superconducting electrons is proportional tothe excitation probability @T and energy gapΔ at T=0

BCS Theory vs. London Eq. Fundamental of SRF Cavityfor v/c≈1 Accelerator

Fundamental of SRF Cavity

Copper cavity with nose cone and small beam tube to enhance the shunt impedance. This enhances the HOM impedance too

Fundamental of SRF Cavityfor v/c≈1 Accelerator

• Accelerating mode: TM010-like;• Resonance frequency (f0); • Ohmic quality factor (Q0);• Surface resistance (Rsur);• External quality factor (Qext);• Shunt impedance (Rsh);• Accelerating voltage (Vc); Note: Pc=Vc

2/2Rsh

• Accelerating gradient (Eacc=2Vc/λo);• Ratio Rsh /Q0;• Cavity geometry constant (G, Q0 =G/Rsur).


Fundamental of SRF Cavity- CESR’s 500 MHz SRF Module


SRF Performance:Achievement and Challenge

• Theoretical performance limitation• Thermal Defect• Q-virus (disease)• Q-switch• Multipacting• Field emission• Global heating

Theoretical Performance Limitations• Surface magnetic field limitation

– lower than rf critical magnetic field Hsh;– for niobium at 0K, Hpeak < Hsh (0) = 2300 Oe;

or Eacc < 49 MV/m for typical SRF elliptical cavity.

• Surface electric field limitation– larger than 120 MV/m demonstrated

• TESLA cavity– larger than 30 (20) MV/m for 1.3GHz single cell cavity

(multicell structure) demonstrated.

Limitations of the SRF Cavity Performance

• Existence of normal conducting area in a size of sub-millimeter.

• Surface power dissipation on the defect increases with the Eacc and results in quench after reaching threshold of thermal instability.

• Temperature instability can be suppressed by using superconducting material with higher thermal conductivity (larger RRR!)

Thermal Defect or Thermal Instability

Thermal Defect

Measured Thermal Conductivity in Niobium Samples

Thermal DefectLow RRR means

low thermal conductivity at low temperature.

RRR Dependence of Quench Field

Thermal Defect

Hydrogen Q-Virus (Q-Disease)- Residual Resistance due to Excess H in the Nb Bulk

• Excessive hydrogen can be condensed intothe RF surface of the cavity made by high purity niobium.

• Q-virus results in Q0 drop at low field but without field emission.

• The hydrogen contamination may be avoided by controlling the temperature of acid etch (lower than 15oC) and other factors which is used to prepare a sufficiently clean surface.

Q-Virus

• Solutions:– Baking of niobium cavity in a

vacuum oven to 900oC is sufficient to remove the hydrogen from the niobium.

– Quick cool down between 150K to 60K can minimize the extend of hydride formation and the accompanying residual losses (problem with thermal stress owing to fast cool-down needs be considered).

Hydrogen Q-Virus (Q-Disease)- Mechanism and Symptom Explanation

• At room temperature hydrogen (H) moves freely inside the niobium bulk.

• When a cavity is cooled, the dissolved hydrogen precipitates as a hydride phase.

• At room temperature, the required concentration of hydrogen to form hydride phases is very high, e.g. 4600 wt ppm, but the required concentration drops to < 10 wt ppmbelow 150 K.

• The hydride phase has high rf loss. This explains the Q-drop of a Q-disease (Q-virus) cavity.

Q-Virus

Multipacting

• Multipacting is– an avalanche effect under high vacuum rf/e--beam environment; – a resonance process between secondary emission electrons and RF

field oscillations (dependent on RF frequency and ratio of forward to reverse power level, etc.);

– Number of secondary emission electrons per impact is very sensitive to the surface conditions (pre-treatment, baking, post rf processing, and post gas condensation);

– not always processable (soft barrier and hard barrier);• Multipacting was a problem of the SC cavity but now is

more a headache problem of the in-vacuum RF main power coupler (for both coaxial and waveguide types) owing to the requirements of high power transmitting.

Multipacting

Multipacting in SRF Cavity

Multipacting

Multipacting in SRF Cavity• One-point MP

• Mulit-point MP

Multipacting

Multipacting in SRF Cavity

Multipacting

SEC of Niobium at Warm

Multipacting

Multipacting Simulation for Cornell’s 500 MHz SC Cavity

Multipacting

MUPAC Simulation by Guillaume Devanz

Different impact energies of primary electrons

Multipacting on RF Power Coupler

• Multipacting needs be processed away off-line (w/o beam) for reliable operation.– How long will the surface remain in multipacting-free for a

specific MP barrier after processing?

• Without beam, the RF feed-line is always operated in the standing wave condition.– How to process the multipacting barrier in the mixed wave

condition (partially reflected)? • DC bias (only available for coaxial coupler); • magnetic coil; • high pulsed power processing;• helium processing;

Multipacting

Coaxial Power Coupler

Multipacting

Waveguide Power Coupler

Multipacting

Why Klystron is Multipacting-Free?- KEK Klystron Output Coupler (1.3 MW)KEK Klystron Output Coupler (1.3 MW)

Multipacting

Beam Current Dependent VSWR- The SRF Operation for TLS

Multipacting

SEC of Copper at Cold

Multipacting

• Multipacting may be enhanced after cryogenic condensation/adsorption of water/gas on the cold metal surface.

Multipacting Suppression and Excitation by Magnetic Field

Multipacting

excitation

suppression

Multipacting

• Characteristic: Q-drop at some discrete field levels, X-ray at the levels.

• Diagnostics: Temperature mapping & X-ray mapping

Multipacting

T-mapping of Tow-point MP

Multipacting

“Conventional” Field Emission

• Thermal emission:– Electrons pass over the

barrier (work function).– Possible to explain by

classical mechanism.

• Field emission:– Electrons pass through the

barrier because of its wavelike properties (tunneling effect).

– A pure QM mechanism.

Field emission

SRF Field Emission

• SRF field emission is normally caused by foreign particle contamination– Emitted electron current grows exponentially with field– Reaching the surface accelerated electrons produce cryo-losses and

quenches– Part of the electrons reaches high energies: Dark Current

• Theoretically, electric fields of 3-5GV/m are needed for field emission, according to Fowler and Nordheim (FN) theory.

• Observation of field emission in SRF cavity started at Epk of 10-20MV/m (factor of few hundred!)

• Characteristic: Q-drop at some field levels, X-ray at the levels.• Diagnostics: Temperature mapping & X-ray mapping

Field emission

SRF Field Emission: Simulations

Field emission

(H. Padamsee)

Solutions to SRF Field Emission

• Different chemical treatment: EP vs. BCP

• Minimize foreign particle contamination– apply high pressure rinsing (HPR);– cavity assembly in a class 10-100 clean room;

• Apply high pulsed power processing (HPP);– damage activated emitters;

• Apply helium processing (HP);

• Apply cavity baking;

Field emission

Chemical Treatments

• BCP (Buffered Chemical Polishing)– HF (48%) : HNO3(69%) : H3PO4 (84%) with volume ratio 1:1:2 or 1:1:1

• EP (Electropolishing)– HF : H2SO4 with volume ratio 1:9

Nb NbBCP EP

Electropolishing

• EP makes an improvement on the onset level of field emission– Much smooth surface → Less local field enhancement;– Better cleaning with high pressure water rinsing;

(Carlo Pagani, 2005)

Solution for High Field Gradient- High-pressure water rinsing in Clean Room

Solution for High Field Gradient - SRF Assembly in Class-10 or Class 100 clean room

DESY

Solution for High Field Gradient - SRF Assembly in Class-10 or Class 100 clean room


Cornell University


Source: Hasan Padamsee and Jens Knobloch

High Pressure Water Rinsing

High Pressure Rinsing

Solution for High Field Gradient- High-pressure water rinsing in Clean Room High Pulsed Power Processing

High Power Processing

High Pulsed Power Processing

High Power Processing

SRF Field Emission: Cornell Model I- Tip-on-tip Model

• Theoretically, electric fields of 3-5GV/m are needed for field emission, according to Fowler and Nordheim (FN) theory.

• Observation of field emission in SRF cavity started at Epk of 10-20MV/m (factor of few hundred!)

• Modifying the FN equation with an artificial field enhancement factor β(between 50 and 500) for experimental fitting => Tip-on-tip model

Field emission

Field emission

f=352MHz

SRF Field Emission: Simulations SRF Field Emission w/ a Puzzle Nature

• Metallic particles of irregular shape; typical size: 0.5-20μm• Only 5-10% of the total number of foreign particles actually present on the surface were found

to emit (H. Pademsee), or condensed residual gas can…• The activated/deactivated emitters can become deactivated/activated after warm-up and cool-

down thermal cycle (Q. S. Shu et al.), or condensed residual gas can…

Field emission

SRF Field Emission & Oxygen Condensation SRF Field Emission: Cornell Modeling II- A “Concluding Picture”?

Field emission

• local heating• liberation of gas from

surroundings - up to 1 bar!

• generation of plasma in stationary ion cloud

• ‘uni-polar’ arc • local melting• characteristic times are

very short compared to (our) pulse lengths (SRF tests at ~CW)Ref: Marc Ross

Volcanoes of the Deep Sea Helium Processing- a) Modification of the Adsorbed Gases; b) Explosive Destruction

• Fill helium gas into the SRF cavity up to 10-6 Torr and then RF processing.

• Risk of vacuum accident.

• Many possible mechanisms had been proposed.

• Sometimes helpful for Multipacting (MP) and field emission (FE) suppression, but not always!

Helium Processing

Problem w/ High Field Q-Slope- When Field Emission Free

Cure of Q-drop at high field- Recent Advances toward High Gradient

• Electropolishing (from KEK) plus in-situ baking ata round 120-1400C, in 48-60 Hr (from CEA-Saclay);

Recent Advances toward High Gradient- In-Situ Baking (not heat treatment > 900o C)

Effects of Cavity Baking on Q-Slopes

• Enhancement of Q-slope at low field level;• Minor flattening of the Q-slope at medium field;• Strong improvement of Q-slope at high field;

High Field Q-Slope- Is High Field Q-Slope due to Surface Electric Field or Magnetic Field?

Q0 vs. Bp measured in the TM010 mode (1.47 GHz) and in the TE011 mode (2.82 GHz) of a post-purified single cell cavity before and after baking at 120 C for 30 h [G. Ciovati et al., 2006] Note that there is no surface electric field in the TE011 mode.

Cavity Baking: BCP vs. EP- Increase of the Quench Field Level if EP Adopted Instead of BCP.

Differences Made by Cavity Baking

Rres: temperature independent

RBCS

- Baking causes a reduce of BCS resistance in a factor of about 2.

- Baking does not change or increase slightly the residual resistance.Cavity Baking


Cavity Baking


• The natural oxide layer of Nb2O5

may play a role on Q-drop at high field...

Role of Baking Time and Baking Temperature?

Cavity Baking

Diffusion Mechanism of Cavity Baking

• The natural oxide layer of Nb2O5may play a role on Q-drop at high field...

• Baking results in decomposition of Nb2O5 into sub-oxides (NbO, Nb2O)…

• Make the Nb2O5 layer thinner will reduce the normal conducting dissipation , because it is a dielectric layer.

• Baking will result in high oxygen concentration in the superconducting layer…

• Therefore, an optimal baking temperature for minimum oxygen concentration in the superconducting layer…

One possible explanation: Consequence of oxygen diffusion process by baking.

Cavity Baking

Eliminating the Effects of Cavity Baking

Cavity Baking

The SRF Project at NSRRC(1997-2005)

Replacing the Doris Cavities with The SRF Module of CESR-III Design

The SRF Project at NSRRC(1997-2005)

• Project initialed in winter, 1997;• Approved in summer, 1999;• Budget available since 2000 in 4 years;• Commissioning originally scheduled for summer,

2003, and done in end of 2004 owing to production problem;

• Spare SRF module available in summer of 2005; • Routine operation since then.

– Test run at 400 mA in decay mode;– Routine operation at 300 mA in top-run mode;

Component Specifications and System Integrations

Decision Making for the SRF Project at NSRRC

• Guarantee the photon light stability at a higher beam current– Transverse feedback system is absolutely required to stabilize the photon

beam;

• Doubling the photon intensity by operating at 500 mA in decay mode– Doubling of photon intensity has been achieved at a beam current less than

280 mA in top-up mode.

• Reduce the cavity occupied-length– Superconduting wiggler is installed in down-stream of the same straight

section.

• Simplify the configuration of RF plant– Low-level rf system is challenged for operation with heavy beam loading.

• Increase the Touschek lifetime– Enlarging the RF gap voltage (from currently 0.8 MV) to its optimal value, i.e.

1.6 MV

• Build-up the application of SRF technology to accelerators in Taiwan.– We are still in the learning stage…

The SRF Module

The SRF Module – S1 & S2

We had a very difficult time during the production of the SRF module S2. We learned a lot from this SRF module.

LHe Cryogenic Plant for SRF Operation

• Requirements:– long term continuous operation– vibration free

– fast cool down capacity

– high redundancy

– energy saving and load matching

– easy operation

– SRF operating LHe pressure as low as possible

• Solution:– turbine machine

– large helium inventory (2000 liter main dewar)

– capacity safety factor of 1.5

– frequency driver

– fully automatic control

– Cold box located to the SRF module as close as possible

Liquid N2 Dewar

He gas storage tank

He Compressor

Liquid He DewarRefrigerator and

LHe Cryogenic Plant for SRF Operation LHe Cryogenic Plants- One for SRF (2003) and the Other One for SMAG (2005)

SRF Module and LHe Cryogenic Plant LHe Cryogenic Plant

Storage Ring RF System (2004-)Operational Challenges

- Margin with DC Robinson Stability

Machine Performance with the SRF Module at TLS

• Much better HOM damping performance;• No more rf modulation applied/required to stabilize the longitudinal

coupler-bunch instability;• Maximum beam current up to 400 mA demonstrated;• Top-up mode at 300 mA in routine operation;• Transverse feedback is required to stabilize the residual instabilities; • Margin benefit from the longitudinal feedback in this moment (I0

fluctuations decreases from 0.08% to 0.06%); IR beam line might be happy with the longitudinal feedback.

• Intensive manpower required for the srf maintenance – not to make mistake!

Touschek Lifetime & Bunch Lengthening

• Particles in the bunch are subjected to Betatron oscillations.• Coulomb scattering between the particles can transfer transverse

momentum to longitudinal plane.• If this extra momentum brings the two scattered particles beyond the

momentum acceptance of the ring, then the particles are lost.• Increasing the bunch length will reduce the peak charge density and

therefore decrease the collision possibility for increasing the Touschek lifetime.

Px

-Px

Ps=-Px Ps=Px

Px

-Px

Ps=-γPx Ps=γPx

Beam frame Laboratory frame

Harmonic Cavity for Bunch Lengthening

Optimum Conditions

Unexpected Performance Degradation w/ Passive Harmonic Cavity due to Partial Beam Fielling

Usage of Superconducting Passive Harmonic Cavity to Reduce the R/Q

• Owing to AC Robinson instabilities, the passive harmonic cavity cab only be operated far away from the resonance –at a large detuning angle.

• The beam induced voltage in a passive warm (copper) harmonic cavity is insufficient for flattening of rf gap voltage at synchronous phase.

• Passive superconducting harmonic cavity has a much high shunt impedance and can much easily provide large beam induced voltage at a large detuning angle. Therefore, the total R/Q can be reduced by using superconducting cavity.

• The lower the R/Q, the less the variations of synchronous phase – understanding from the tracking code.

• Superconducting harmonic cavity is superior than warm (cooper) harmonic cavity for effective bunch lengthening.

Passive Superconducting Harmonic Cavity for SLS and Elettra

Passive Superconducting Harmonic Cavity for SLS and Elettra Perspective: Next Era for

Superconducting Light Sources?

• Superconducting RF– Accelerating Cavity – Passive harmonic cavity– Crab cavity

• Superconducting Magnets– Superconducting insertion device (wiggler and undulator)– Superconducting magnet (wave-length shifter and bending magnet)

Further Reading

• H. Padamsee et al., RF Superconductivity for Accelerators, John Wiley & Sons, Inc.

Documents

Outlines Introduction to Superconducting RF - 上海光源ssrf.sinap.ac.cn/ocpaschools06/School Courses/2.7-SupercondutiongRF.pdf · Introduction to Superconducting RF Chaoen Wang