Transcript Superconducting materials R&D: RRCAT

Superconducting Materials R&D: RRCAT-JLAB Collaboration
S B Roy
Materials & Advanced Accelerator Science Division
RRCAT, Indore
Collaborators: M. K. Chattopadhyay, V. C. Sahni, G. R. Myneni , P Prakash
Superconducting Materials R&D : Overall Aim
• Tuning superconducting properties of a suitable material for
fabrication of an energy efficient and cost effective SC-RF
accelerator structure.
• Achieving reliability and reproducibility in the SC-RF cavity
performance.
• Gain knowledge and experience to venture into newer energy
efficient and superior materials.
Superconducting Radio Frequency (SCRF) Cavity
‘An RF power source’ fills the
RF cavity via a ‘coupler’.
EM field will accelerate & impart
energy to the charge particles if they
are in phase with the electric field.
What do we want from a good cavity ?
High Quality Factor: Q = (Stored energy)/(Dissipated power)
As high a gradient as possible
Dissipated power:
For copper at 300 K 1.3 GHz, Rs Copper= 9.4 mΩ
For bulk Nb at 2K RBCS  10 n
Superconducting RF cavities excel in applications where
one needs ‘continuous wave or long-pulse’ acceleration
with gradients above a few million volts per meter (MV m-1)
Materials and surface issues in Niobium SCRF cavities: Extrinsic effects
• Surface roughness, grain boundaries  Electrical break down; ↓Gradient
• Impurities  Depress superconductivity, increase Rresidual.
• Surface Oxides  Suspected to degraded SC response?? NO
• Field emission and multipacting  Quenching of the Cavity.
Most of these problems are solved
with proper cavity shape, and chemical
treatment and cleaning of cavity surface.
Field emission free cavities reaching up to
30-35 MV m-1 are obtained regularly in
various labs.
But Nb elliptical 1.3 GHz SCRF cavities at
2K are supposed to give 45 MV m-1?!
Two fundamental limits for a SC-RF cavity:
(1) A critical rf magnetic field above which the perfect SC state is destroyed
-- limits the Accelerating Field or Gardient.
(2) The surface resistance as predicted by the microscopic BCS theory.
-- limits Q.
Nb for SCRF cavity fabrication: Material qualifying criterion
*Current approach mainly relies on improving the residual resistivity ratio
(RRR) of the Nb. Involves expensive Niobium refinement process.
*With high RRR Nb + right cavity shape + chemical treatment
 Extrinsic (+ surface) defects are low & so cavity loss reduces.
*But high RRR does not necessarily say how good are the SC properties of
Niobium & at best gives indirect information on thermal conductivity.
*All cavities fabricated in the same way do not give high gradients.
* Cavity gradient seldom reaches above 40 Mv m-1
Two Major Open Issues in RF Superconductivity of Niobium:
(1) What is the RF critical magnetic field in Niobium? Is it
– Thermodynamical critical field-Hc or field for first flux line
penetration-HP?
– How does it depend on temperature?
(2) Why does the RF surface resistance of niobium increase
sharply at high RF magnetic field?
-- High-field slope in the quality factor-Q-slope
HC1
Nb SCRF cavities working at 2K are supposed to give 45 MV m-1?!
SCRF Materials R&D
HC1<H<HC2 => Abrikosov lattice or Vortex state => important for high critical
current (Jc) applications e.g. SC magnets
HHC1(T) => important for RF superconductivity applications
Points we are examining (in Nb & other SC materials)
Role of HP and how it may be varying with,
(1) the methods of Nb materials preparation, grain size ?
(2) the surface chemical treatment of Nb: Electropolishing
versus BCP ?
(3) thermal treatment -- annealing temperature and time ?
Through an understanding of the microscopic properties of the
materials treated differently we can possibly identify SC materials,
which will give best performance.
A better qualification scheme is needed using HC1 or Hp and RS since
those set limits on achievable SC-RF accelerating gradients.
Magnetization Measurements
The basic VSM measurement is accomplished
by oscillating the sample near a detection(pickup)
coil and synchronously detecting the voltage induced.
By using a compact gradiometer pickup coil
configuration, a relatively large oscillation amplitude
(1-3mm peak) and a frequency of 40 Hz, the system
is able to resolve Magnetization changes of less than
10-6 emu at a data rate of 1 Hz. The sample is
attached to the end of a sample rod that is driven
sinusoidally. The center of oscillation is positioned at
the vertical center of a gradiometer pickup coil. The
voltage induced in the pickup coil is amplified and
lock-in detected in the VSM detection module.
Effect of BCP treatment on Nb samples
Nb samples obtained from the same batch that was used for making SCRF
cavities at JLab, USA, subjected to same BCP and annealing treatments as
was given to the SCRF cavities.
Estimated HC1 (or Hp) of the BCP treated samples correlates well with the
reported surface magnetic fields above which a severe degradation of the Q-factor
is observed in the BCP treated Nb SC-RF cavities.
Effect of BCP treatment on the TC of Nb samples
Samples from Jlab, USA.
Conclusions
BCP degrades SC properties, Tc, HC1 and HC2 , significantly, hence not
quite desirable.
Main results in a nutshell
BCP treatment lowers the field at which magnetic
flux lines enter the material as compared to that in
pristine Nb.
→
RF cavity prepared with
such BCP Nb would reach
maximum 30-32 MV/m
Effect of EP treatment on Nb samples
Samples from IUAC, New Delhi
Effect of EP treatment on Nb samples
Samples from IUAC, New Delhi
Conclusion: Effect of EP treatment on SC properties of
Nb is rather small. EP treatment is therefore preferable
for processing Nb-SCRF cavities.
Effect of Ta impurities on the SC properties of Nb
Samples from JLab
Conclusions
Higher Ta impurity only marginally affects SC properties. One can possibly
use less pure materials to make cavities. Will reduce the cost of Nb refinement.
–“By now there exists compelling evidence that the BCP process limits the attainable
accelerating fields of multicell cavities to about 30 MV/m even if niobium of excellent
thermal conductivity is used.” L. Lilje et al (DESY) arXiv Physics:0401134v1
Ongoing and future works : Fundamental physics &
newer SCRF materials:
• Which one is most influential: HC1or HP ?
• Does upper critical field HC2 (or HC3) play any role in the SCRF cavity ?
• Thermal instability in superconducting properties of Nb
– flux jump in Nb → role of thermal conductivity
• Detailed study of surface resistance of superconductors RBCS in applied
magnetic fields.
• Nb thin films  Nb-coated Copper cavities.
• Newer materials : MgB2 ,Nb-Zr, Nb-Al, Mo-Re alloys etc.
Thank You
Single crystal
Nb
Large grain
Nb