1. Materials, Advanced Accelerator Science & Cryogenics Division
SCRF Materials R&D activities at RRCAT
S B Roy
Materials, Advanced Accelerator Science & Cryogenics Division
RRCAT, Indore
Collaborators:
RRCAT-- M. K. Chattopadhyay, A. Putambekar, P. Kush and V. C. Sahni
IUAC, New Delhi– R. N. Prakash and D. Kanjilal
Jefferson Lab– G. R. Myneni

2. Linear Accelerators (LINAC) are made of Radio Frequency (RF) Cavities
‘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.

3. Two fundamental limits for a SC-RF cavity:
What do we require from a good cavity ?
High Quality Factor: Q = (Stored energy)/(Dissipated power)
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 requiring continuous wave
or long-pulse accelerating fields above a few million volts per meter (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.
(2) The surface resistance as predicted by the microscopic BCS theory.

4. Nb SCRF cavities working at 2K are supposed to give 50 MV m-1?!
Materials and surface issues in superconducting Niobium: Extrinsic effects
Surface roughness, grain boundaries  Lower break down field
Impurities  Depress superconductivity, increase Rresidual.
Surface Oxides  Suspected to depress SC ?? 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
30-32 MV m-1 are obtained regularly in
various labs.
Nb SCRF cavities working at 2K are supposed to give 50 MV m-1?!

5. Two Major Open Issues in RF Superconductivity of Niobium:
(1) What is the RF critical magnetic field in Niobium? Is it
Superconducting critical field-Hc1 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
Nb SCRF cavities working at 2K are supposed to give 50 MV m-1?!

6. Qualification of Nb materials for SCRF cavity fabrication:
*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 + BCP/EP treatment
 Extrinsic (+ surface) defects are low & so multipacting reduces.
*But high RRR does not say how good are the SC properties of Nb & 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

7. Questions we are examining in Nb and other SC materials:
Is there any dependence of Hcritical on,
(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 since that
sets an upper limit on achievable SC-RF accelerating gradients.

8. SCRF Materials R&D
HC1<H<HC2 => Vortex state => important for high critical current (Jc) applications SC magnets
HHC1(T) => important for RF superconductivity applications

9. Effect of BCP treatment on Nb samples
Nb samples used here are obtained from the same batch of materials used
for making SCRF cavities at Jlab, USA, and they are subjected to same
BCP and annealing treatments as usually given to the SCRF cavities.
Estimated HC1 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.

10. Effect of BCP treatment on the TC of Nb samples
Controlled samples from JLab

11. Main results in a nutshell
Magnetic flux lines penetrate at lower H in a large grain chemically treated Niobium sample than in an pristine Nb sample.
RF cavity prepared with
such BCP Nb would reach
maximum Mv m-1 →

12. Effect of EP treatment on Nb samples
Controlled samples from IUAC, New Delhi

13. Effect of EP treatment on Nb samples
Controlled samples from IUAC, New Delhi

14. Effect of Ta impurities on the SC properties of Nb
Controlled samples from JLab

15. Fundamental physics & newer SCRF materials: ongoing and future works
. Which one is most influential: HC1or HP ?
. Does upper critical field HC2 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.
. Newer materials : Alloys of Niobium, MgB2 ,Nb3Al, MoRe-alloys etc.

16. RRR measurement on Niobium samples (A. Puntambekar)
As part of development programme we have established a RRR test facility for Niobium at RRCAT.
This facility is for measurement of bulk Resistivity mainly aimed for gross qualification of vendor material and is also used for qualification of indigenous niobium development.
The facility is similar to the facility at FNAL. It uses a dip stick holding the sample & inserting it in to standard LHe Dewar.
The RRR defined here is the ratio of the resistances at room temperature (300 K) and just above the super- to normal-conducting transition ~10 K.
In order to calibrate the test set up some reference samples taken from FNAL & CERN were measured again.
The test results are compared with that of measurements done at RRCAT.
The test results are summarised in the next slide.

18. SUMMARY & COMPARISION OF TEST RESULTS
S.NO.
source
SAMPLE ID
REPORTED
RRCAT RESULTS
DEVIATION%
CODE
RRR
TEST 1
TEST 2
OBSERVED 1
FNAL
Nb/FNAL/Dec-07/15
339
335
323
4.70%
4-Apr-08
7-Apr-08 2
Nb/FNAL/Dec-07/16
342
352
2.90%
8-Apr-08 3
CERN
Nb/CERN/July-04/05
182
175
170 7%
30-Jul-04
8-Jul-05 4
Nb/CERN/July-04/10
190
185
187.7
2.60%
27-Jul-04
29-Jul-04 5
NFC
Nb/NFC/IC-1/A * 34
11-Apr-08
This list to be updated …………

19. Electrical & Magnetic properties testing Nb-NFC

20. Thank You

21. Single crystal Nb
Large grain Nb