easitrain...easitrain-wp2-d2.2 date: 01/02/2021 grant agreement 764879 public 1 / 25 grant agreement...

Quality parameters for Nb3Sn, NbN and Tl thin- films

EASITrain-WP2-D2.2

Date: 01/02/2021

Grant Agreement 764879 PUBLIC 1 / 25

Grant Agreement No: 764879

EASITrain European Advanced Superconductor Innovation & Training

DELIVERABLE REPORT (D2.2)

QUALITY PARAMETERS FOR NB3SN, NBN AND TL THIN- FILMS

Document identifier: EASITrain-WP2-D2.2 EDMS 2041968

Due date: End of Month 40 (January 31st, 2021)

Report release date: 01/02/2021

Work package: WP2 (Materials)

Lead beneficiary: TUW

Document status: RELESED (V0100)

Abstract: The requirements on thin films are quite specific in applications such as radio frequency (rf) cavities or beam screen coatings in accelerators. The figure of merit is the surface impedance which generally benefits from a clean flat surface. However, also thermal properties, the behaviour in vacuum or the interaction with magnetic flux is of central importance. We report on the preparation and characterization of NbN and Tl-1223 films and discuss or model the expected behaviour under the conditions relevant for these applications.


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Copyright notice:

Information provided with this document is subject to the H2020 EASITrain – European Advanced Superconductivity Innovation and Training grant agreement. This Marie Skłodowska-Curie Action (MSCA) Innovative Training Network (ITN) receives funding from the European Union’s H2020 Framework Programme under grant agreement no. 764879.

Delivery Slip

Name Partner Date

Authored by Stewart Leith1, Dmitry Tikhonov2, Michael Vogel1, Oliver Kugeler2, Aisha Saba3, Emilio Bellingeri3, Alice Moros4, Johannes Bernardi4

1USI, 2HZB, 3CNR, 4TUW 19/01/2021

Edited by Michael Eisterer TUW 22/01/2021 Reviewed by Michael Benedikt CERN 31/01/2021 Approved by Michael Benedikt CERN 31/01/2021


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TABLE OF CONTENTS 1. NBN BASED THIN FILM SYSTEMS ........................................................................................................................ 4

1.1. INTRODUCTION ...................................................................................................................................................... 4 1.2. DEPOSITION .......................................................................................................................................................... 4 1.3. RF PERFORMANCE MEASUREMENTS ......................................................................................................................... 11 1.4. REFERENCES ........................................................................................................................................................ 13

2. TL-1233 COATINGS ........................................................................................................................................... 14 2.1. INTRODUCTION .................................................................................................................................................... 14 2.2. PREPARATION ..................................................................................................................................................... 14 2.3. CHARACTERISATION ............................................................................................................................................. 16 2.4. RESULTS ............................................................................................................................................................. 22 2.5. REFERENCES ........................................................................................................................................................ 25


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Since the requirements for rf cavities are different from those for the beam screen, the reported is divided into two respective parts.

1. NBN BASED THIN FILM SYSTEMS 1.1. INTRODUCTION At USI, the work on NbN on copper has been continued and the performance of these films has been improved. We report here on the developments and summarize the results of NbN single layer films on various pretreated copper substrates as well as SIS systems of the form: copper substrate/Nb/AlN/NbN. The film systems have been characterized by a broad range of materials characterization techniques including most importantly FSEM, TEM, EDX, and XRD. In addition to these, the DC and RF superconducting (SC) performance has been investigated. In the first section we summarize the results of small, flat samples including their DC SC performance (Vibrating Sample Magnetometer (VSM) option of a commercial Physical Property Measurement System (PPMS)). The second part shows the results of QPR s measured at HZB. The superconducting properties of NbN are strongly dependent on the formation of the correct crystallographic phase. NbN forms in three superconducting phases, with Tc’s ranging from 11.6 K for the hexagonal ϵ-phase, 12-15 K for the tetragonal γ-phase and up to 17.3 K for the cubic δ-phase. All other phases are non-superconducting and include the hexagonal β (Nb2N), δ´ (NbN) and further higher order nitrides such as Nb4N5 and Nb5N6. Because of this, controlling the crystallographic phase is key for a successful outcome of a NbN based film system. Besides this, the results shown here hint to a possible effect of nano-scale surface topography on the early flux penetration of high-κ materials. To prevent early flux trapping, SIS film systems have been investigated. The sought after shielding effect purported to be provided by SIS structures requires the deposition of NbN films with thicknesses less than their penetration depth. The compatibility of AlN in terms of the deposition process and its crystallography makes it an ideal candidate for the insulation layer. The preparation and pretreatment of the copper samples, including the QPR sample, as well as the development of a good performing Nb base layer have been reported earlier [1][2].

1.2. DEPOSITION In the following, the experimental conditions for the fabrication of NbN films are summarised. The NbN thin films and the multilayer SIS films were deposited onto electropolished OFHC copper substrates, as well as Si witness samples, via reactive DC magnetron sputtering (DC MS) with a fixed target to substrate distance of 5.5 cm. The films were deposited using a 100 x 100 mm² Nb (RRR 300) target in a commercial, high-volume, fully automated coating system (CemeCon CC800) with a base pressure of 5.0·10-7 mbar. Based on the results of previous studies reported earlier [3], the films were deposited using a mixture of argon (99.999 Vol-%) and nitrogen (99.999 Vol-%) gases. The N2/Ar ratio was maintained via flow rate control of the two gases. Prior to deposition, the system, including the substrates, was passively baked at 280°C for 6 hours. Following this, the substrates were subjected to a medium frequency (MF) etching process as a final surface treatment and the Nb target was sputter cleaned for 10 minutes to ensure all possible contaminants were removed. The parameters of selected optimised samples are summarized in Table 1.


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Table 1: Deposition parameters, superconducting test results (transition temperature, Tc, flux entry field Hen) and AFM surface roughness (Sq) results for the three highest Tc, optimised NbN thin film samples. Reproduced from [4].

Sample Temperature (°C)

Pressure (mbar)

N2%

Bias (V)

Power (W)

Hen (mT)

Tc (K)

Sq (nm)

Opt-1 280 1.2·10-2 8 0 400 2.0 15.9 6.36±0.42 Opt-2 280 1.4·10-2 8 0 500 2.0 15.4 7.39±2.34 Opt-3 280 1.4E·10-7 8 75 500 5.0 16.1 14.05±2.32

The microstructure of a representative NbN sample is displayed in Figure 1. The general columnar nature of the structure has been maintained while the film density has been improved. All samples possess a dense columnar nature of structure and a faceted grain peak, homogeneously deposited across the sample surface. The SAED pattern indicates the δ-NbN (111) orientation of the film, while the spot-like pattern shows the restricted fiber texture of the film.

Figure 1: SEM images of the best performing NbN sample, Opt-3. (a) shows the NbN film cross section on Si and (b) shows the NbN film surface on Cu. (c) shows the TEM image of the film cross section on Cu as well as the accompanying SAED pattern. Parts of this figure reproduced from [4]. The crystallographic relationship between the NbN film and the Cu substrate was also explored through the use of HRTEM shown in Figure 2. From (a) we see that the Cu lattice planes are found to lie oblique to the surface. It is also evident that the NbN film initially grows epitaxially along the crystal orientation of the Cu substrate. Following growth of a few nanometers, the crystal orientation is no longer influenced by the Cu substrate and the NbN grows in a separate crystal orientation. The lattice parameter of Cu is listed as a = 3.615 Å, while that of this NbN film was calculated to be a = 4.3829 Å. This results in a lattice parameter difference of 17.52 %. From (b) it is evident that the film is epitaxially grown for the initial few nanometers of the film. In these first few nanometers the film is subjected to compressive strain. As the film thickness increases, this strain is released through the formation of dislocations, indicated by the red marks in (b). Based on the lattice mismatch, and the HRTEM image, one dislocation will form for every five Cu lattice planes.


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Figure 2: HRTEM image of (a) The interface between the NbN film grown with 8 % N2 and the Cu substrate. (b) Magnified image of the area shown in red in (a) displaying the epitaxial relationship between Cu and NbN. The XRD scans of the optimised samples, presented in Figure 3 indicate a general δ-NbN (111) orientation for the samples. Samples Opt-1 and Opt-3 also display a very low presence of δ´-NbN. The lattice parameter of the samples remains around the value for bulk NbN (4.396, 4.397, 4.395 Å for Opt-1, 2, 3, respectively) while the average crystallite size of the samples has further increased to 26.55, 32.15 and 26.46 nm for Opt-1, 2 and 3 respectively. Based on initial DC SC measurements, these three samples displayed the highest critical temperatures of all deposited NbN samples so far. For a more accurate assessment, they were also measured in an AC susceptometer option of the PPMS. The measured mre (T) curves, normalised to the mre value at the low temperature plateau, are displayed in Figure 4. The changes in the deposition parameters for these three samples culminated in the highest critical temperature (Tc = 16.1 K) of all the NbN samples for sample Opt-3 while sample Opt-1 and Opt-2 still displayed a Tc > 15 K.


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Figure 3: XRD spectra of the three optimised samples (normalised to δ-NbN) and the best performing SIS sample (SIS-1), which was coated with the same NbN film as Opt-1 (normalised to Nb). The spectra are plotted on a logarithmic scale. Reproduced from [4].

Figure 4: Normalised AC susceptometry results indicating the critical temperature of the three optimised samples. Reproduced from [4]. Following the successful optimisation of the NbN single layer films, a series of SIS films with a Nb/AlN/NbN structure were also deposited onto OFHC Cu substrates. This structure is shown in Figure 5 (a). A corresponding SEM image of a test film on Si, including layer thicknesses, is displayed in Figure 5 (b). The SIS film coating was set up in such a way that the sample did not require removal from the deposition chamber in between coatings. In the following we show four selected SIS film samples (SIS-1, SIS-2, SIS-3, SIS-4) deposited with NbN thicknesses of 200, 200, 150 and 250 nm respectively, based on the work by Kubo [5].


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Figure 5: (a) Illustration of a multilayer SIS structure to be deposited onto electropolished copper. (b) SEM image of the pre-trial SIS film deposited on Si with a minimal thickness Nb (30 m) base layer. Reproduced from [4]. Table 2: Resultant values for the surface roughness, entry field and NbN film critical temperature values of the SIS films, pure Nb film, Nb/AlN film and pure NbN film. Reproduced from [4].

Sample (S-Layer Thickness) (nm)

RMS Surface Roughness (nm)

μ0Hen (mT)

Tc (Nb) (K)

Tc (NbN) (K)

SIS-1 (200) 12.79±2.49 64.5 9.4 14.7

SIS-2 (200) 13.16±1.61 14.5 9.2 14.5

SIS-3 (150) 15.74±1.22 24.0 9.2 14.0

SIS-4 (250) 13.00±2.15 26.5 9.4 14.9

Nb 19.38±4.33 52.0 9.4 -

Nb/AlN 15.79±1.70 - - -

Opt-1 (NbN) 6.36±0.42 2.0 - 15.9

The critical temperature of the SIS films was also measured using an AC susceptometer, as it is able to detect the transition of both the underlying Nb layer as well as the outer NbN S-layer. The results for the four SIS films are illustrated in Figure 6. The critical temperature of the outer superconducting layer in the SIS films is lower than the single NbN layer with the best performing SIS film (SIS-1) displaying a Tc = 14.7 K, compared to Tc = 15.9 K for the single NbN layer (Opt-1). This Tc reduction is believed to be due to a combination of reduced film thickness in SIS structures and the increased roughness of the underlying Nb and AlN layers. Further results of the superconducting measurements as well as roughness measurements are also displayed in Table 2.


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Figure 6: Normalised AC susceptibility measurements detailing the transition points of the Nb and NbN layers of the SIS films. Reproduced from [4]. Figure 7 presents the DC magnetisation curves as a function of the applied field (Ha), measured at 4.2 K for the three component films of the SIS coatings: the single DC MS Nb layer, the single NbN layer used in the SIS films (Opt-1) and the best performing SIS film (SIS-1). In (a) the virgin magnetisation curves are presented, in a plot where the measured sample magnetic moment m(Ha) is divided by a linear field dependence of the magnetic moment followed in the Meissner shielding state mlin = c·Ha (c – constant, different value for each sample). Figure 7 (b) displays the full magnetization loops measured on the three samples. The deviation from the Meissner state, marked by the decline in m/mlin, appears significantly earlier for the single NbN layer than the Nb and SIS films. The SIS film displays very similar curves to the Nb film. The magnetization loops in (b) indicate the presence of the surface barrier to the magnetic flux penetration for both the SIS and Nb films, although there is no visible barrier in the single NbN film itself. The NbN layer instead presents a smooth curve, indicative of a stable flux pinning state.

Figure 7: (a) Normalised virgin DC magnetisation curves and (b) full magnetisation loops as a function of magnetic field, at 4.2 K. Data for the single Nb layer (black), SIS-1 film, (red) and single NbN film Opt-1 (green) are plotted. Reproduced from [4]. Samples SIS-2, SIS-3 and SIS-4 display entry field values significantly lower than the individual Nb layer sample. On the other hand, sample SIS-1 has the highest Hen = 64.5 mT, surpassing the value obtained with the pure Nb sample, Hen = 52.0 mT. This result indicates the possibility of delaying the field penetration into the Nb layer, as predicted for SIS film structures. The poorer entry field results for samples SIS-2, SIS-3 and SIS-4 are believed to be due to the lower surface quality of these films. However, damage created at the thin film edges in the cutting process could also play some role, although the same procedure was followed, and the same cut-off machine was used in preparation of all the investigated samples. In any case, the homogeneity and density of the surface layer is of utmost importance in order to provide the sought-after shielding effect wanted from SIS films.


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In light of this, an investigation into the deposition of HiPIMS NbN films was completed using the same system as described above. In terms of the correct NbN phase formation, the results show a marked reliance on the cathode power and deposition pressure, similar to the DC MS films, and a new reliance on the substrate bias. The increased effects of the substrate bias stem from the ionisation of sputtered particles, characteristic of HiPIMS coating systems. This is shown in the XRD patterns displayed in Figure 8. (a) shows an increasing presence of the non-superconducting hexagonal phase at bias levels > 50V, while (b) shows a shift to δ-NbN (111) orientation with increasing deposition pressure. Further to this, the purported densification of deposited layers through the use of HiPIMS was found for all films. This is reflected in the SC performance of these films. All HiPIMs NbN films displayed significant increases in the entry field value while maintaining the high Tc of the DC MS films. The effects of the substrate bias are also noticed in the topography of the film surface, which is now comprised of rounded grain peaks as opposed to sharp surface features, typical of most DC MS NbN films.

Figure 8: XRD spectra of the (a) HiPIMS NbN films deposited with varying substrate bias values. (b) HiPIMS NbN films deposited with varying deposition pressure values. The spectra are plotted in log scale.


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Based on the results of the small sample investigation, the optimised HiPIMs NbN coating was also deposited in a series of HiPIMS ML SIS films. A selection of the characterisation results for these films are displayed in Table 3. The samples have not been measured in the AC susceptometer as yet and therefore the Tc of the NbN films is as yet, unknown. However, the Tc of the individual layer was 14.6 K. As is evident, these films display a significant increase in the entry field value over the previous DC MS based SIS films, with the best performance provided by an outer layer thickness of 200 nm. Table 3: Resultant values for the surface roughness, entry field and Nb film critical temperature values of the HiPIMS SIS films

Sample (S-Layer Thickness) (nm)

RMS Surface Roughness (nm)

μ0Hen (mT)

Tc (Nb) (K)

HP SIS-1 (200) 12.48±0.36 88.0 9.3

HP SIS-2 (157) 14.11±3.24 57.0 9.3

HP SIS-3 (232) 12.01±2.03 80.0 9.3

1.3. RF PERFORMANCE MEASUREMENTS At HZB, the true SRF performance was measured with the quadrupole resonator on two different superconductor-insulator-superconductor samples. The chemical composition of the layers was Cu(bulk)-Nb-AlN-NbN (from bottom to top). The first Nb layer had 3-4 µm thickness and was coated on a polished OFHC-copper substrate. The samples varied in insulator thickness with 30 nm in sample-1 and 8 nm in sample-2. The thickness of the top NbN layer was again varied with 200 nm in sample-1 and 180 nm in sample-2.


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Figure 9: Images of the HiPIMS SIS film coating on the QPR sample-2. (a) and (b) show the QPR sample before and after SIS film coating. (c), (d) and (e) show the coating process during Nb, ALN and NbN deposition respectively. Sample-1 was prepared with DC-magnetron sputtering and a full rotation of the table during coating, as described elsewhere [see ARIES Deliverable Report 15.3: Evaluation of System 3 and SIS]. The Nb layer was exposed to air between coating runs. Sample-2 was prepared with HiPMS without intermediate air exposure, see Figure 9, using the same procedure as detailed for the small sample, HP SIS-1 (200). The results of the RF measurements are illustrated in Figure 10 which shows the measured surface resistance vs. peak field on the sample in the first row and surface resistance vs temperature in the second row. While sample-1 performed poorly, sample-2 exhibited a much smaller surface resistance, almost as good as a reference bulk Nb sample, which is very likely due to the improved preparation conditions. The resistance vs field behaviour is also much better and monotonic for sample-2 which also likely indicates good films adhesion on the sample-2. The temperature dependence of the surface resistance exhibits a peak around ~5.5 K. Contradictory to that, the theory of SRF behavior of multilayer structures predicts a monotonic increase of surface resistance with temperature. However, the fact that this peak vanishes when a smaller insulator layer thickness is used, like in sample-2, could mean that the thermal properties of the involved materials play a major role. A possible explanation could be the strong temperature dependence of the thermal conductivity of insulator materials, such as AlN, that biases the heat removal at lower temperatures which in turn leads to a systematic error in the temperature measurement. In order to further investigate this effect, a follow-up measurement with intermediate insulator layer thickness is planned.


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a)

b)

Figure 10: Results of the RF test of (a) Nb-AlN-NbN multilayer sample-1 (SIS 4um-35nm-200nm) and (b) Nb-AlN-NbN multilayer sample-2 (SIS 4um-8nm-200nm) film

Acknowledgements Part of this work was performed at the Micro- and Nanoanalytics Facility (MNaF) of the University of Siegen. Special thanks to Ying Li for the TEM analysis. We also acknowledge Eugen Seiler and Rastislav Ries (IEE Bratislava) for the small sample sc measurements.

1.4. REFERENCES [1] S. B. Leith, A. S. H. Mohamed, Z. Khalil, M. Vogel, and X. Jiang, “Initial results from investigations into different surface preparation techniques of OFHC copper for srf applications,” in 19th International Conference on RF Superconductivity (SRF 2019), 2019, pp. 941–944. [2] C. Pira et al., “ARIES Deliverable Report D15.1 - Evaluation of cleaning process,” 2018. [3] S. B. Leith, M. Vogel, X. Jiang, E. Seiler, and R. Ries, “Deposition parameter effects on niobium nitride (NbN) thin films deposited onto copper substrates with DC magnetron sputtering,” in 19th International Conference on RF Superconductivity (SRF 2019) 947–951 [4] S. Leith, M. Vogel, J. Fan, E. Seiler, R. Ries, and X. Jiang, “Superconducting NbN thin films for use in superconducting radio frequency cavities,” Supercond. Sci. Technol., vol. 34, no. 2, p. 025006, Feb. 2021 [5] T. Kubo, “Multilayer coating for higher accelerating fields in superconducting radio-frequency cavities - a review of theoretical aspects,” Supercond. Sci. Technol. 30, (2016) 023001


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2. TL-1233 COATINGS 2.1. INTRODUCTION Studies of Tl(1223) coatings have been conducted for the realization of a superconducting layer in the inner part of the radiation beam screen of future high energy particle accelerator in order to minimize the losses due to the eddy currents induced in the conducting part of the screen. These losses reduce energy from the beam and, even worse, generate beam instability effects that are difficult to control. The quality parameters of Tl(1223) material and coatings have been evaluated in this respect. Unfortunately, the most important parameter i.e. the radio frequency surface resistivity at 50K and 16 T is not directly measurable up to now (experimental facilities are being realized) and has to be estimated from other transport properties by a recently developed model. The samples are then characterized in term of vacuum compatibility, secondary electron emission and an estimation of the thermal conductivity required for the thermal stability of the screen is proposed.

2.2. PREPARATION The films are realized by electrochemical deposition in a 3-electrode system on suitable substrates: silver and strontium titanate (STO, SrTiO3) intentionally metallized via sputtering; a flat cell is employed. The working, counter and reference electrodes are the chosen substrate, platinum grid, and Ag/AgNO3 0.1 M in Dimethyl Sulfoxide (DMSO, Sigma-Aldrich, 99.9%, anhydrous, sure-sealed grade), respectively. Nitrates have been dissolved in 250 ml of DMSO. Due to the large difference in the overpotential for the different ions, the stoichiometry of the solution is very far from the one required on the substrate. By iterative analysis of deposited precursors with energy dispersive x-ray spectroscopy (EDX) we obtained the element contents required as follows: 0.25 g TlNO3, 0.18 g Bi(NO3)3 5H2O, 0.18 g PbNO3, 2.73 g Sr(NO3)2, 1.52 g Ba(NO3)2, 1.63 g CaNO3 H2O and 1.33 g Cu(NO3)2 H2O. Typically, the film deposition is performed between −2.9 V and −3.1 V for 600 s with a commercial potentiostat. Since DMSO is a high boiling point solvent, the samples are dried in vacuum at 120 °C before performing the final high temperature heat treatment. The samples are processed in a three-zone tube furnace in a partially closed system inside a gold foil crucible for 10 min at 885 °C. Since thallium oxide is volatile above 710 °C it is mandatory to keep the sample in a thallium atmosphere. To overcome this restriction Tl2O3 powder (4 mg) and Tl-1223 pellets are used. The main scope is to find the best compensation between losses and reabsorption of thallium-bismuth-lead. The precursor-pellet releases with an already balanced stoichiometry that favors the growth of the Tl-1223 phase. As shown in the inset of Figure 11, a certain degree of texturing can be observed from the rocking curve of the Tl-1223 (002) peak, tentatively evidenced by the arrows in Figure 12. However, the control of the thallium atmosphere is still under optimisation. SEM-EDX analysis, shown in Figure 11, reveals that the plate-like grains do not achieve uniform coverage of the substrate yet and evidences that the transformation of the precursors into the desired phase is not complete, since the Tl-1212 phase and complex oxides are detected as well. Under this condition, it seems that Tl-1212 grains preferentially grow on top of Tl-1223 grains


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Figure 11: X-ray diffractogram of a thallium-based HTS coating. Phases and orientation are shown in black for Tl-1223 and in red for Tl-1212. The inset shows the rocking curve of the Tl-1223 (002) peak.

Figure 11: SEM image of an electrodeposited sample after heat treatment. The arrows are guides to visualize the in plane texturing.


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2.3. CHARACTERISATION The temperature dependence of the resistance of one Tl(1223) sample in magnetic fields up to 9 T is shown in Figure 12. The shift of zero-resistivity caused by the applied field gives the estimate of the irreversibility and the upper critical fields, Hirr and Hc2, (Figure 13) adopting the 10% RN and 90% RN method, where RN is the normal state resistance. The measurements have been performed by a Quantum Design PPMS.

Figure 12: 4-probe resistance measurement on varying the temperature at different applied magnetic fields as reported in the legenda by different colours.

Figure 13. Upper critical field (Hc2) and irreversibility field (Hirr) extracted from the data presented in Fig. 13. The dashed red line gives a prediction at high field.


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The extrapolation shows a very high Hc2 and, more importantly, a very high Hirr. An estimation of the Hirr dependence for higher fields is inserted as dashed line confirming the suitability of Tl-1223 for the FCC-hh environment.

Figure 14: AC susceptibility of thin film on Ag substrate. Two transitions are identified at approximately 107 K and 75 K, which correspond to the Tl-1223 and Tl-1212 phase, respectively.

The superconducting transition temperature was determined by AC susceptibility measurements with an amplitude of 30 µT and a frequency of 1 Hz in a SQUID magnetometer (MPMS, Quantum Design). The measurement of a thin film on Ag substrate, as seen in Figure 15, reveals two transitions. The first, with Tc,onset = 107 K, can be attributed to the Tl-1223 phase, while the second transition with Tc,onset = 75 K shows the contribution of the Tl-1212 phase. The high Tc of the Tl-1223 is promising; with further optimisation of the heat treatment, a better phase purity is expected. The remnant field profiles were mapped with a Hall scanner in an 8 T cryostat setup with micrometer resolution. Everything but a round spot of superconducting film was etched off the Ag substrate with hydrochloric acid in order to fit into the SHPM setup.


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Figure 15: Image of grains on Ag substrate (a) and corresponding area scan of the trapped field (b). An overlay of the two images (c) shows perfect agreement between optical image and the magnetic measurement. High resolution scan of a smaller area (d) and corresponding critical current density distribution (e).

An optical image obtained with a digital microscope (VHX-6000, Keyence) is given in Figure 15(a), the dark spots indicate the superconducting areas. The sample was cooled to 5 K in zero field and then magnetized with 1.5 T. The area scan shows many individual grains and grain clusters with high trapped field, indicated by bright colors, as can be seen in Figure 15(b). The optical image and magnetic field map can be directly compared, and an overlay of the two images shows a perfect agreement, seen in Figure 16(c). In order to get a better insight into the currents within single grains and across grain boundaries, a high-resolution scan of a smaller area was performed. Figure 15(d) shows the trapped field of a few grain clusters, which amounts to 60 mT. By inversion of Biot-Savart's law, the spatial distribution of Jc can be calculated from the measured trapped field. The white arrows in Figure 15(e) show the direction of current flow in grains and across grain boundaries for the measured grain clusters. The average magnitude of the critical current density in these grain clusters amounts to Jc = 8·1010 Am-2 at 5 K. The same experiments were also performed on a thin film on STO substrate.

Hall mapping shows the trapped field of a larger grain cluster. In this case, the maximum trapped field amounts to 100 mT at 5 K. Calculation of the Jc distribution gives the same value as on the Ag substrate but on a larger area.


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Figure 16: Overlay of trapped field and SEM image of a thin film on STO substrate (a), and preparation of two TEM lamellas with a Focused Ion Beam (b). TEM images of areas with high trapped field (c) and no trapped field (d) show the desired Tl-1223 phase with good and bad grain alignment, respectively.

The microstructure of the measured thin film on STO substrate was further investigated by means of SEM (FEI Quanta 200 FEGSEM) and TEM (FEI TECNAI F20) imaging. Through precise comparison of SEM images and magnetic maps, the areas with the highest trapped fields can be identified. For such an area the overlay of magnetic map and SEM image is given in Figure 16(a). With a focused ion beam (FEI Quanta 200 3D DB-FIB), two TEM lamellas were prepared from this sample, one lamella taken from an area with high trapped field and one from an area with no trapped field, as indicated by the arrows leading from Figure 16(a) to (b). In this process, a protective Platinum layer is deposited on the sample surface, the lamella is cut out and the thickness of the lamella is reduced to about 100 nm. In the area with high trapped field, the TEM image shows one large grain with a thickness of 1.8 µm flat on the substrate surface, seen in Figure 16(c). With EDX analysis, the different areas can be identified: on the top, we find the Pt protective layer, next, a thin layer of SiO, which stems from small contaminations in the fabrication process, and then a large Tl-1223 grain on top of the STO substrate. We find the same compositions in the lamella taken from the area with no trapped field, shown in Figure 16(d). In this case however, we find multiple Tl-1223 grains, randomly oriented on the substrate surface. This underlines the necessity for grain orientation, as the current flow is blocked by the misalignment of the Tl-1223 grains. Coating the beam screen mandates compliance with certain requirements inherent of the particle accelerator environment. For example, the vacuum base pressure must not be deteriorated by outgassing from the layer, and the secondary electron generation probability of the top material should be low enough to avoid formation of an electron cloud during circulation of the proton beam in order


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to avoid beam instabilities and excessive heat load to the cryogenic system. Thallium compounds, and in particular Tl-1223, have never been used in particle accelerators, leaving a lack of knowledge in literature about its intrinsic properties concerning any possible release of gaseous species in vacuum, or the secondary electron yield (SEY) of this material. In this section we discuss outgassing and residual gas analysis (RGA) measurements, and a first x-ray photoelectron spectroscopy (XPS) and SEY analysis performed on some samples. One sample (sintered pellet) was placed onto an aluminum foil for a standard vacuum characterization measurement. The specific H2O outgassing rate after 10 h of pumping was 2·10-7 mbar l−1 s−1 cm−2, ~500 times the reference for unbaked, clean copper surfaces. The vacuum pump-down exhibited a linear behaviour in logarithmic time/total pressure scale, with a slope very close to −1, indicating open porosities responsible for the large outgassing rate. The RGA after 24 h of pumping was readily within CERN's acceptance criteria for unbaked components, except for a rather pronounced O2 release and some F contamination, the latter barely exceeding the acceptance criteria probably due to packaging. In addition, XPS analysis of the aluminum foil itself revealed that no high atomic mass elements (Tl, Pb, Bi) are released by the pellet. XPS analysis was performed on the surface of another sample. The surface composition indicates a heavily oxidized surface with more than 50 at.% of oxygen due to air exposure. The surface carbon content is ~15 at.%, showing a rather clean surface, in good correlation with the RGA analysis. Tl, Pb and Bi appear to exist in a single oxidation state, while the constituents Ca, Sr and Ba exhibit different chemical states due to their high surface reactivity.

The SEY δ is the ratio between the number of generated secondary electrons per single electron impinging on a surface. It can be measured in lab experiments using a tunable electron source considering δ = Iδ/Ii, where Iδ is the total emission current and Ii is the current of the incident electron beam. For Tl-1223, the SEY coefficient was determined within this study for precursor-pellets with different surface topography. Figure 17 includes the dependency of the SEY on the primary electron energy for the bare Tl-1223 surfaces. The SEY maximum is between 2.05 and 2.25 for all investigated samples, while the curve distribution depends on the surface roughness.


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Figure 17: Secondary electron yield curves of bare Tl-1223 surfaces as a function of primary electron (PE) energy.

The surface of one selected pellet (#1) was modified by an amorphous carbon (a-C) coating utilizing DC magnetron sputtering according to a procedure described in detail in [1]. First a 150 nm thick Ti buffer layer was deposited followed by the addition of a 100 nm thick a-C top layer. A comparison of the SEY curves before and after coating is shown in Figure 18, revealing that the SEY maximum (δmax) has considerably dropped to 0.77, exhibiting the typical SEY energy dependence of a-C coatings (δmax at 250 eV).

Figure 18: Secondary electron yield of a Tl-1223 pellet in dependence of primary electron (PE) energy before and after Ti/a-C coating.


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Consequently, the bare Tl-1223 surface has a rather high initial δmax at room temperature. Nevertheless, the application of a thin amorphous carbon coating on top of the superconducting layer reduces this value to below unity, which is a very promising result for a further development of this approach.

2.4. RESULTS In this deliverable, we presented the progress on development of Tl-1223 HTS coatings for future implementation as low impedance layers on particle accelerator components, specifically the FCC-hh beam screen. We extrapolated a very high irreversibility line, while for Tl-1223 coatings; large plate-like grains have been obtained on silver and STO substrates, even if a fully superconducting coverage is still not achieved. A quite high critical current density of Jc ~ 8·1010 Am−2 at 5 K in self-field was calculated from SHPM measurements. We used this set of data to extrapolate the surface resistance of the coatings under the operative condition in the FCC beam screen by realizing a semiclassical model of the behaviour of a type II superconductor in the high frequency high field regime. Based on literature reviews [2], it is safe to assume that the intrinsic surface resistance of HTS and in particular Tl(1223), is lower than that of copper at frequencies below 10 GHz. Nevertheless, a strong magnetic field such as in the FCC-hh will significantly increase the surface impedance of the HTS, thus limiting its advantages. The effect of an applied magnetic field B0 > Bc1 on the surface impedance can be described in terms of the oscillations of the rigid fluxon lattice driven by the RF currents, which lead to a dissipation of energy by viscous motion [3]. For large applied magnetic fields B0, i.e. of the same order as the upper critical field Bc2, the intrinsic surface impedance of the HTS can be neglected, and the surface impedance as a function of frequency ν is dominated by its field-dependent component [4]:

(1) The above equations make use of the usual notation , where 𝑍! = 𝑅" + 𝑖𝑋!. 𝑅! = #𝜇"𝜌!𝜋𝜈 is the normal-state surface resistance of the HTS. The so-called depinning frequency ν0 is given by

where ρn is the normal-state resistivity of the HTS, Jc(B0) is its critical current density as a function of the applied field B0, and Φ0 is the flux quantum. It is commonly reported in literature that the depinning frequency of HTS films is significantly above 1 GHz [2] in absence of an applied magnetic field, while the influence of strong magnetic fields is less documented. However, the depinning frequency can in principle be calculated based solely on results from DC transport measurements. The surface impedance can then be evaluated using the simplified above cited equations or, for a frequency regime overlapping with the depinning frequency, using the full model described in [Error! Bookmark not defined.]. Based on our results for Tl-1223, we can assume a normal-state resistivity 𝜌! = 40𝜇Ω, and an upper critical field Bc2 of 70 T at 50 K (H//c, conservative estimate). In addition, we shall assume a range


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for the critical current density from 108 to 109 A m−2 (conservative estimate) in order to evaluate the surface resistance at the FCC injection field of 1.06 T, which should correspond to the most unstable condition, and at the nominal maximum field of 16 T, respectively. The depinning frequency ranges from 12 MHz up to 480 MHz, depending on applied field and critical current density, and we thus make use of the full formulas to evaluate the surface resistance over the entire frequency spectrum of interest, the results are shown and compared to copper in Figure 9. At low frequencies, where the most unstable modes are predicted for a copper beam screen, a substantial gain of several orders of magnitude is clearly apparent.

Figure 9: Induced current power spectrum (a) and predicted surface resistance at 50 K of Tl-1223 (gray band) compared to Cu (orange line) at 1.06 T (b), and at 16 T (c). The band corresponds to the range of critical currents mentioned in the text at low frequencies, where the most unstable modes are predicted for a copper beam screen, a substantial gain of several orders of magnitude is evident.


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Another calculation predicting properties of Tl(1223) coatings using the measurements performed in this program as the input parameters is about the thermal runaway effect in a possible FCC beam screen. Each beam will emit about 28Wm−1 of synchrotron radiation in the arcs that cannot be reasonably absorbed in the magnet at a temperature of 1.9 K. The radiation can be instead conveniently absorbed by a 3 cm diameter beam screen, kept at 50 K. The Rs(f ,T) dependence, for T > 50 K, calculated using equations (1) considering the temperature dependence of the involved quantities and parameters generically valid for HTS (very conservative assumption for Tl(1223) that has higher Tc and Hc2), is reported in Figure 21 as a solid line. Full details of the calculations and assumptions can be found in Ref. [5]. The strong temperature dependence of Rs can easily trigger thermal runaway problems. In fact, the surface temperature in the presence of a high rf power, Prf (T), dissipated at the surface can be written as: T = To + RT Prf (T) where To is the copper beam screen temperature kept at the fixed temperature of 50 K (To = 50 K). An increase in T due to the radiofrequency power increases the superconductor surface resistance Rs(f,T) that produces an increase in Prf(T) (proportional to Rs(f ,T)) and then a further increase in T. The process leads to a surface equilibrium temperature T > To or can lead to thermal runaway. The surface equilibrium temperature can be determined graphically in the following way. First, given the proportionality of dissipated power and surface resistance, we can rewrite the previous relation as:

This implies a linear relation between Rs(f ,T) and T whose slope is determined by the value of the thermal resistance RT. This linear relation is plotted in Figure 21 for two different values of RT. The surface equilibrium temperature is found as the crossing point of this line with the Rs(f ,T) curve. For RT1=1 m2K/W, the equilibrium temperature is T = 50.5 K, whereas RT2=10 m2K/W represents the maximum transverse thermal resistance value for which an equilibrium temperature does exist. For RT > RT2 there is no crossing point and the system goes in the thermal runaway regime. This results shows that thermal runaway only occurs for very high thermal resistivity (RT ≥ 10 m2K/W), and should not represent a serious problem for the case of HTS coated conductor and even less a problem for Tl(1223) coated on pure silver substrate that has an expected RT value at least one order of magnitude lower.


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Figure 10: Solid curve: temperature dependence of the HTS surface resistance as a function of the temperature. Dashed lines calculated from Equation (7) for two different values of the transverse thermal resistance RT .

Furthermore, Tl-1223 is found to be applicable in vacuum environments. With an additional amorphous carbon top coating, it exhibits a remarkably low coefficient of secondary electron generation (δmax = 0.77) making it compatible with accelerator environments. In light of the results obtained, we can affirm, even though there are critical issues mainly due to the volatility of thallium, that the Tl-1223 phase can fulfil the requirements for this kind of application.

2.5. REFERENCES [1] T. Puig et al “Coated conductor technology for the beamscreen chamber of future high energy circular colliders”, Supercond. Sci. Technol. 32 (2019) 094006 [2] M. Hein, High-Temperature-Superconductor Thin Films at Microwave Frequencies (Berlin: Springer, 1999) [3] J. I. Gittleman and B. Rosenblum, “The Pinning Potential and High‐Frequency Studies of Type‐II Superconductors”, J. Appl. Phys. 39 (1968) 2617 [4] S. Calatroni and R. Vaglio, “Surface resistance of superconductors in the presence of a DC magnetic field: Frequency and field intensity limits”, IEEE Trans. Appl. Supercond. 27 (2017) 3500506. [5] R. Vaglio and S. Calatroni, “Advances in the study of HTS superconductors for the beam impedance mitigation in CERN-FCC: the thermal runaway problem”, Eur. Phys. J. Special Topics 228, (2019) 749–754

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