Nb/Cu, Nb 3 Sn/Cu Thin Film Cavity

Transcription

1 Nb/Cu, Nb 3 Sn/Cu Thin Film Cavity Genfa Wu Abstract A thin film deposition method inside an elliptical cavity is proposed. The system uses the substrate copper cavity as the vacuum chamber. The ECR plasma will be created to produce direct niobium ion deposition, thus realize the energy controlled vacuum deposition. An extended use of such deposition system to study the niobium-tin alloy (Nb 3 Sn) is also proposed. JLAB-TN

2 Nb/Cu, Nb 3 Sn/Cu Thin Film Cavity Genfa Wu Table of content: 1. Introduction 2. Current thin film effort 3. Copper based niobium thin film cavity by energy-controlled deposition 4. Copper based Nb 3 Sn thin film cavity 5. Cost and schedule 6. Conclusion 1. Introduction Recent development in RF superconductivity (SRF) is both exciting and gloomy. The single cell elliptical RF cavity based on solid niobium is reaching accelerating gradient of 40 MV/m [1]. Multicell cavity is not far behind, which is 35 MV/m for TESLA 9-cell cavity [1]. While the solid niobium based SRF is reaching its theoretical limit. The cost of the solid niobium remains much too high. The future development of SRF technology much becomes reducing cost or finding alternative material to exceed the 40-50MV/m limit due to the theoretical magnetic field limitation. The answer could well be the SRF based on thin film technology: the copper based niobium thin film cavity for near term cost reduction and Nb 3 Sn material based cavity for the accelerating gradient to reach well beyond the MV/m limitation. The early successful LEP-II [2] proved the viability of thin film technology in particle accelerator. Since copper material can be one tenth of the cost of niobium, plus the potential for much lower manufacturing cost, the thin film technology has potential to reduce the particle accelerator cost dramatically, which could have been beneficial for TESLA proposal. The technology would also be a real benefit for muon storage ring, which called for 200MHz large cavities [3]. The existing magnetron sputtered niobium thin film cavity achieved 20 MV/m for single cell elliptical RF cavity at 1500MHz [4]. From the perspective of thin film deposition process, the magnetron sputtering technique has some limitations to achieve certain film structure. The process has relatively low impacting energy and has difficulty to form high quality thin film for some areas inside an elliptical cavity due to the low deposition angle [5]. The Nb 3 Sn cavity effort was pioneered at Univ. of Wupertal. The new layer of Nb 3 Sn was grown inside the high purity niobium cavity at high temperature up to 1200 C. The 1.5GHz single cell cavity achieved 10MV/m accelerating gradient with Q 0 of 10 9 at 4.2K and 20MV/m with Q 0 of 10 9 at 2K [6]. The result was promising at that time, since the accelerating gradient was comparable to that of solid niobium and had much potential to exceed. 1

3 2. Current thin film effort For the case of niobium thin film, different processes are being tried to improve niobium thin films, including post-deposition laser annealing [7], DC post-magnetron sputtering deposition [8], biased DC magnetron sputtering [9] and vacuum arc deposition [10-11]. For the aspect of thin film growth, the higher surface adatom mobility [12] and the vacuum condition are believed helpful to get better film quality as required for superconducting material like niobium. For a niobium thin film on a copper substrate, increasing the substrate temperature is not an option to achieve greater surface adatom mobility. Some processes such as ion-assisted deposition, biased magnetron sputtering, ionized magnetron sputtering, vacuum arc deposition and energetic cluster deposition, are expected to have higher impact energy during film growth, thus increasing the surface adatom mobility. These techniques either require a working gas or entail risk of microparticle contamination and lack good control of the deposition energy. To take advantage of the high vacuum condition and the capability of controllable deposition energy, the electron cyclotron resonance (ECR) plasma metal ion source [13,14] is selected as an energetic deposition system to study the niobium thin film deposition at different deposition energies [15]. Table 1. Comparison of niobium films on sapphire by several coating processes Coating process T c (K) T c (K) RRR *** Crystallization measured by (X-ray diffraction) Film structure by XTEM analysis Magnetron Sputtering Range from oriented to less oriented, depends on deposition angle. Columnar growth Some voids present at high deposition angle Biased Magnetron Sputtering 9.6 >1K 7-15 N/A Columnar growth Vacuum Arc deposition * 9.25 < Preferred orientation, other orientations exist N/A Energetic Vacuum deposition ** Perfectly oriented Epitaxial in some films. * T c measured by different process ** Sample made at deposition energy around 123 ev on sapphire. *** RRR as measured on sapphire substrate From the several parameters for different processes listed in Table 1, one can see vacuum arc deposition and energetic vacuum deposition have clear advantages. While macro particle contamination remains the major challenge for vacuum arc deposition, the energetic vacuum deposition has better chance to achieve the high performance thin film cavity. According to Matricon and Saint-James [16], the superheating critical field for Nb 3 Sn can be as high as 4000 Oe, which suggests up to 100MV/m for a typical elliptical cavity. But the best superheating critical field obtained for Nb 3 Sn was around 1033 Oe [17], which is 2

4 much lower than previous predicted. It could simply be the reason of different quality of Nb 3 Sn, or more seriously the wrongly predicated superheating critical field [18]. Never the less, the Nb 3 Sn s ability to achieve high Q above 4.2K makes it quite attractive. And the possibility to use copper as substrate cavity can further reduce the cost of particle accelerator just likes that of the copper based niobium thin film cavity. Due to the University of Wupertal s quitting the SRF business, the Nb 3 Sn thin film effort was virtually stopped. The vapor diffusion process to make the Nb 3 Sn cavity was based on solid niobium technology. Even though the high accelerating gradient at 4.2K is rewarding, the cost issue always comes into minds. Early Nb 3 Sn was prepared through carefully monitored two-source evaporation [19]. Other than the co-evaporation and vapor diffusion process [6,20], the Nb 3 Sn thin film actually can be made though RF magnetron sputtering process in which a Tin embedded Nb [21] or reacted Nb 3 Sn [22,23] is the target. 3. Copper based niobium thin film cavity by energy-controlled deposition Based on successful ECR plasma operation result [15], it is natural to extend this energetic vacuum coating technique to the elliptical cavity as a substrate. In this case, the cavity itself would serve as a vacuum chamber. To generate niobium plasma inside the cavity vacuum, at least three components are needed: neutral niobium vapor, RF power at certain frequency, proper static magnetic field which is perpendicular to the electric field of RF power and satisfies the ECR condition. To create energy controllable depositing niobium ions, a cylinder grid will be inserted into the cavity vacuum. The grid confines the RF field, floats at certain potential above the cavity to provide the controllable accelerating voltage for niobium ions. The concept is illustrated in figure 1. Figure 1. Illustration of ECR plasma creation inside an elliptical cavity. 3

5 (a) (b) (c) Figure 2. The permanent magnet configuration (a), axial field distribution (b), and midplane field distribution (c) 4

6 A 1.3 GHz single cell elliptical cavity is envisioned for the cavity deposition. The RF power would be commercial grade 2.45 GHz power source, which needs to be able to provide 500 W at least. The cylinder grid would be made out of niobium, both to reduce contamination and provide high heat tolerance. To meet ECR condition, the magnetic field needs to be around 875 Gauss. The actual field was designed as shown in figure 2. The field is configured to have relatively large area uniform field of 875 Gauss. The RF power is fed into the cylinder grid in TE 11 mode. The cavity, RF field and ECR magnetic field is superimposed in figure 3. Figure 3. A 1.3GHz cavity with permanent magnet and RF power. From the above discussion, one can see the clear advantages for this deposition process: 1. No working gas like argon 2. High vacuum means reduced impurities 3. Controllable deposition energy, and 90-degree deposition flux 4. Excellent bonding 5. No macro particles 4. Copper based Nb 3 Sn thin film cavity Based on the survey discussion in section 2, several coating processes are imagined to form the Nb 3 Sn inside the copper cavity. One is certainly the magnetron sputtering process; the target can either be reacted Nb 3 Sn, or the geometrically separated Nb and Tin target. Second process would be the Nb and Tin co-evaporation. These two processes use little of the proposed niobium cavity deposition system. The following two processes 5

7 are based on the energetic vacuum deposition principle, i.e. the direct ion deposition through ECR plasma process. The sequential process creates the niobium film on copper first, and then second energetic tin ions are delivered to the newly formed niobium surface to allow the diffusion process to occur. Since Tin only starts to diffuse into niobium around 1100 C, it is not acceptable to heat the copper substrate to that high temperature. The energetic vacuum deposition has shown that the energetic impinging ions do enhance the surface adtom mobility [24], which is equivalent to surface layer temperature. It is possible to use the same energetic deposition system to create high enough surface layer temperature to form niobium-tin alloy on cold copper surface. The nature of ionization also provides the novel method to deliver the vapor to inner surface of an elliptical cavity. The parallel process actually tries to create plasma of both niobium and tin. The twosource e-beam gun is commercially available to provide accurate amount of niobium and tin vapor. It is convenient to extend this cavity deposition system to study the feasibility of the plasma of co-existing niobium and tin, then subsequent Nb 3 Sn coating on copper substrate. 5. Cost and schedule The deposition system is designed to use many off the shelf commercial product to reduce the equipment cost. The elliptical copper cavity can be obtained through lab collaboration. The only part that needs some attention is the niobium cylinder grid. Table 2 lists the estimated cost for several subsystems. The author assumes most of the labor. Total one man/year s technician support should help to expedite the project. Table 2. Estimated cost to build the energetic vacuum deposition Equipment/activity Estimated cost Note: Microwave power source $3,000 Includes waveguide Microwave accessory $2,000 Includes power meter, adapters, etc. Vacuum pump $5,000 Vacuum getter pump Vacuum system accessory $5,000 Includes RGA, vacuum gauge, etc. Vacuum Tees, supporter $3,000 Permanent magnet $5,000 Includes Nd-Fe-B and supporting structure Magnetic coil $2,000 Coils and power supply E-beam gun system $15,000 Includes power controller E-beam gun second source $7,000 Bias voltage control $1,000 Includes power supply, insulator assembly Cylinder grid assembly $3,000 Total $51,000 6

8 The goals and milestones for thin film cavity deposition Activity 1st 2nd 3rd 4th 5th. 6th 7th Design review and finalize Order PM, grid RFpowerSource Making flanges, Tees, vacuum Assembly Vacuum test, Grid test with RF power Magnet, coil assembly E-gun, niobium feeder Test the plasma Test bias voltage Deposition in cavity The goals and milestones for thin film cavity deposition (continued) Activity 8-9th 10-11th 12-13th 14-15th 16-17th 18-19th 20 th month and beyond Deposition in cavity & testing Nb 3 Sn sample deposition & characterization Nb 3 Sn Cavity deposition & testing 6. Conclusion Following the ever-increased size and cost of modern particle accelerator, both the cost reduction and future technique to achieve even higher energy is critical for the future of particle accelerators. The proposed research topic can be both feasible and rewarding. Acknowledgement The author would thank W. Funk, R. Rimmer and C. Reece for their constant support, L. Phillips, P. Kneisel and A-M. Valente for very constructive discussions. 7

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