May 11, 2012

 

Plasma  Science  and  Fusion  Center  Massachusetts  Institute  of  Technology  

Cambridge,  MA    02139    

 

This  work  was  supported  by  the  U.  S.  Department  of  Energy,  Office  of  Fusion  Energy  Science  under  Grants:  DE-­‐FC02-­‐93ER54186,  and  partially  DE-­‐SC0004062  and  Supercon  DOE  STTR  Phase  II  DE-­‐SC0004269.      Reproduction,  translation,  publication,  use  and  disposal,  in  whole  or  in  part,  by  or  for  the  United  States  government  is  permitted.  

PSFC/JA-­‐12-­‐65      

 

Conductor Characterization of HTS Twisted Stacked-Tape Cable

Makoto Takayasu, Luisa Chiesa*, and Joseph V. Minervini

 

*  Tufts  University,  Mechanical  Engineering,  Medford,  MA  02155  USA    

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ABSTRACT

A flat tape cabling method of Twisted Stacked-Tape Cable (TSTC) for High Temperature Superconductor has been investigated. A 32-tape YBCO tape conductor with a 4 mm width and 200 mm twist pitch has been tested at 77 K under various bending diameters after the cable was soldered. It has been confirmed that the soldered cable can be bent on a diameter as tight as 0.14 m within a critical-current degradation of 6%. The 32-tape YBCO TSTC cable mounted on a 250 mm diameter disk was successfully charged to 10 kA in a one-second ramp up in liquid helium (4.2 K). Various cable fabrication methods have been investigated using a copper former having one and three helically grooved channels on 9.5 mm diameter copper rod for embedding a stacked YBCO tape conductor in the twisted structure channels. Scalability investigations of the fabrication methods to make a long TSTC conductor and bendability of the grooved copper rod former also have been performed. Three TSTC samples fabricated by Supercon have been tested. .

KEYWORDS: Twisted stacked tape cable, HTS cable, and YBCO tape, critical current, and high temperature superconductor.

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TABLE OF CONTENTS

 1   INTRODUCTION  ...............................................................................................................................................  4  

2   TSTC SOLDERED CONDUCTOR TEST  .................................................................................................  4  2.1   SOLDERING OF TSTC CONDUCTOR  .......................................................................................................  4  2.2   SOLDERED TSTC CONDUCTOR TEST RESULTS  .................................................................................  5  

3   TSTC CONDUCTOR FABRICATION METHOD DEVELOPMENT  ...........................................  9  3.1   INTRODUCTION  .............................................................................................................................................  9  3.2   ONE- AND THREE-HELICAL-GROOVE ROD FORMERS  ........................................................................  9  3.3   ANNEALING  .................................................................................................................................................  10  3.4   LONGER CONDUCTOR SCAL-UP INVESTIGATION  ............................................................................  12  3.5   FUTURE WORK  ...........................................................................................................................................  13  

4   SUPECON TSTC SAMPLE TEST RESULTS  ......................................................................................  14  

5   SUMMARY  ........................................................................................................................................................  16  

6   REFERENCES  ...................................................................................................................................................  16    

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1 INTRODUCTION  We have reported on a cabling concept of a Twisted Stacked-Tape Cable (TSTC) for flat tapes of High Temperature Superconductor (HTS) [1], [2]. The development of the TSTC method has been continuing. This cabling method is suitable and scalable to high current multiple cable conductors. We have been developing TSTC methods for YBCO coated tapes that have excellent high current performance at high magnetic fields [3]-[6]. The cabling method is very attractive for large-scale fusion magnets as well as various other magnet applications, such as SMES, compact superconducting cyclotron, etc. Our most recent investigations have been focused on mechanical integrity evaluation for a high field magnet application. In order to improve mechanical and electrical performance, we have soldered the HTS tape cables. A soldered cable of HTS tapes can be rigidly supported against a large electromagnetic Lorentz force. Additionally, the current sharing capability and current distribution of the cable can be enhanced by soldering the tapes. Soldering, however, could limit the bendability of the cable. Therefore we investigated bendability of a TSTC cable 2 m long and composed of 32-YBCO tape TSTC after soldering.

2 TSTC SOLDERED CONDUCTOR TEST

2.1 Soldering of TSTC Conductor A soldering station was developed for continuously soldering a Twisted Stacked-Tape conductor (TSTC) as shown in Figure 2-1. The TSTC conductor with a cross-section of 4.8 mm x 4.8 mm was composed of 32 YBCO tapes (Superpower YBCO tape of 4 mm width and 0.1 mm thickness). To solder the conductor, it was passed through a 60%Sn-40%Pb melting bath by hand. Figure 2-2 shows a part of the conductor before and after soldering.

Figure 2-1 Continuous cable soldering station with an enlarged insert picture of a solder melting bath with a TSTC conductor.

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!  

(a)  

 

(b)  

Figure 2-2 2 m 32-tape YBCO TSTC conductors with a cross section 4.8 mm x 4.8 mm before (a) and after (b) soldering.

 

2.2 Soldered TSTC Conductor Test Results 2.2.1 Bending tests

Critical current of the straight soldered TSTC conductor with 32 YBCO tapes was first tested in liquid nitrogen, and then it was mounted on a side surface of disks with various diameters. The diameter was reduced stet by step from 1 m to 0.14 m. Figure 2-3 shows the sample mounted on 0.25 m diameter disk with the current terminations. The terminations were made using our YBCO-BSCCO tape terminators that were develop earlier [5], [6]. The winding diameter was reduced down to 0.14 m as shown in Figure 2-4.

The measured critical currents of the soldered 32-tape YBCO TSTC conductor with various diameters are plotted in Figure 2-5 as a function of bending strain, which was converted from the bending diameter using the following:

Bending strain = Nominal cable diameter of 4 mm / Bending diameter.

The straight cable was bent to the following diameters (m) in the order of 1, 0.8, 0.6, 0.5, 0.46, 0.4, 0.36, 0.33, 0.3, 0.27, 0.25, 0.5, 0.25, 0.25 (in liquid helium), 0.25, 0.23, 0.21, 0.19, 0.17, 0.16, 0.15, 0.14, 0.25, 0.5, and then re-straight.

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Figure 2-3 Soldered 32-tape YBCO TSTC conductor mounted on a side surface of 0.25 m diameter disk.

Figure 2-4 Soldered 32-tape YBCO TSTC conductor mounted on a side surface of 0.14 m diameter disk.

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Figure 2-5 Preliminary results of measured critical currents of a soldered 32-tape YBCO TSTC conductor as a function of the bending strain. The arrow lines with numbers indicate the test order. The straight cable was bent to 2.86% using the 0.14 m bending diameter disk, and after a partial cyclic tests between 1.6% (0.25 m diameter bent) and 0.8% (0.5 m diameter bent), it was straightened again.

The critical currents were degraded by about 1.9% at the first 0.25 m diameter bending (the bending rate of 1.6%), about 5.4% degradation at the 0.14 m diameter bending (bending rate of 2.86%). After partial cyclic tests the degradation was between 1.6% (0.25 m diameter bend) and 0.8% (0.5 m diameter bend). The sample was then straightened again. The re-straightened cable resulted in 3.6% degradation from the initial critical current. As seen in Figure 2-5, the test results show that the critical current degraded at the same rate as the bending rate up to about 1% bending, and then the degradation increased to double the bending rate (for example critical current degradation of 5.4% at 0.14 m diameter corresponding to 2.8% bending strain).

2.2.2 TSTC cable operation test at 4.2 K

The 32-tape YBCO TSTC conductor was tested in liquid helium. The sample was mounted on a 0.25 m diameter disk as shown in Figure 2-6. The current leads were mounted on copper lead extensions of 10 kA vapor cooled current leads. A 10 kA charge test was performed without an external magnetic field using a 10 kA DC power supply at

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MIT PSFC. The current was initially increased very slowly with 20 A/s to achieve 10 kA the first time, and then it was charged to 10 kA more quickly. Finally it was charged to 10 kA in one second without any resistive voltage signals for the 1 m and 1.7 m voltage tap signals. The charge current and the hall sensor voltage are shown in Figure 2-7. The hall sensor was mounted parallel to the cable axis, so that the hall voltage was proportional to the sample current.

 

Figure 2-6 Soldered 2 m long, 32 tapes, YBCO stacked-tape cable twisted with a twist pitch of 200 mm, mounted on a side of 0.25 m diameter disk.

Figure 2-7 The charge current and the hall sensor voltage proportional to the conductor current as a function of time.

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3 TSTC Conductor Fabrication Method Development

3.1 Introduction We have demonstrated fabrication of a short length of one or three helical grooves of 254 mm twist pitch on a copper rod that is used as a sheath of a TSTC conductor [2]. This groove is obtained using a four-axial CNC milling machine with a 1” machinable working length. We now have a four axis CNC machine that can machine helical grooves on a rod of 20” long at our research center of MIT PSFC, as shown in Figure 3-1. We used this machine to fabricate one and three helical grooves.

 

Figure 3-1 Four axis CNC milling machine fabricating 20” long, three helical grooves on a 9.5 mm diameter copper rod.

3.2 One- and three-helical-groove rod formers

Figure 3-2 shows the fabricated one- and three- helical groove rods of 20” length on 3/8” (0.95 mm) diameter copper rods. The depth of the groove of the one-channel rod was 0.285”. Each groove of the three-channel rod had a depth of 0.110”. The width of the groove of both rods was 0.188”. The twist pitch of these helical grooves was 10” (254 mm).

The depth 0.285”of the one-groove rod was limited by machining. Since the groove was much deeper than the center of the rod, the groove cross-section along the twist pitch of 10” on a 3/8” rod was distorted from a simple square shaped groove as shown in Figure

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3-3. The distortion is a nature of helical groove made by the four-axis milling machine. The distortion becomes severe, if the twist pitch is short. We will be able to obtain a better groove shape by using a five-axis milling machine which allows adjusting a cutting tool angle to make a deep helical groove smoothly.

 

Figure 3-2 Fabricated one (upper) and three (lower) helical grooves of 20” length on 0.95 mm diameter copper rod. The inserts show a close-up view of the rods of one and three grooves, and the cross-sections.

 

Figure 3-3 Cross-section of one-helical-groove machined on a 0. 375” copper rod.

3.3 Annealing

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After machining the one- and three-groove rods were annealed to make them softer to improve bendability. Each rod was supported within a quartz tube and heat-treated at 500 °C for 5 hours. Figure 3-4 shows the rods taken from the furnace after the annealing.

 

Figure 3-4 One- and three--groove rods taken out from furnace after annealing at 500 °C for 5 hours.

 

Figure 3-5 Three-groove copper rod of 10” length bent with 250 mm diameter.

In order to test the bendability, we attempted to bend an annealed, three-grove copper rod of 10” length, to a 250 mm diameter. It was easily bent by hand the first time. Then the sample was straightened, and it was bent again into the same radius to test cyclic effects. Each bending of the sample became harder due to cold work. After the second bending

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cycle it was difficult to straighten the rod back to its original shape. During a halfway bending process it was bent back to 250 mm diameter which is shown in Figure 3-5.

3.4 Longer conductor scal-up investigation  

To make a longer conductor, the 20” length, machined copper rods (after annealing) were joined by brazing. Two grooved copper rods were mounted on a specially made fixture and brazed to join them together. Figure 3-6 shows the brazing process of test pieces of three-grove copper rods

 

Figure 3-6 Brazing to join two test pieces of three-groove rods.

Figure 3-7 shows one- and three-groove copper rod formers of 40” length joined from two 20” length copper rods by brazing. This confirmed we can make a (relatively) long conductor by brazing together pieces that were machined and annealed individually . We also confirmed that it is possible to easily bend a 3/8” copper rod former after annealing.

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Figure 3-7 One- (right) and three-groove (left) copper rods of 40” length, made of two 20” length copper rods by brazing. These copper pieces were annealed at 500 °C for 5 hours before brazing.

3.5 Future work

Using the 40” long one- and three-groove copper rod formers described above, we will fabricate two different cables: one using the three-helical-groove rod and one using the one-channel rod. The first one is a 60-tape conductor that uses three stacks (20 tape each). The tape used is YBCO 4 mm wide. The second cable is a 40-tape TSTC conductor with a single stack of 40 YBCO tapes 3 mm wide. The latter sample will be pre-twisted with a shorter twist pitch (about 150 mm ), and then it will be embedded into a groove of 254 mm twist-pitch groove. We expect this sample to have better mechanical bendability and AC losses. We will also investigate the critical current degradation of TSTC conductors on the grooved copper rod formers due to bending.

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4 SUPECON TSTC SAMPLE TEST RESULTS

Two TSTC samples of 40 YBCO tapes (4029-2 and 4029-3) fabricated by Supercon were tested in liquid nitrogen. Stacked tapes of these cables were first inserted in a round tube with a square hole made in a special method, and then twisted after drawing to compact. The compaction rates were 3.2% for Sample 4029-2 and 7.0% for 4029-3. Figure 4-1 shows one of the samples in a test cryostat for liquid nitrogen tests. The sample length was about 1.1 m.

 

Figure 4-1 Supercon Sample #2 (4029-2) in a liquid nitrogen cryostat.

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Figure 4-2 Critical current test result of Supercon sample 4029-2. V-I curve obtained for 0.6 m voltage taps on the copper sheath.

 

 

Figure 4-3 V-I curves of Sample 4029-3: Three curves were obtained for the voltage tap separations of 0.6 m, 0.8 m and 0.92 m. The first two voltage taps were mounted on the copper sheath, and the third one was mounted directly on a tape of the stacked tape cable.

 Figure 4-2 and Figure 4-3 show test results of Samples 4029-2 and 4029-3, respectively. The critical current of Sample 4029-2 at the criterion of Ec=100 micro-V/m was 1531 A with n=13.3. The n-value is much lower than the single tape value, which might have resulted from the copper sheath. The I-V curve showed a small linear resistive component of about 10 nano-V/m. This might indicate a slight mechanical damage due to compaction. The sample 4029-3 was resistive. It seems to have been severely damaged by swaging during sheath compaction.

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5 SUMMARY  1) A soldered, twisted, stacked-tape conductor of 32-YBCO tapes with 200 mm twist pitch can be bent on a diameter as tight as 0.14 m with a critical-current degradation less than 6%.

2) The 32-tape YBCO TSTC cable mounted on a 250 mm diameter disk was successfully charged to 10 kA in a one-second ramp up in liquid helium (4.2 K).

3) We also investigated cable fabrication methods using a copper former having one and three helically grooved channels on 0.953 mm diameter copper rods for embedding a stacked YBCO tape conductor in the channel.

4) We have investigated scalability to make a long TSTC conductor and bendability of the grooved copper rod former.

5) We have also tested two of three TSTC samples fabricated by Supercon.

 

6 REFERENCES

1. Makoto Takayasu, Luisa Chiesa, Leslie Bromberg, and Joseph V. Minervini, “Investigations of HTS Twisted Stacked-Tape Conductor,” MIT PSFC report, PSFC/JA-11-4, March 25, 2011. http://www.psfc.mit.edu/library1/catalog/reports/2010/11ja/11ja004/11ja004_full.pdf

2. Makoto Takayasu, Joseph V. Minervini, and Leslie Bromberg, “HTS Twisted Stacked-Tape Cable Development,” MIT PSFC report, PSFC/JA-11-10, May 31, 2011. http://www.psfc.mit.edu/library1/catalog/reports/2010/11ja/11ja010/11ja010_full.pdf

3. M. Takayasu, J.V. Minervini, and, L. Bromberg “Torsion Strain Effects on Critical Currents of HTS Superconducting Tapes,” Adv. Cryo. Eng., 56, Plenum, N.Y., 337-344, 2010.

4. M. Takayasu, L. Chiesa, L. Bromberg, and J.V. Minervini, “Cabling Method for High Current Conductors Made of HTS Tapes,” IEEE Trans. Appl. Superconduct., 21, no. 3, 2340-2344, 2011.

5. M. Takayasu, L. Chiesa, L. Bromberg, and J. V. Minervini, “HTS twisted stacked-tape cable conductor,” Supercond. Sci. Technol., 25(2012) 014011.

6. M. Takayasu, J.V. Minervini, L. Bromberg, M.K. Rudziak, and T. Wong, “Investigation of twisted stacked-tape cable conductor,” to be published at Adv. Cryo. Eng., 57, Plenum, N.Y., 2012.