Paschen Tests Research Articles

Recent progress of JT-60SA project toward plasma operation

Abstract Superconducting tokamak JT-60SA plays an essential role in fusion research and development by supporting and complementing ITER project, providing directions to the DEMO design activity and fostering next generation scientists and engineers. Since the incident of the Equilibrium Field coil #1 during the Integrated Commissioning (IC) in March 2021, both EU and JA Implementing agencies (IAs) have examined how to ensure safety operation of JT-60SA by mitigating the risk of possible discharge occurrence inside the cryostat. Based on the experience of the Global Paschen tests, the IAs have established a strategy of risk mitigation measures, which is a combination of (i) reinforcement of insulation, (ii) avoiding unnecessary voltage application to the coil systems and (iii) immediate de-energization of the coils when deteriorated vacuum condition is detected. Thanks to the considerable efforts of the Integrated Project Team (IPT) members, the IC restarted in May 2023. After the confirmation of superconducting state of coil systems (TF, EF and CS), the coil energization test and the plasma operation (OP-1) starts. The first plasma was successfully achieved on 23 October 2023 with a limited value of applied voltage and current to the coils. The plasma configuration control will be also confirmed with low plasma current and low auxiliary heating power conditions. Based on the IO-F4E-QST collaboration, activities of JT-60SA have been shared with the IO and provided an important lesson learned for ITER assembly and commissioning, and will provide an outstanding contribution to fusion research at large. After OP-1, Maintenance & Enhancement phase 1 (M/E-1) starts from January 2024, in which in-vessel components are installed, and heating system and diagnostic system are extensively upgraded to allow high power heating experiment planned in OP-2. In order to make the best use of JT-60SA, newly organized JT-60SA experiment team will refine the research plan in the future high heating power operation phase.

Paschen Tests at Cryogenic Conditions: Experimental Results

Paschen Tests at Cryogenic Conditions: Experimental Results

Global cryogenic Paschen test of the JT-60SA magnet system

Superconducting fusion magnets feature large inductances, and operate at high currents and at cryogenic temperatures. In case of a quench, the stored electromagnetic energy need to be immediately discharged, thereby inducing high voltages. High quality electrical insulation withstanding cryogenic temperatures is therefore essential for the safe operation of the magnet system. Local and integral Paschen tests at ambient temperature have become a standard to validate the electrical insulation of electromagnetic components of fusion devices. In order to mitigate the risk from a degradation of the electric insulation during cool-down, a global Paschen test of the complete magnet system at cryogenic temperatures is suggested. This paper focuses on the specific design of JT-60SA fusion device, analyses the boundary conditions for a cryogenic global Paschen test, and demonstrates its feasibility at limited risk during a future cooldown.

Configuration of instrumentation wire extraction for ITER magnet feeder system

For the quench detection, the high voltage (HV) instrumentation wires should be led out from the electrical insulation layers of ITER magnet feeders. Based on the latest design, at least four HV instrumentation wires need to penetrate from the busbar joint, which break the integrity of the intact insulation layer. A perforated crack may generate along the instrumentation wire in the insulation layer. At some specific ambient pressure and operational voltage, the crack will cause Paschen discharge inevitably, which is the most serious risk threatening the safe and steady operation of the feeders. There are over 200 busbar joints with the instrumentation wires penetration need to be manufactured on the ITER site, and a reliable and durable insulation structure and technology need to be developed for the extracting HV instrumentation wires of ITER magnet feeder. In this paper, the HV instrumentation wire extraction and the specimen design are presented. The electrical properties of the specimen, including the DC hi-pot test, Paschen test and Partial Discharge (PD) test, are reported and discussed.

Testing of the ITER Central Solenoid Modules

The ITER Central Solenoid is under fabrication by the U.S. ITER organization and its subcontractors. U.S. ITER will supply seven modules to ITER IO, six of which will be assembled in a stack that forms the ITER Central Solenoid. The first modules that were built by GA at their facility, went into high voltage testing, including Paschen testing in the vacuum, and then they were tested at 4.5 K and up to 40 kA to demonstrate compliance of the coil with the ITER requirements. In this article, we present the Test Plan and results of the central solenoid (CS) module’s performance, especially at the full current. We measured critical temperatures in several pancakes, we measured ac losses, joint resistance, and hydraulic characteristics of the coils. We also measured displacements of the coil height and hoop strain of the CS module (CSM) to verify the structural mechanical characteristics of the coil along with the cooldown shrinkage of the coil. We studied the performance of the cowound quench detectors and confirmed their effectiveness in the suppression of inductive noise. This information is necessary for verification of the stack behavior of CS in ITER operation. The test results and preliminary analyses are presented, compared to expectations, and discussed.

Insulation of Current Leads for Superconducting PF-3 Coils of SST-1

Conventional copper current leads (CLs) for superconducting magnets of SST-1 are fabricated to operate in dc condition for Toroidal field coils and fast current ramp up to 10 kA for Poloidal field (PF) coils at low temperature under vacuum. Two pairs of CLs are required to be installed with PF-3 coil terminals in current feeder system chamber (CFSC) for shaped plasma operation in SST-1.The PF coils of SST-1 are subjected to the induced voltages of around 1 kV due to Ohmic coil discharge. In past few campaigns, high induced voltages on PF coils due to Ohmic coil discharge have caused arcing inside the CFSC between high potential PF CLs and grounded thermal shield. This arcing resulted in severe damage to the PF current leads and helium hydraulic lines. We report the development and implementation of Paschen leak tight electrical insulation for cylindrical CLs with variations in diameter and geometry along its length. The developmental procedures include the sample preparations and optimization of insulation processes. The scales down version of CLs samples were prepared after the investigation of curing kinetics of insulation resin at 140 °C and 110 °C. The high voltage testing of these insulation samples was carried out up to 28 kV at room temperature (RT) before and after multiple thermal shocks. These insulation samples were also tested up to 5 kV dc and 80 K temperature in Paschen test setup. The insulation resistance and breakdown voltage of these samples found to be more than 100 GΩ and 28 kV, respectively at RT.

Thermal Hydraulic Behavior of the First ITER CS Module

The ITER Central Solenoid (CS) modules are under fabrication by the US ITER organization and its subcontractors. US ITER will supply seven modules to the ITER Organization (IO), six of which will be assembled in a stack that forms the ITER Central Solenoid, the last one being spare. The first module, namely CSM 1 was manufactured by General Atomics (GA) and went through Factory Acceptance Tests (FAT) including high voltage testing, Paschen testing and then cold test at 4.5 K and up to 40.0 kA in order to demonstrate compliance with coil performance requirements. The paper focuses on the results of the first CS Module thermal hydraulic characterization without current (and field). Pressure drops, transit time, and thermal coupling between pancakes are presented. Analysis results and tests are also compared.

First ITER CS module test results

The ITER Central Solenoid (CS) is under fabrication by the US ITER organization and its subcontractors. US ITER will supply seven modules to ITER IO, six of which will be assembled in a stack that forms the ITER Central Solenoid, with one as a spare. The first module fabrication has been completed by General Atomics (GA) at their facility and has begun testing including high voltage testing, Paschen testing in the vacuum and then testing at 4.5 K and up to 40 kA in order to demonstrate compliance of the coil to ITER requirements. In the paper we present the Test Plan and results of the CS Module performance tests, especially at 40 kA current. AC losses, joint resistances and hydraulic characteristics of the coil are all measured. Displacements of the coil height and hoop strain of the CS Module are also measured to verify structural and mechanical characteristics of the coil along with cooldown shrinkage of the coil. This information is used for verification of the stack behavior of CS in ITER operation. The test results and preliminary analyses results are presented, compared to expectations, and discussed. All measured parameters suggest that the CS module will perform well in ITER machine.

Electrical performance characterization of helium inlet of the ITER PF6 coil double pancakes

The helium inlet is one of the most important components of the ITER Poloidal Field (PF) coils. The insulation structure of the helium inlet is critical to provide sufficient electrical and mechanical properties for a practical application. In this paper, an ITER PF6 coil double pancake helium inlet trial mock-up was designed and manufactured by simulating the actual manufacturing process. A thermal cycling test on the sample was carried out from 280 K to77 K, also before and after the thermal cycling, a turn-to-turn DC HV test and Paschen test were performed. The test results satisfied the ITER requirements. It has therefore been verified that the PF6 coil double pancake helium inlet insulation has good electrical properties, and can be applied to the formal production of the PF6 coil double pancakes.

Extraction of instrumentation wires through the high-voltage insulation of the ITER magnet feeder system

The ITER magnet quench detection system relies on voltage taps to detect the development of resistive voltages across the coils and busbars. These voltage taps may be at high voltage, up to 30 kV during high voltage tests, and the signals from them must be led from the coils and feeders to the quench detection electronics, requiring that they must penetrate the ground insulation on the coils and feeders. This paper describes the R&D effort to develop and qualify the technology to extract these wires through the feeder busbar ground insulation. The voltage tap wires are 3 mm in diameter, and are a significant perturbation to the 6 mm thick glass/Kapton ground insulation. Techniques to fill the void between wire and ground insulation are presented, as well as techniques to inhibit unwanted resin flow and locally reinforce the wires. Specimens have been thermally cycled and qualified through highvoltage Paschen tests.

Study on Electrical Tests of Superconducting Busbar Protection Insulation of ITER Feeder PF4 CFT

Poloidal field coil no. 4 (PF4) cryostat feedthrough (CFT) is a key subcomponent of ITER feeder system. The superconducting busbars of PF4 CFT have insulation protection layer, which consists of several half-overlapped pre-preg and glass Kapton for main structure. Base on the special busbar structure, how to do the electrical tests for production final acceptance was studied. In this paper, three kinds of electrical tests for PF4 CFT busbar insulation acceptance were introduced, include direct current high-voltage test, alternating current partial discharge test, and Paschen test. On-site environment control, electrodes connection configuration, and some detail processes of these electrical tests were introduced. Some test results of leakage current were showed in this paper to guarantee the high quality of PF4 CFT superconducting busbar protection insulation.

Experimental results on Paschen tests on the electrical insulation of the terminal service Box of the correction coils of ITER

The insulation performance of wet wrapping of terminal service box pipework is investigated by the Paschen discharge test in this study. The test mock-up was designed with a U-shaped pipework as the fundamental structure, consisting of insulator breaks, voltage taps, and high voltage wires. In addition, the design of the test mock-up and insulation structure is studied, and the investigation method used in this study is described in detail. A high voltage test was conducted at room temperature for various degrees of vacuum. The test results demonstrated that the insulation (wet wrapping) remains undamaged, and leakage current is less than 10 μA. Further, the test results conformed to the requirements of the ITER Organization. Finally, the performance of the insulation was determined, and the feasibility of the wet wrapping process was verified indirectly.

Manufacture and electrical characterisation of high voltage insulation for superconducting busbars – (Part 2) application techniques and electrical characteristics

It is planned that the high voltage electrical insulation on the ITER feeder bus bars will consist of interleaved layers of epoxy resin-pre-impregnated glass tapes (‘pre-preg’) and polyimide. The development and characterisation of these materials was detailed in Part 1 of this report [1]. This paper provides details of the techniques adopted to fully insulate the complex shape of the busbars, forming insulation to fit the many changes in section. Additionally and so far as is reasonably practicable, the insulation should be void and wrinkle free around the helium inlet and outlet pipes that protrude from the surface of the current leads.High voltages will be developed during the pulsed operation of some ITER magnets and in a fault condition, voltages are likely to rise even further. It is therefore necessary to ensure that the insulation can reliably withstand high voltages over the estimated 30,000 operational cycles of the ITER device.The selected materials were processed and techniques developed via three stages (a) short length but full section of current lead (so called, mini mock-up), (b) longer lengths of full cross-section current lead and joint (mock-up) and (c) full sized, busbar. Trials on (a) and (b) were carried out at Rockwood Composites in the UK, tested at the ITER Organization in France and the full sized busbar insulated and tested at the Institute of Plasma Physics (ASIPP), Hefei, PR China.Insulation effectiveness was assessed by measuring the insulation resistance (IR), and performing partial discharge and Paschen tests, before and after thermal cycles to 77 K. All of the criteria established by the ITER organization were met.

Performance of the insulation mock-ups for the ITER PF6 coil

The ITER PF6 coil is manufactured by winding, insulating the superconducting conductors, and further connecting them electrically and hydraulically. To qualify the insulation design and manufacturing process to be applied to the ITER PF6 coil, two beam-shaped mock-ups taking the form of a 3 × 3 array of conductors combined with the same insulation as in the production coil have been tested in simulated Tokamak operating conditions. All the results of the electrical and mechanical tests, including DC and AC high-voltage withstanding at both room and cryogenic temperature, AC partial discharge, Paschen tests, cryogenic compressive fatigue, push-out, and thermal cycling, met the ITER requirements. Performance of the mock-ups confirmed that the insulation design and manufacturing process are suitable for the ITER PF6 coil, based on that the qualification of the insulation manufacturing process for the double pancake can be started.

Superconducting Feeder System for ITER Central Solenoid Module Final Test Facility

General Atomics has commissioned the final test facility at the central solenoid module (CSM) manufacturing site in Poway, CA, USA. The facility includes a number of critical subsystems including the feeder system (FS), supplied by the Institute of Plasma Physics Chinese Academy of Science (ASIPP), that connects the test chamber to the cryogenic and electrical systems. It includes a coil termination box, high-temperature superconducting current leads, and a feeder duct. The FS carries the current (50 kA) and supercritical helium (4.5 K, 5.5 bara) to the CSM and its supporting structure, while monitoring and controlling the temperature, pressure, flow, and voltage drop through all elements of the superconducting components. It functions as an integral part of the system to rapidly (6-s decay time) dissipates 1-GJ energy stored in the coils, and to protect the cryogenic system. The system is complex, requiring multidisciplinary engineering including: high-temperature superconductivity, cryogenic-temperatures (77, 50, and 4.5 K), high vacuum (1.0 × 10−5 Pa), redundant quench detection (voltage, temperature), high-voltage insulation (15 kV), low thermal loads (70 W), low electrical joint resistance (2 nΩ), Paschen testing, high-voltage signal measurements, and high-current electronics. The system presented significant challenges for design and analysis, complex manufacturing assembly processes, measurement and control, and stringent quality and safety requirements.

Experimental Study on Vacuum Control Method for Paschen Tests of the Superconducting Magnet Mockup

In order to verify the integrity of the insulation of a superconducting magnet, the Paschen test is needed to perform on the insulation after its manufacture. The vacuum vessel for the Paschen test is required to keep for an adequately long time under the pressure values of different degrees. To achieve the low pressure in the vacuum vessel, isolating the pumping unit from the vessel is not applicable as degassing inside the vessel will eventually ruin the pressure. Therefore, one dynamic balance control system for low pressure control is designed. The feature of the system is adjusting the opening of intake valve by proportional-integral-differential control system automatically, while the vacuum pump is working constantly. The results show the dynamic balance control system can keep the pressure values of 1 ± 0.05, 10 ± 0.15, 100 ± 0.2, and 1000 ± 0.5 Pa, respectively, and the holding time of each vacuum degree is more than 2 h, which satisfies the basic vacuum requirement for the Paschen test of the tested magnet mockup sample, and can also be extended to the Paschen test on the insulation of future actual superconducting magnet.

The ITER Central Solenoid Module final test facility

General Atomics (GA) is currently fabricating the ITER Central Solenoid Modules (CSM) under contract to US ITER at Oak Ridge National Laboratory, under the sponsorship of the Department of Energy Office of Science. The contract includes the design and qualification of manufacturing processes and tooling necessary to fabricate seven CSM (6+1 spare) that constitute the ITER Central Solenoid. The modules will be produced and delivered to the ITER site during 2018–2021.Each CSM will undergo final testing at GA to verify performance. Testing includes helium leak testing, high voltage insulation testing, cooldown to 4.5K and charging to 48.5kA followed by a series of tests designed to measure as-built performance of the superconductor.GA has completed the design and is now installing and commissioning the Final Test Facility at the CSM Manufacturing site in Poway, California. The facility includes a number of critical subsystems. The test chamber system consists of a 160m3 liquid nitrogen shielded cryostat to support the 110-t CSM, a vacuum pumping and leak detection system and Paschen testing equipment. The feeder system connects the test chamber to the cryogenic and electrical systems. It includes a coil termination box (CTB), high temperature superconducting current leads and a superconducting feeder duct. The cryosystem provides refrigeration and circulates supercritical helium at 4.5K for cooling the CSM and 50K helium gas for cooling the current leads. The electrical system includes a DC power supply providing 48.5kA of current and a fast discharge system used for quench protection. The fast discharge system uses redundant DC breakers based on vacuum circuit breaker switching and a 1 Giga-Joule discharge resistor to absorb the stored energy in the coil with a 6s decay time.

Influence of void defects on partial discharge behavior of superconducting busbar insulation

For a magnetic confinement fusion device, the superconducting magnets and busbars need to be insulated with one layer of solid insulation to isolate the high voltage potential from the ground. The insulation layer commonly consists of several interleaved layers of epoxy resin-impregnated glass fiber tapes and polyimide films. The traditional electrical inspection methods for such solidified insulation on the magnet and busbar are a DC voltage test or a Paschen test. These tests measure the quality of the insulation based on the value of leakage currents. However, even if there is a larger quantity of high dielectric strength material implemented, if there are some microcavities or delaminations in the insulation system, the leakage current may be limited to microampere levels under testing levels over dozens of kilovolts. Therefore, it is difficult to judge the insulation quality just by the magnitudes of leakage current. Under long-term operation, such imperceptible defects will worsen and finally completely break down the insulation because of partial discharge (PD) incidents. Therefore, a PD detection test is an important complement to the DC voltage and Paschen tests for magnet and busbar insulations in the field of fusion.It is known that the PD detection test is a mature technique in the electric power industry. In this paper, the PD characteristics of samples containing glass fiber-reinforced composite insulations for use with the superconducting busbar were presented and discussed. Various samples with different void contents were prepared and the PD behaviors were tested.

Reaction Heat Treatment and Epoxy Impregnation of a Large <formula formulatype="inline"> <tex Notation="TeX">$\hbox{Nb}_{3}\hbox{Sn}$</tex></formula> CICC Coil for a Series-Connected Hybrid Magnet

Recent activities that were conducted on the Helmholtz Zentrum Berlin Series-Connected Hybrid outsert coil included the reaction heat treatment and vacuum-pressure impregnation. A thermocouple was wound into the windings for diagnostic purposes during processing of the coil. To maintain a reasonable temperature differential across the coil, the ramp rate was limited to approximately 2.7°C/hr. Epoxy impregnation was conducted with an epoxy developed in-house. The epoxy is transferred in vacuum but cured with one atmosphere gage pressure applied. Subsequently, the winding hardware was removed including the stainless steel mandrel through a machining operation. After assembly of voltage breaks and voltage taps, Paschen tests to 4 kV were conducted successfully that qualified the ground insulation. In addition, surge tests to qualified the internal insulation with 3 kV applied across the leads were conducted. Impedance measurements were conducted to confirm the internal insulation integrity.

Design principles for handmade electrical insulation of superconducting joints in W7-X

The superconducting magnet system of the Wendelstein 7-X (W7-X) experiment consists of 50 non-planar and 20 planar coils, 121 bus bars and 14 current leads. The connection between bus bars, coils and current leads will be provided by 198 joints. The joints have to be insulated manually during the assembly of the machine in constraint positions and a tight environment. In general the insulation is based on glass tapes impregnated with epoxy resin and special G10 insulating pieces embedded in the glass tape insulation. In critical areas Kapton®-foils are embedded in the insulation. All types of insulation were qualified at mock-ups in a 1:1 model of the expected environment in W7-X. The qualification programme comprises thermal cycling between room temperature and 77K and high voltage tests under air, under vacuum and under reduced pressure (Paschen test). The paper describes the main principles used for different types of handmade Paschen-tight insulations in W7-X and the visual and electrical tests during and after assembly.