DC Break Research Articles

Evaluation of a newly developed low reflection dummy load for high power millimeter waves

A low reflection dummy load (LRD) has been newly developed for gyrotron development and operation. This LRD is designed for 0.5 s of high power (∼1 MW) millimeter wave injection with a low reflection. In order to reduce the reflection, the LRD consists of an inner cylinder, a main chamber, and a conical mirror, which results in the efficient absorption by long pass length between the inner cylinder and the main chamber. The characterization of low reflection has been demonstrated by the water temperature rise of the DC break (DCB) section inside the gyrotron, which works as an indicator of the reflected stray millimeter wave from the dummy load (DL). The water temperature rise of DCB with LRD was decreased by 1%, 15% and 17% at three tested frequencies of 82 GHz, 110 GHz and 138 GHz, respectively, compared with the conventional DLs, and the power absorption of LRD is 4–7% larger than the conventional DLs. A thermal analysis by a finite element method (FEM) with experimentally obtained heat load distribution and cooling time constant showed that the maximum temperature at 0.5 MJ injection is below the melting point of the material but it is not acceptable for the cycle operation considering the allowable stress of the aluminum-alloy used in the present design. FEM analysis suggested that changing its material to CuCrZr enable to use for 0.5 MJ injection. The result of FEM analysis also shows that the CW injection with less than 0.15 MW is acceptable in the present design.

DIAGNOSIS OF INSULATION OF ELECTRIC MACHINES USING SWITCHING PROCESSES

Insulation of electric machines is the most vulnerable area in the reliability of operation and use of electric machines. Existing methods of insulation diagnostics do not provide quality testing of winding insulation of windings. The existing normative method provides for exceeding the rated voltage by only 30%, which can detect only multi-closed groups, so the pulse process is provided as a DC break that feeds the winding during testing and then analysis of the transient process indicating the level of defect and diagnostic system module, which can provide a system of planned and preventive repairs.

DC Break Design for a 2.45 GHz ECR Ion Source

DC Break Design for a 2.45 GHz ECR Ion Source

Development of Waveguide DC Break for Power Transmission at 2.45 GHz and 100-kV Insulation

This letter presents a DC break for a waveguide developed for transmission coefficient higher than 90% at 2.45 GHz and for 100 kV of high-voltage insulation. The DC break consists of 15 dielectric sections (G-10) inserted between metal sections (Al) with WR340 flanges. Experimental results demonstrate high-voltage insulation up to 110 kV without any discharges and 97% of power transmission efficiency. The developed DC break achieves three times greater performance than that of previous DC breaks in the perspective of insulation and compactness. The waveguide DC break enables the operation of a cost-effective and compact microwave transmission system in a high-voltage environment while avoiding the usage of a high-voltage isolation transformer.

Circular Corrugated Miter Bend and Gap Losses for Broadband Frequency Applications

Circular corrugated waveguides are often used in fusion applications at single, multiple, or broadband millimeter frequencies due to their low ohmic loss, expected large frequency bandwidth, and direct coupling to free-space Gaussian modes. For single-frequency corrugated waveguide systems, transmission line components can be optimized to the desired frequency. For broadband or multiple-frequency applications, this is not possible. The goal of this paper is to demonstrate that the frequency bandwidth of circular corrugated waveguides can be compromised by diffraction losses to miter bends and gaps. It is shown that if the corrugation depth differs significantly from $\lambda $ /4, a theory can substantially underpredict the gap and miter bend losses. The simulations are also shown to compare favorably to experimental measurements. To improve the transmission line performance in large frequency bandwidth systems, such as 33–165 GHz reflectometry, reducing the number of miter bends may, therefore, be necessary. For the improvement of performance in narrower frequency bandwidth or multiple-frequency systems, especially high-powered systems, wavelength-dependent techniques may be applicable. One such application is shown for a Bragg reflection technique to reduce the thermal load to insulating ceramic rings in a dc break component for multiple-frequency ITER electron cyclotron heating transmission lines.

Radio frequency measurements of energetic-particle-driven emission using the ion cyclotron emission diagnostic on the DIII-D tokamak.

The Ion Cyclotron Emission (ICE) diagnostic on the DIII-D tokamak consists of two outboard midplane systems. In the first system, straps of an ion cyclotron range of frequencies antenna are configured as receiving antennas. For the second system, dedicated magnetic probes incorporated into the outer wall of carbon tiles have recently been restored. These systems collected a large set of radio frequency measurements in the 2015-2018 experimental campaigns by digitizing signals at 200 MSamples/s for ∼5 s per discharge. Each shot typically yields 32 GB of data; techniques for successful handling and analysis of this challengingly large dataset are discussed. The raw voltage fluctuations (<0.2 V and <1 mW) are analyzed in frequency space via fast Fourier transforms. Signals can be analyzed between 1 and 200 MHz with appropriate filtering and aliasing; this frequency range is limited by DC breaks used to provide 5 kV DC isolation. These high-frequency signals are driven by energetic ions and electrons. In particular, energetic-ion-driven ICE occurs at harmonics of the ion cyclotron frequency, enabling the frequency to be mapped to lab space via equilibrium reconstruction. In many DIII-D plasmas, ICE is emitted from the radial center of the plasma.

Waveguide DC breaks for TE01 and HE11 microwave transmission lines

Waveguide DC breaks for TE₀₁ and HE₁₁ microwave transmission lines

42-GHz/500-kW Electron Cyclotron Resonance Heating System on Tokamak SST-1

The 42-GHz electron cyclotron resonance heating (ECRH) system on SST-1 is used to carry out ECRH-assisted preionization, breakdown, start-up, and heating experiments at 0.75-T (second harmonic) and 1.5-T (fundamental harmonic) operation. The gyrotron delivers 500-kW power at 42-GHz frequency at −50-kV beam voltage, 20-A beam current, and 19-kV anode voltage. The gyrotron has been commissioned successfully on dummy load for full parameters (500 kW/500 ms). The transmission line consists of matching optic unit, circular corrugated waveguide, miter bend with bidirectional coupler, waveguide switches, polarizer, bellows, dc breaks, and an uptaper. Approximately 20-m-long transmission line is used to launch the power from gyrotron to tokamak, and the burn pattern at the exit of line near the tokamak ensures a good Gaussian beam. A composite launcher [consisting of four mirrors (two profiled and two plane), two gate valves, and two vacuum barrier windows] is used to connect two ECRH systems (42 and 82.6 GHz). The 82.6-GHz/200-kW ECRH system is also planned for SST-1 to carry out experiments at 3-T magnetic field. The 42-GHz ECRH system has been commissioned with tokamak SST-1, ECRH power has been launched in tokamak, and successful ECRH-assisted breakdown is achieved at second harmonic.

Converters of the Parameters of Multicomponent Two-Terminal Networks with a Dc Short Circuit and a Break in the Circuit Between Poles

The features of algorithms and apparatus for determining the parameters of passive multicomponent two-terminal networks with a short circuit between the poles through an inductive component and a dc break in the circuit are considered. The measuring circuit is supplied with voltage pulses varying as a function of the nth power of time and n-fold differentiation of the signals at the input and output of the circuit. Generalized parameters of the transfer function are calculated in terms of the ratio of the signals in each differentiation channel.

Multi-layered waveguide DC electrical break for the PEFP microwave proton source

A microwave proton source has been developed and is being used for 50-keV proton injection to the 20-MeV proton linac of the Proton Engineering Frontier Project (PEFP). It includes a 2-kW rf system for plasma generation. Because the rf system is electrically connected to a high-voltage beam extraction power supply, it is not easy to operate, control and maintain the rf system on the high-voltage platform. A 50-kV DC break has been developed on the waveguide between the proton source and the rf generator, and has been compared with the waveguide without the DC break. The results show the same operation performances, with very low radiation loss and no electrical breakdown, and will be helpful for easy operation and maintenance of the proton injector for the 100-MeV proton linac.

Compact microwave ion source for industrial applications

A 2.45 GHz microwave ion source for ion implanters has many good properties for industrial application, such as easy maintenance and long lifetime, and it should be compact for budget and space. But, it has a dc current supply for the solenoid and a rf generator for plasma generation. Usually, they are located on high voltage platform because they are electrically connected with beam extraction power supply. Using permanent magnet solenoid and multi-layer dc break, high voltage deck and high voltage isolation transformer can be eliminated, and the dose rate on targets can be controlled by pulse duty control with semiconductor high voltage switch. Because the beam optics does not change, beam transfer components, such as focusing elements and beam shutter, can be eliminated. It has shown the good performances in budget and space for industrial applications of ion beams.

Performance and arcing characteristics of Ag/Ni contact materials under DC resistive load conditions

Silver alloys used in medium duty switching devices exhibit low erosion and anti-weld characteristics along with good electrical and thermal properties. Silver metal-oxides (Ag/MeO) are extensively used as a contact material and silver nickel (Ag/Ni) is also employed that can exhibit improved features in given conditions. Erosion and arc characteristics of Ag/Ni contact materials under DC break conditions, 1–16 A, are presented and these demonstrate several features. (i) Close to 100% of all eroded cathode material is deposited onto the anode. (ii) A cathode mass loss for Ag/Ni appears to be similar for other Ag/MeO materials tested and is a linear function of arc energy. (iii) The resistivity per cross-sectional area of the arc, as the arc is drawn between the opening contacts, is found to be approximately constant during the metallic ion phase of the arc. (iv) With arc resistivity per cross-sectional area assumed to be constant during the metallic ion phase, the arc length on ignition can be determined. This is found to be 0.40±0.02 mm for the 4 A study of the Ag/Ni material.

Development and achievements on the high power ECRF system in JT-60U

An output power of 1.5 MW for 1 s was achieved at 110 GHz in a recent gyrotron development using the JT-60U ECRF system. It is the world's highest power oscillation for a pulse duration of at least 1 s. The achievement was enabled by, in addition to the carefully designed cavity and collector, necessary because of thermal stress, an RF shield for the adjustment bellows and a low-dielectric-loss dc break. The way the power was modulated was improved upon by controlling the anode voltage, with high modulation frequency of 5 kHz being achieved in NTM stabilization experiments. Moreover, as a developmental step to realizing a reliable ECRF system for use in future fusion experiments, a long pulse demonstration of 0.4 MW and a 30 s injection into the plasma was achieved with real time control of the anode/cathode-heater. Confirmation was made that the temperature of the cooled components had been saturated with no evidence of any damage being discovered in the waveguides and antenna without forced cooling. An innovative antenna with a relatively wide range of beam steering capabilities utilizing a linearly moving-mirror concept was also designed for use as an active cooling antenna with longer pulses in the future, e.g. for JT-60SA. The beam profile and mechanical strength analyses proved the feasibility of the antenna.

Commissioning of the 28-GHz Electron Cyclotron Resonance Heating System on ADITYA Tokamak

An electron cyclotron resonance heating system is commissioned on Aditya tokamak to carry out pre-ionization, start-up, and heating experiments. A high-power microwave source (gyrotron), capable of delivering 200-kW cw power at 28 ± 0.1 GHz, is commissioned successfully using a water dummy load for pulsed operation. The output mode of the gyrotron is TE02. The output power of the gyrotron is measured using microwave probe couplers, a spectrum analyzer, and calorimetric techniques. A hardwired interlock operates a rail-gap-based crowbar system in less than 10 μs under fault condition and protects the gyrotron. The rail-gap crowbar operation has been qualified with the high-voltage power supply by performing a 10-J wire-burn test prior to energizing the gyrotron.A transmission line consisting of matching optic units, dc break, polarizer, miter bend, and corrugated waveguides terminates with a boron nitride window. The total attenuation in the line is measured to be less than 1.1 dB. Based on quasi-optical theory, a beam launcher is designed, fabricated, and tested for ultrahigh-vacuum compatibility prior to commissioning on tokamak.After successful operation of the gyrotron on the dummy load, the gyrotron output has been coupled to the ADITYA tokamak, and successful breakdown of neutral gas is observed without assistance from an ohmic transformer.

Design of High Power DC Break for ICRH of EAST

1.5 MW Ion Cyclotron Wave Heating system was developed, the transmitter and the antenna both have their ground loops, which will severely perturb the system's normal operation. To avoid perturbation, a DC break was designed. The S parameter and the VSWR (voltage standing wave ratio) of incident port were calculated; the thermal effect caused by conductor loss and dielectric loss was analyzed.

Test and Commissioning of 82.6 GHz ECRH system on SST-1

Electron Cyclotron Resonance Heating (ECRH) system will play an important role in plasma formation, heating and current drive in the Superconducting Steady state Tokamak (SST-1). Commissioning activity of the machine has been initiated. Many of the sub-systems have been prepared for the first plasma discharge. A radial and a top port have been allotted for low field side (LFS) and high field side (HFS) launch of O and X- modes in the plasma.The system is based on a gyrotron source operating at a frequency of 82.6±0.1GHz (GLGD-82.6/0.2) and capable of delivering 0.2 MW / 1000s with 17% duty cycle. The transmission line consisting of ∼15 meters length 63.5mm corrugated wave guide, DC break, wave guide switch, mitre bend, polariser, bellows that terminates with a vacuum barrier CVD window. A beam launching system used to steer the microwave beam in the plasma volume is connected between the end of the transmission line and the tokamak radial and top ports. A VME based real time data acquisition and control (DAC) system is used for monitoring, acquisition and control. Hard-wired interlock operates a rail-gap based crowbar system in less than 10µs under any fault condition. Burn patterns are recorded at various stages in the transmission line. The gyrotron is tested for ∼200 kW / 1000s operation on a water dummy load. Transmission line is tested at various power levels for long pulse operation. The paper highlights the experimental results of successful commissioning of the system.

Operational progress of the 110GHz-4MW ECRF heating system in JT-60U

The JT-60U electron cyclotron range of frequency (ECRF) heating system is utilized to realize high performance plasma. Its output power is 4 MW at 110 GHz. The output power of the gyrotron used in the heating system can be controlled by changing its anode voltage. Then, a compact anode voltage controller was developed to modulate the injected power into plasmas. This controller achieved the modulation frequency of 12 - 500 Hz with modulation factor of 80 % at 0.7 MW of output power. Additional function of this controller also could make the pulse duration longer from 5 s to 16 s at 0.5 MW. For the long pulse operation, temperature rise of the DC break made of Alumina ceramics in the gyrotron was estimated from the measured temperature rise of the coolant and its maximum temperature was about 140 deg. From the analysis of this temperature rise, DC break materials should be changed to low-loss materials for extending to 30 s of the pulse duration. The stabilization of neoclassical tearing mode (NTM) was demonstrated by ECRF heating using the real-time system in which ECRF beams were injected to the NTM location predicted from ECE measurement every 10 ms. The ECRF beams were scanned poloidally with the steering mirrors of the antennas.

Considerations in Selection of ECH System Transmission Line Waveguide Diameter for ITER

The reference diameter for the ECH transmission lines is presently 63.5 mm. Analyses of the heat generation and removal in 63.5 mm corrugated waveguide components were reported [R.A. Olstad, et al., Proc. of IAEA TM on ECRH Physics and Technology for ITER, Kloster Seeon, Germany, 2003, http://ipp.mpg.de/tmseeon; R.A. Olstad et al., ‘‘ECH MW-level CW Transmission Line Components Suitable for ITER,’’ to be published in Fusion Eng. & Design (2005)]. Those analyses concluded that the temperature of all components could be kept to acceptable levels, even with operation at 2 MW cw per transmission line. Recently interest has been expressed in the community about the possible advantages of using a smaller diameter waveguide for ITER, particularly because of limited space available at both the equatorial and upper launchers. In addition to ameliorating the space constraints, there could be large cost savings for a modest diameter reduction in certain transmission line components, particularly gate valves and CVD diamond window assemblies at the entrance to the launchers.Results of a tradeoff study on ITER waveguide diameter are reported. Diameters considered range from 45 mm to 63.5 mm; consideration is also given to the possibility of tapering down to 31.75 mm at the launchers. The most critical issue for smaller diameter components is the increased losses and increased power densities. These lead to more demanding cooling provisions and higher operating temperatures for components such as miter bends, power monitor miter bends, bellows, dc breaks, waveguide switches, and waveguide sections adjacent to miter bends. In addition, the overall transmission efficiency of the ITER transmission lines would be reduced.

An ITER-relevant evacuated waveguide transmission system for the JET-EP ECRH project

An over-moded evacuated waveguide line was chosen for use in the transmission system for the proposed JET-enhanced performance project (JET-EP) electron cyclotron resonance heating (ECRH) system. A comparison between the quasi-optical, atmospheric waveguide and evacuated waveguide systems was performed for the project with a strong emphasis placed on the technical and financial aspects. The evacuated waveguide line was chosen as the optimal system in light of the above criteria. The system includes six lines of 63.5 mm waveguide for transmitting 6.0 MW(10 s) at 113.3 GHz from the gyrotrons to the launching antenna. The designed lines are on average 72 m in length and consist of nine mitre bends, for an estimated transmission efficiency of ∼90%. Each line is designed to include an evacuated switch leading to a calorimetric load, two dc breaks, two gate valves, one pumpout tee, a power monitor mitre bend and a double-disc CVD window near the torus. The location of waveguide support is positioned to minimize the power converted to higher-order modes from waveguide sagging and misalignment. The two gate valves and CVD window are designed to be used as tritium barriers at the torus and between the J1T and J1D buildings. The last leg of the waveguide leading to the torus has to be designed to accommodate the torus movement during disruptions and thermal cycles. All lines are also designed to be compatible with the ITER ECRH system operating at 170 GHz.

Steady state high power test system at 3.7 GHz

A Lower Hybrid Current Drive (LHCD) system would be the prime method to drive current in SST-1. The LHCD system is based upon two TH2103D klystrons of M/s Thomson CSF at 3.7 GHz capable of delivering 500 kW CW microwave power. The klystrons have been commissioned and deliver the microwave power through two standard WR-284 waveguides. It is electrically isolated from the main system using DC breaks. Two high power circulators protect the tube from high reflection. Directional couplers measure forward and reflected power in the system. The output power is deposited on two matched dummy loads during testing. In the event of arcing or any other fault, the high voltage from the klystron is removed in less than 5 μs, with the help of a crowbar system, based on a pressurized rail gap switch, for the protection of the klystron. High power (up to 200 kW), 1000-s tests of two klystrons have been conducted and detailed test results are described in this paper. It also describes the high power microwave system at 3.7 GHz that is being used to test the high power microwave components, being designed and fabricated for use in the LHCD system on SST-1.