Depressed Collector Gyrotrons Research Articles

Advances in technology and high power performance of the ECH system on DIII-D

The gyrotron complex on DIII-D has been updated and consists of six gyrotrons installed and routinely operating reliably for injection of up to 3.6MW into the plasma. The eight sets of real time steerable mirrors that are installed on DIII-D inject the rf power from the tokamak low field side. The transmission line was upgraded with new rf power monitors at the last miter bend before the tokamak, which can measure wave polarization, mode content, and rf power reflected from the plasma. A new design depressed collector gyrotron in the 1.5MW class, operating at 117.5GHz, is expected to be installed in 2017. Plans call for an expansion of the ECH system to ten gyrotrons.

DIII-D Electron Cyclotron Heating System Status and Upgrades

The DIII-D electron cyclotron heating (ECH) system consists of six 110-GHz gyrotrons with corrugated coaxial 31.75-mm waveguide transmission lines, and steerable launching mirrors. The system has been gradually updated, leading to an increased experimental flexibility and a high system reliability of 91% in the past year. Operationally, the gyrotrons can generate up to a total of 4.8 MW of RF power for pulses up to 5 seconds. The maximum ECH energy injected into the DIII-D is 16.6 MJ. The HE1,1 mode content is over 85% for all the lines, and the transmission coefficient is better than −1.1 dB for all the transmission lines, close to the theoretical value. A new depressed collector gyrotron was recently installed and was injected up to 640 kW of power into the plasma during 2014–2015 tokamak operations. Four dual waveguide launchers, which can steer the RF beams ±20° poloidally and toroidally, are used for real-time neoclassical tearing mode control and suppression. The launchers now have increased poloidal scanning speed and beam positioning accuracy of approximately ±2 mm at the plasma center. Two more gyrotrons are expected to be installed and operational in 2015–2016. The first is a repaired 110-GHz, 1-MW gyrotron that had a gun failure after more than 11 years of operation at DIII-D. The second is a newly designed depressed collector tube in the 1.5 MW class, operating at 117.5 GHz, manufactured by Communications and Power Industries.

Final design and test results of a high voltage amplifier for a gyrotron body power supply

The design of a high voltage amplifier was being developed that uses series-connected transistors to control the output [Tooker and Huynh, 2013]. Although having many potential applications, this amplifier is designed to meet at a minimum the requirements of a body power supply for depressed collector gyrotrons that are used in electron cyclotron systems on the DIII-D tokamak and other devices worldwide. Therefore, the output voltage needs to be adjustable up to 35kV DC and be capable of square-wave modulation at frequencies up to 5kHz. Continuous operating body currents for a gyrotron are on the order of a few tens of milliamps. However, to attain the required speed for modulation with load capacitances as high as 2.5nF, the ability to both source and sink peak currents as high as 5–10A is required.The design of a 40kV, 250mA high voltage amplifier has now been completed and the amplifier was fabricated. The transistors are configured into modules and the modules are mounted in four card cages within an enclosure that has a size equivalent to a standard electronics rack. The high voltage DC power supply, that provides the input voltage to the amplifier section, and the controls for the amplifier are housed within a separate electronics rack. An 800kΩ, 2.5nF dummy load for testing the amplifier was also fabricated, and the amplifier was been tested into this load up to its full output.The description of the recently completed amplifier and the test results will be presented.

Performance and Upgrades for the Electron Cyclotron Heating System on DIII-D

The electron cyclotron heating (ECH) system on the DIII-D fusion reactor consists of six 110-GHz gyrotrons with 6 MW installed power for pulses limited administratively to 5 s in length. The transmission coefficient is better than -1.1 dB for four of the transmission lines, which is close to the theoretical value. A new depressed collector gyrotron was recently installed and is injecting up to 720 kW of power into DIII-D during 2013 tokamak operations. Three of the four dual waveguide launchers, which can steer the RF beams ±20° both poloidally and toroidally, were used for real-time neoclassical tearing mode control and suppression with increased poloidal scanning speed up to 60°/s and positioning accuracy of the beams of ±2 mm at the plasma center. The ECH capabilities on DIII-D are being steadily updated, leading to increased experimental flexibility and high reliability of the system. In the past year, the ECH system reliability reached 87% for 2352 successful individual gyrotron shots into DIII-D. Planning is under way for the addition of two new depressed collector gyrotrons, one at 110 GHz, 1.2 MW and another at 117.5 GHz, 1.5 MW generated power, both of which are in the test stage at Communications and Power Industries.

Gyrotron and power supply development for upgrading the electron cyclotron heating system on DIII-D

An upgrade of the electron cyclotron heating system on DIII-D to almost 15MW is being planned which will expand it from a system with six 1MW 110GHz gyrotrons to one with ten gyrotrons. A depressed collector 1.2MW 110GHz gyrotron is being commissioned as the seventh gyrotron. A new 117.5GHz 1.5MW depressed collector gyrotron has been designed, and the first article will be the eighth gyrotron. Two more are planned, increasing the system to ten total gyrotrons, and the existing 1MW gyrotrons will subsequently be replaced with 1.5MW gyrotrons.Communications and Power Industries completed the design of the 117.5GHz gyrotron, and are now fabricating the first article. The design was optimized for a nominal 1.5MW at a beam voltage of 105kV, collector potential depression of 30kV, and beam current of 50A, but can achieve 1.8MW at 60A. The design of the collector permits modulation above 100Hz by either the body or the cathode power supply, or both, while modulation below 100Hz must use only the cathode power supply.General Atomics is developing solid-state power supplies for this upgrade: a solid-state modulator for the cathode power supply and a linear high voltage amplifier for the body power supply. The solid-state modulator has series-connected insulated-gate bipolar transistors that are switched at a fixed frequency by a pulse-width modulation regulator to control the output voltage. The design of the linear high voltage amplifier has series-connected transistors to control the output voltage, which was successfully demonstrated in a proof-of-principle test at 2kV. The designs of complete power supplies are progressing.The design features of the 117.5GHz 1.5MW gyrotron and the solid-state cathode and body power supplies will be described and the current status and plans are presented.

Performance, diagnostics, controls and plans for the gyrotron system on the DIII-D tokamak

The DIII-D ECH complex is being upgraded with three new depressed collector gyrotrons. The performance of the existing system has been very good. As more gyrotrons having higher power are added to the system, diagnostics of gyrotron operation, optimization of the performance and qualification of components for higher power become more important. A new FPGA-based gyrotron control system is being installed, additional capabilities for rapid real time variation of the rf injection angles by the DIII-D Plasma Control System are being tested and infrastructure enhancements are being completed. Longer term plans continue to include ECH as a major component in the DIII-D heating and current drive capabilities.

ECH system developments including the design of an intelligent fault processor on the DIII-D tokamak

A new generation fault processor is in development which is intended to increase fault handling flexibility and reduce the number of incomplete DIII-D shots due to gyrotron faults. The processor, which is based upon a field programmable gate array device, will analyze signals for aberrant operation and ramp down high voltage to try to avoid hard faults. The processor will then attempt to ramp back up to an attainable operating point. The new generation fault processor will be developed during an expansion of the electron cyclotron heating (ECH) areas that will include the installation of a depressed collector gyrotron and associated equipment. Existing systems will also be upgraded. Testing of real-time control of the ECH launcher poloidal drives by the DIII-D plasma control system will be completed. The ECH control system software will be upgraded for increased scalability and to increase operator productivity. Resources permitting, all systems will receive an extra layer of interlocks for the filament and magnet power supplies, added shielding for the tank electronics, programmable filament boost shape for long pulses, and electronics upgrades for the installation of the advanced fault processor.

Recent Upgrades and Extensions of the ASDEX Upgrade ECRH System

The multi-frequency Electron Cyclotron Heating (ECRH) system at the ASDEX Upgrade tokamak employs depressed collector gyrotrons, step-tunable in the range 105–140 GHz. The system is equipped with a fast steerable launcher allowing for remote steering of the ECRH RF beam during the plasma discharge. The gyrotrons and the mirrors are fully integrated in the discharge control system. The polarization can be controlled in a feed-forward mode. 3 Sniffer probes for millimeter wave stray radiation detection have been installed.

Progress and First Results With the New Multifrequency ECRH System for ASDEX Upgrade

A multifrequency electron cyclotron resonance heating (ECRH) system is currently under construction at the ASDEX Upgrade tokamak experiment. The system employs depressed collector gyrotrons, step tunable in the range of 105-140 GHz, with a maximum output power of 1 MW and a pulse length of 10 s. One two-frequency GYCOM gyrotron has been in routine operation at ASDEX Upgrade since 2006. A further extension of the system with three more gyrotrons is underway. An in situ calibration scheme for the broadband torus window has been developed. The system is equipped with fast steerable mirrors for real-time MHD control. The gyrotron and the mirrors are fully integrated into the discharge control system. The ECRH system turned out to be essential for the operation of H-modes after covering the plasma facing components of ASDEX Upgrade with tungsten. Deposition of ECRH inside rho <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">tor</sub> < 0.2 is necessary to prevent accumulation of W in plasmas with high pedestal temperatures. With respect to the limited loop voltage available in ITER, the use of ECRH for neutral-gas preionization to facilitate plasma breakdown and its application during the current ramp-up to increase the conductivity in order to save transformer flux have been demonstrated successfully for 105 GHz, 3.2 T (O1-mode) and 140 GHz, 2.2 T (X2-mode), corresponding to 170 GHz at ITER with the full and half values of its foreseen toroidal field of 5.3 T.

Experimental observation of the effect of aftercavity interaction in a depressed collector gyrotron oscillator

This paper presents the experimental observation of the effect of an aftercavity interaction (ACI) in a depressed collector gyrotron oscillator. The gyrotron generates an output power of 1.5MW at 110GHz in 3μs pulses with a 96kV and 40A electron beam and has a single-stage depressed collector. The ACI arises from an unintended cyclotron resonant interaction between the microwave beam traveling out from the cavity and the gyrating electron beam. The interaction occurs in the uptaper of the launcher, immediately downstream from the cavity, where the magnetic field is slightly lower than its value in the cavity region. The ACI results in a reduction in efficiency since the electron beam tends to extract power from the wave. There is also a broadening of the spent beam energy profile, which reduces the effectiveness of the depressed collector and in turn limits the overall efficiency of a gyrotron. Measurements of the maximum depression voltage of the collector vs beam current at 96kV are compared with simulations from the MAGY code [M. Botton et al., IEEE Trans. Plasma Sci. 26, 882 (1998)]. Excellent agreement is obtained between theory and experiment but only if the ACI is included. In the present experiment, it is estimated that the observed efficiency of 50% would have been about 60% in the absence of the ACI. These results verify the role of the ACI in reducing the efficiency of the gyrotron interaction.

The upgrade of the DIII–D EC system using 120 GHz ITER gyrotrons

The planned growth in the EC system on DIII–D over the next few years requires the installation of two depressed collector gyrotrons, a high voltage power supply, two low loss transmission lines, and the required support equipment. This new DIII–D EC equipment could be made identical to the ITER EC system requirements. By building the DIII–D hardware to the ITER specifications, it will allow ITER to gain beneficial prototyping experience on a working tokamak, prior to committing to building the hardware for delivery to ITER.

Design study of a test stand for ITER gyrotron

In the frame of development of the ITER electron cyclotron wave (ECW) system, a two MW CW coaxial cavity gyrotron will be developed during the Sixth Framework Program (2003–2006). Such development relies on the availability of a test stand capable of providing the electrical energy and cooling capacity. This test stand will possibly be used, in a later stage, for the component test of the ITER ECW system. This paper will first present the main parameters of this new coaxial gyrotron. Then we describe the test stand itself, including the general requirements for testing and evaluating the behaviour of the RF source and then a description of the electrical system design. Compared to the ITER reference design, the test stand emphasises the requirement of flexibility, which is necessary during the development of the gyrotron. The additional electrical equipment is included in the overview of the electrical system. The cooling system will be an important part of the design study. Indeed, the design efficiency of such a depressed collector gyrotron is ≈50%, implying >4 MW of continuous heat dissipation and evacuation by the cooling equipment. The specifications of the cooling system must also comply with ITER reference design values.