An integrated gyrotron controller

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Abstract

The ECRH system of W7-X is composed of 10 independent gyrotron modules. Each module consists of one gyrotron and its peripherals such as power supplies, cooling plants and distributed PLC systems. The fast real-time control functions such as the timing of the two high voltage supplies, trigger pulses, protection, modulation and communication with the central control of W7-X, is implemented in an integrated controller which is described in this paper.

As long-term maintainability and sustainability are important for nuclear fusion experiments, the choice fell on an FPGA-based design which is exclusively based on free (as in “freedom”) software and configuration code. The core of the controller consists of a real-time Java virtual machine (JVM) that provides the TCP-IP connectivity as well as more complicated control functions, and which interacts with the gyrotron-specific hardware. Both the gyrotron-specific hardware and the JVM are implemented on the same FPGA, which is the main component of the controller.

All 10 controllers are currently completed and operational. All parameters and functions are accessible via Ethernet. Due to the open, FPGA-based design, most hardware modifications can be made via the network as well. This paper discusses the capabilities of the controllers and their integration into the central W7-X control.

Introduction

The ECRH system of W7-X is equipped with 10 independent 1 MW, 140 GHz, CW gyrotron modules [1]. Apart from the gyrotron itself, each module consists of the necessary peripherals such as cooling circuits, the HV power supplies, protection and control systems. The main (cathode) HV power supplies [2] are located in a different building. These power supplies are floating multi-purpose devices which can also be used for NBI and ICRH. Due to the different polarity requirements of the individual heating systems, the grounding of these power supplies is carried out on the consumer side. The W7-X gyrotrons [3] operate in the depressed collector regime, which means that a second low power HV supply of opposite polarity provides the so-called body (anode) voltage of the gyrotron. The total acceleration voltage is the difference between the two voltages. Both voltages determine the operating point and the emission of the gyrotron and must be controlled safely on a fast timescale. The timing, interlock and modulation of the HV supplies is the task of the gyrotron controller which will be described in the following. All other control tasks such as magnet and heater settings, cooling circuits etc. operate on slow timescales (≫1 ms) and are handled by conventional industrial PLC’s (programmable logic controllers). Nevertheless, the gyrotron controller has the capability to also handle slow control tasks as well because its basis is a general purpose real-time system.

Section snippets

Functionality

As stated above, the controller’s task is the management of the anode an cathode voltage of the gyrotron. In the simplest form, this comprises timer functionality, as the gyrotron must be switched on and off at predefined time intervals at which the two voltages have to be turned on and off in the right order with a certain delay. In addition to the proper timing, there are many conditions requiring an immediate (<1 μs) shutdown of the gyrotron. The controller should be able to collect the OK

Design of the controller

It is a difficult task to find the proper balance between hardware and software for the implementation of a certain functionality. This is also a subject of current research. For the gyrotron controller we have chosen a simple paradigm: implement as much as possible in hardware, and use software only to read and write parameters. The reason for this is that hardware is usually faster and its real-time behavior is easier to predict, while software is arguably more error-prone.

In order to be

Conclusion

Fig. 4 shows a photograph of the controller. We see some LED indicators at the front panel, a network socket, two fiber optics sockets for the anode power supply, and a sub-D socket for the remaining signals. The ten controllers were manufactured and tested, and are now routinely used for the installed gyrotrons. The non-linear HV control is still under development and has so far been used as an arbitrary waveform generator.

It has been demonstrated that the controller is also able to receive

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