Powerful Vacuum Electron Tubes used in European Particle Accelerators and Thermonuclear Fusion Systems

Manfred Thumm

Forschungszentrum Karlsruhe, IHM, 76021 Karlsruhe, Germany
and Universität Karlsruhe, IHE, 76128 Karlsruhe, Germany

E-mail: manfred.thumm@ihm.fzk.de, Phone: ++49 7247 822440, Fax: ++49 7247 824874

Introduction

High-power vacuum electron tubes are key components in large scientific installations like particle accelerators and experimental thermonuclear fusion devices with magnetic plasma confinement. Unlike the needs in telecommunication systems, these scientific applications require tubes that generate and transmit large amounts of pulsed or continuous wave (CW) RF power (from about a megawatt in CW to 100 MW in m s-pulse mode) over a frequency range from 20 MHz to 170 GHz. These applications demand powerful, efficient, cost effective and reliable tubes which are easy to start and to operate.

Three families of powerful vacuum electron tubes cover the entire frequency range: tetrodes, diacrodes (and IOTs) for the 20-500 MHz band, klystrons from 300 MHz to 11.4 GHz and gyrotrons from 8 GHz to 170 GHz. This paper reports on high-power RF tubes used in different types of European particle accelerators and magnetic confinement fusion plasma devices like tokamaks and stellarators.

RF particle accelerators

The accelerator application requires high RF phase stability (typically <1) between all the accelerating cavities. When the total RF power requirement exceeds the capabilities of a single microwave source, this generally requires the use of RF-amplifier tubes operating in phase synchronism.

Particle accelerators can be characterized into several categories, depending on their geometry (linear or ring), on the nature of the particles (electrons or protons and their antiparticles, heavy ions) and on the final objective (high energy particle physics or radiation source).

Multibeam klystron: 1.3 GHz, 10 MW, 1.5 ms, 10 Hz (for TESLA in Europe) and
PPM-focused klystron: 11.4 GHz, 75 MW, 1.5 m s, 60 Hz (for NLC in USA).

 

Fusion plasma experiments

There are three types of RF methods for thermonuclear fusion devices like the tokamaks JET in Culham, ASDEX-U in Garching, Tore Supra in Cadarache, FTU in Frascati and Textor in Jülich and the stellarators W7-AS in Garching and W7-X in Greifswald available: Ion- and electron cyclotron systems which heat ions (ICH) and electrons (ECH) by using RF waves which match the natural frequency (or its second harmonic) of the ion gyration fi = ZieBo/2p mi and electron gyration fe = eBo/2p me in the confining magnetic field Bo where e, Zi, mi and me are the elementary charge, the ion charge number, the ion mass and the electron mass, respectively. The ICH frequencies are in the 20-120 MHz range (fi [MHz] = 7.63 Bo [T] for D) with vacuum wavelengths of a few meters and the ECH frequencies (fe [GHz] = 28 Bo [T]) correspond to millimeter waves (28-170 GHz). The third frequency, the lower hybrid frequency for noninduc-tive current drive (LHCD), is between the ICH and ECH frequencies and range from 1-8 GHz.

For fusion reactors there are two great advantages of such RF systems compared to Neutral Beam Injection (NBI): (1) dielectric windows serve as tritium barriers between the plasma chamber and the transmission lines and power sources (2) there is no particle refuelling by the heating system, so that these two functions are clearly decoupled.

The characteristics of the three RF systems are summarized in Table 1:

H&CD

Method

f

(GHz)

Power

Source

Transmission

System

Antenna

Coupling Medium

Electron Cyclotron (EC)

 

 

Lower Hybrid

(LH)

 

Ion Cyclotron

(IC)

28-170

 

 

 

1-8

 

 

0.02-0.12

0.4 (high field)

Gyrotron

 

 

 

Klystron/ Gyrotron

 

Tetrode

Klystron

Oversized Circular

Waveguides

or Mirror Lines

Standard Rectangular

Waveguides

Coaxial Lines

Oversized Waveguides

and Reflectors

(100 MW/m2)

Waveguide Grill

(25 MW/m2)

 

Loop Antenna

Structure with Faraday Shield

(10 MW/m2)

Vacuum

 

 

 

Low Density

Plasma*

 

Low Density

Plasma*

* Antenna guard limiter is required

Table 1: Characteristics of RF and current (H&CD) drive systems.

The needed power levels range from a few MW up to 50 MW so that CW or longpulse ³ 1 MW RF sources are required. The tetrode power sources that exist today are adequate, with minor upgrading (2 MW diacrodes), for fusion plant applications in the case of ICH. However, the klystrons and gyrotrons that will be required for reactor applications of LHCD and ECH represent a major technological development.

The paper reports on the present state-of-the-art and future developments of the different types of vacuum electron tubes employed in ICH, LHCD and ECH systems of European plasma fusion devices.