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

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.

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).

• Linear electron accelerators, like LAL in Orsay, and linear accelerators used in injectors of large circular accelerators employ high peak power klystrons, mainly S-band, at a power level of 40 to 65 MW with a pulse duration of 3 to 4 m s.
• Depending on the nature of the accelerating cavities (copper or superconductive material) the RF-tubes requirement for future linear TeV-supercolliders are different, but in all cases at the ultimate limit of the state-of-the-art:

• Large ring-storage colliders with usually two beams (e.g. e^- and e⁺ or e^- and p⁺) turning in opposite directions, like LEP at CERN or HERA at DESY are characterized by energies in the range of 30 to 800 GeV per beam. The energy needed to compensate the synchrotron radiation losses is provided by 1 MW, CW klystrons at a frequency of up to 500 MHz. With a much smaller diameter, such ring-shape electron accelerators are used as synchrotron radiation sources (like ESRF in Grenoble, DORIS III at DESY, ANKA in Karlsruhe, SLS in Villigen, BESSY II in Berlin, etc.). Their RF power is provided by the same class of klystrons.
• Proton ring accelerators for high energy particle physics, like the LHC to be installed at CERN, but also for intense neutron sources, like IPHI in Europe, require 1 MW, CW or very long pulse klystrons with a frequency of up to 800 MHz.

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 f_i = Z_ieB_o/2p m_i and electron gyration f_e = eB_o/2p m_e in the confining magnetic field B_o where e, Z_i, m_i and m_e 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 (f_i [MHz] = 7.63 B_o [T] for D) with vacuum wavelengths of a few meters and the ECH frequencies (f_e [GHz] = 28 B_o [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:

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.