The gyro oscillators or simply gyrotrons are used in a variety of applications where high electromagnetic power is required at millimeter/submillimeter wave frequencies. The research on the gyrotron microwave tube was initiated by the demand of high power, high frequency electromagnetic wave source in the magnetically confined plasma fusion application. Since the initial phase of gyrotron development, new thrust areas have been explored by the several research groups. The gyrotron shows several unique advantages as a high power source compare to the other millimeter/submillimeter wave sources either semiconductor based devices or vacuum based tube devices. At present gyrotron is used successfully in the field of plasma heating, plasma diagnosis, medical spectroscopy, material processing, whether monitoring etc. discussed in detail in this article. Several new fields of technology like security, metal joining, planetary defense etc. are under exploration for the futuristic use of gyrotron. In this review article, the applications of gyrotron, various issues remained in the further modification of the device, global scenario of the gyrotron development are discussed in detail.

A Review on the Applications of High Power, High Frequency Microwave Source: Gyrotron

The goal of the Lower Hybrid Current Drive (LHCD) system on the Alcator C-Mod tokamak [Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] is to investigate current profile control under plasma conditions relevant to future tokamak experiments. Experimental observations of a LHCD “density limit” for C-Mod are presented in this paper. Bremsstrahlung emission from relativistic fast electrons in the core plasma drops suddenly above line averaged densities of 1020 m−3 (ω/ωLH∼3–4), well below the density limit previously observed on other experiments (ω/ωLH∼2). Electric currents flowing through the scrape off layer (SOL) between the inner and outer divertors increase dramatically across the same density range that the core bremsstrahlung emission drops precipitously. These experimental x-ray data are compared to both conventional modeling, which gives poor agreement with experiment above the density limit and a model including collisional absorption in the SOL, which dramatically improves agreement with experiment above the observed density limit. These results show that strong absorption of LH waves in the SOL is possible on a high density tokamak and the SOL must be included in simulations of LHCD at high density.

/pdf/absorption-of-lower-hybrid-waves-in-the-scrape-off-layer-of-53p4iutlbh.pdf

Absorption of lower hybrid waves in the scrape off layer of a diverted tokamak

On ITER, the LH system aims at supplying 20 MW CW for controlling the q-profiles in partially and fully non-inductive steady-state operation (9 MA). The designed LH system relies on a transmitter made of 24 5 GHz 1 MW klystrons linked to one antenna based on the passive active multijunction (PAM) concept, via a 60 m long oversized circular transmission line. Through a common European effort including several associations (CEA, ENEA, UKAEA, IST, TEKES and IPP-CZ) many outstanding problems have been solved: coupling has been achieved on JET in a physics scenario similar to ITER, a PAM has been tested with success on the FTU plasma. The reliability of LH for long pulse operation has been ascertained on Tore Supra. In order to routinely realise long pulse operation (1000 s), the Tore Supra LH system is being refurbished within the CIMES project framework where the transmitter is to be equipped with 16 klystrons 700 kW 1000 s at a frequency of 3.7 GHz. A new LH launcher based upon the PAM concept has been studied, designed and is actually under realisation.

Toward a LHCD system for ITER

The electron cyclotron (EC), ion cyclotron (IC), heating-neutral beam (H-NB) and, although not in the day 1 baseline, lower hybrid (LH) systems intended for ITER have been reviewed in 2007/2008 in light of progress of physics and technology in the field. Although the overall specifications are unchanged, notable changes have been approved. Firstly, it has been emphasized that the HC the possibility to operate IC with 2 antennas with increased robustness (no moving parts); the possible increase to 2 MW of key components of the EC transmission systems in order to provide an easier upgrading of the EC power as may be required by the project; the addition of a building dedicated to the RF power sources and to a testing facility for acceptance of diagnostics and heating port plugs. Thirdly, the need of a plan for developing, in time for the active phase, a CD system such as LH suitable for very long pulse operation of ITER was recognised. The review describes these changes and their rationale.

Progress on the heating and current drive systems for ITER

The DTT device: System for heating

One of the main Tore Supra (TS) objectives is to produce long and performing non-inductive discharges whose studies are crucial for the next step. The Lower Hybrid (LH) system is routinely used on TS to provide such discharges. The CIMES project will improve the TS LH power injection facilities leading to a transmitter power of 10 MW CW. A new klystron, TH2103C has been developed at Thales Electron Devices since November 2001. The specified RF output power is 620 kW CW on plasma (Voltage Standing Wave Ratio (VSWR) = 1.4) and 700 kW CW on matched load. Six klystrons have been successively manufactured and tested to define the good design. The last one was successfully tested on both TED and CEA test beds. It shows adequate margins of its design towards specifications. These margins ensure high reliability needed for long pulse operation (LPO) foreseen in the frame of the CIMES project. The procedure used to test the klystron and the safety interlocks carried out to avoid klystron defaults are detailed. The results obtained with prototype are presented and analyzed.

Progress in the high power CW klystron development for Tore Supra

A 3.7-GHz 700-kW klystron in continuous-wave (CW) operation has been developed to upgrade the lower hybrid RF plasma heating power in a tokamak up to 10 MW. The klystron is equipped with a diode gun, a five-cavity RF structure, two BeO-disk RF windows, and a large-size X-ray-shielded hypervapotron collector. The output power is recombined in a four-port junction which we also developed. The tube is designed to deliver 620-kW CW RF power with a mismatched load (VSWR = 1.4) and 700-kW CW with a matched load. Several prototypes have been built with successive design improvements. The major improvement was to change one single RF output into two RF outputs. The most recently built prototype meets all design specifications at 73.1 kV and 20.7 A, with an efficiency of 47% on a matched load and 40% with a 1.4 : 1 VSWR load, worst case phase. The power losses dissipated in the body have been measured as low as 17 kW, which corresponds to the RF heating and implies low beam interception. The measured temperatures of the output cavity noses and collector wall have been kept below 130degC and 200degC, respectively, which results in large thermal margin.

High-Power CW Klystron for Fusion Experiments

We are developing a high-efficiency klystron to perform particle accelerators cavities conditioning. We first verify this concept with an enhanced version of the Thales TH2166 klystron. After we designed the new interaction line, we focused on the development of the cavities' tuning system and the study of the multipactor phenomenon.

Development of a high-efficiency klystron based on the kladistron principle

Thales Electron Devices has been developing and manufacturing 10MW L-band vertical Multi Beam Klystrons (TH1801) for TTF, FLASH and PITZ machines at DESY for more than 10 years. At this point, 8 tubes have been installed at DESY with more than 80 000 hours of cumulated RF operation. All over these years, THALES has been collecting data on MBK tubes behavior which permitted to improve continuously the design margins and reliability. In order to allow horizontal installation in the XFEL tunnel, a horizontal version (TH1802) was fabricated, tested and qualified. The key performance of this tube is its high stability under all various operating conditions.

1.3 GHz, 10 MWp long pulse multibeam klystrons: A long heritage and proven reliability

The CLIC project requests new type of RF sources for the high power conditioning of the accelerating cavities. We are working on the development of a new kind of high-efficiency klystron to fulfill this need. This work is performed under the Eucard-2 European program and involves theoretical and experimental study of a brand new klystron concept.

/pdf/high-efficiency-klystron-design-for-the-clic-project-3ypwc62yk5.pdf

High-efficiency klystron design for the CLIC project

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