Passive Active Multijunction Research Articles

Overview of the FTU results

An overview of the FTU results during the period 2003–4 is presented. A prototype ITER-relevant lower hybrid current drive (LHCD) launcher, the passive active multijunction, has been successfully tested (f = 8 GHz) showing high power handling and good coupling properties and current drive comparable to those of a conventional launcher. Effective electron and ion heating (via collisions) have been achieved with the 140 GHz ECRH systems up to 1.5 MW, as well as current drive (25 kA at ne0 = 8 × 1019 m−3). The mitigation of disruptions has been studied with on-axis ECRH. Ion Bernstein wave studies have shown the importance of recycling in achieving improved confinement plasmas. Advanced tokamak scenarios are presented including repetitive pellet enhanced plasmas and electron internal transport barriers (e-ITBs). Very peaked density profiles have been achieved with a low speed vertical pellet injector located at about mid-radius on the high field side. The performance is comparable to that achieved with a high-speed horizontal pellet injector. Possible reasons for this behaviour are discussed, among them the presence of an ‘MHD’ drift once particles reach the q = 1 surface. The effect of peaked density profiles on confinement is discussed. Electron ITBs can be produced at high density in FTU with LHCD only and with the combined use of LHCD and ECRH: Te0 = 6 keV with ne0 = 1.4 × 1020 m−3 and H97 = 1.6. Turbulence is strongly reduced. Ions are heated by collisions with ΔTi/Ti up to 35% showing that e-ITBs are not degraded by the electron–ion collisions. Particle pinch studies have been made at high densities in full current drive conditions where the Ware pinch plays no role. An anomalous inward pinch exists even at these high densities (ne0 = 1.5 × 1020 m−3). Despite the absence of energetic particles in FTU, MHD spectroscopy has revealed high frequency modes (30–80 kHz) that might have consequences for burning plasmas.

LHCD and coupling experiments with an ITER-like PAM launcher on the FTU tokamak

A prototype passive active multijunction (PAM) launcher for lower hybrid (LH) waves conceptually similar to that foreseen for ITER has been successfully tested on FTU at frequency f = 8 GHz. The power routinely and safely managed for the maximum time allowed by the LH power plant (0.9 s) without any fault in the transmission lines is 250 kW, corresponding to 75 MW m−2 across the antenna active area and very close to the design value of 270 kW or 80 MW m−2. The achieved value is at least 1.4 times larger than the ITER request, which would be only 52 MW m−2, if the 33 MW m−2 required to the ITER grill in order to couple 20 MW to the plasma, are scaled up linearly with f, from fITER = 5 GHz. This linear scaling of the power handling capability of the LH antennae is indeed conservative with respect to the available data. The test results validate also the other two main expectations relevant to ITER, foreseen by the codes, namely to operate with the grill entirely in the vessel shadow and to still preserve good current drive (CD) efficiency. Even with the grill mouth retracted 2 mm inside the port shadow and with density in front of the launcher very close or even lower than the cut-off value, the PAM reflection coefficient is always ⩽ 2.5%, if the antenna has been properly conditioned. The CD efficiency is comparable to that of a conventional grill, once the lower directivity is taken into account. Flexibility in determining the N|| spectrum is also maintained, according to hard x-rays and electron cyclotron emission spectra. Conditioning the PAM in order to operate at the ITER equivalent power level required only one day of radio-frequency operation, without a previous baking of the waveguides.

Toward a LHCD system for ITER

Abstract 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.

ITER-like PAM launcher for Tore Supra's LHCD system

Abstract In the frame of the Tore Supra Composants d’Injection de Matiere et d’Energie Stationnaire (CIMES) Project, consisting in particle fuelling via a pellet injector and in energy powering using Lower-Hybrid Current Drive (LHCD) for pulses lasting 1000 s, complemented by the transmitter refurbishing, a new launcher, based on the Passive-Active Multijunction (PAM) concept already proposed for ITER, has been developed and is currently under realisation. It was designed for a power injection capability of 2.7 MW CW at 3.7 GHz, a power density of 25 MW/m 2 , to radiate a power spectrum peaked at N // = 1.7 with a maximum power directivity near the electron cut-off density and with very good coupling properties on a wide range of electron densities. In this paper, an overall description of the antenna as well as the foreseen manufacturing and assembling processes are given, followed by the results from studies and optimisations of its RF components, by a stress analysis of its thermo-mechanical behaviour, taking into account the plasma radiated flux at the mouth and the RF losses, and complemented with additional mechanical stresses due to the combined action of the eddy current induced in the launcher by disruptions and the residual toroidal magnetic field.

The test of a PAM launcher on FTU: The first step toward the LHCD launcher for ITER

The 20 MW, 5 GHz, LHCD system for ITER aims mainly at controlling the q-profiles in partially (hybrid regime at IP = 12 MA) and fully non-inductive steady-state operation (IP = 9 MA). In the last few years several European associations have given their contribution to the solution of many prominent problems on the road to the design of an efficient and reliable system. LH coupling has been achieved on JET in physics scenarios relevant for ITER, while the reliability of the LH for long pulse operation has been ascertained on Tore Supra. A downscaled prototype of the Passive Active Multijunction (PAM) launcher presently proposed for ITER, has successfully demonstrated on FTU its good coupling characteristics, power handling capability and current drive efficiency. An ITER relevant launcher, based on the PAM concept is presently under construction to be installed on TORE SUPRA to investigate the technological solutions (materials, cooling, etc.) envisaged for this launcher.

Chapter 11: The Heating and Current Drive Systems of the FTU

High-frequency wave systems with high-power density launching capability have been the preferred choice to heat the Frascati Tokamak Upgrade (FTU) because of physics arguments (electron heating at very high density) and space constraints from the compactness of the machine design (8-cm-wide port). They do include an 8-GHz lower hybrid current drive (LHCD) system, a 140-GHz electron cyclotron resonance heating (ECRH) system, and a 433-MHz ion Bernstein waves system (IBW). The technical aspects of these systems will be reviewed in this article. The main features of the design include the following: (a) a very compact conventional LHCD grill with a compact window to keep the vacuum on 48 (12 columns, 4 rows) individual waveguides allowing the maximum flexibility in spectra generation to be achieved; power handling up to [approximately equal to]10 kW/cm2 has been achieved, (b) ECRH launchers designed as a quasi-optical system (implementing ITER relevant solutions) retaining the maximum flexibility in the equatorial launcher (poloidal/toroidal steerability) to exploit a variety of scenarios, (c) a two-waveguides launching array making the IBW experiment on FTU unique. Other technical aspects (sources, transmission lines, etc.) are also reviewed. The development of a new ITER relevant lower hybrid launcher, the passive active multijunction, is described.

Overview of the ITER-FEAT LH system

LH is considered to be used in ITER-FEAT steady-state scenario thanks to its highest current drive efficiency in the plasma outer part. The design of a 5 GHz, 20 MW CW LH system has been realised. That relies on a transmitter of 24 1-MW klystrons, and a circular oversized 60-m long transmission line feeding an antenna, based on the passive active multijunction concept. In order to have a high reliability, the power density is limited to 33 MW/m 2. After the system description, the results of different studies are given: RF computation, launcher coupling properties, thermal analysis, mechanical stress caused by disruption, neutron activation, acceleration of electrons in the antenna near field.

The PAM launcher for FTU: results of the preliminary tests

The Passive active multi-junction (PAM) launcher is a robust coupling structure, able to withstand the strong thermal loads and the electromechanical stresses of ITER-like tokamaks. In the frame of a collaboration between ENEA, Frascati and CEA, Cadarache, a PAM antenna has been designed and built to validate on FTU the concept of this launcher. All the technological solutions of an ITER-like launcher will be tested in a full rated experiment to be performed, possibly, on Tore Supra. This paper reports the results of the low power measurements of the microwave characteristics of the PAM for FTU.