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.

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

The development of electron cyclotron resonance heating (ECRH)-electron cyclotron current drive (ECCD) as a tool for suppression of plasma instabilities requires that the millimeter-wave beams used...

A New Launcher for Real-Time ECRH Experiments on FTU

Disruption avoidance by stabilization of MHD modes through injection of ECRH at different radial locations is reported. Disruptions have been induced in the FTU (Frascati Tokamak Upgrade) deuterium plasmas by Mo injection or by exceeding the density limit (D gas puffing). ECRH is triggered when the ${V}_{\mathrm{loop}}$ exceeds a preset threshold value. Coupling between MHD modes ($m/n=3/2$, $2/1$, $3/1$) occurs before disruption. Direct heating of one coupled mode is sufficient to avoid disruptions, while heating close to the mode leads to disruption delay. These results could be relevant for the International Thermonuclear Experimental Reactor tokamak operation.

Disruption avoidance in the Frascati Tokamak Upgrade by means of magnetohydrodynamic mode stabilization using electron-cyclotron-resonance heating.

The turbulence in the scrape-off layer (SOL) plasma of FTU is characterized in order to assess its effect on the current drive efficiency of the lower hybrid (LH) waves. Amplitude, frequency and perpendicular wave vector of the fluctuations are measured for a variety of the main plasma conditions in front of the LH antenna together with the temperature and density in the SOL and used as inputs for the linear scattering theory of the LH waves developed many years ago. This theoretical model can account for both the frequency spectral broadening of the LH pump and the variations of the driven current, inferred by the perpendicular fast electron bremsstrahlung signals. The fraction of the LH power that is then deduced to be effective for current drive appears to be well related to the calculated optical thickness τ of the SOL. It drops as low as 40% as τ increases, consistent with the model prediction. Possible ways to control the SOL optical depth are investigated and a clear relation of the fluctuation level with the collisionality is found.

Lower hybrid current drive efficiency in tokamaks and wave scattering by density fluctuations at the plasma edge

In this paper we will report on the experimental results arising from the implementation of extremum seeking techniques to maximize in real time the RF power coupled to the plasma in the Frascati Tokamak Upgrade (FTU) experimental facility. The RF under consideration is the Lower Hybrid (LH) and the coupling issue consists in the match between an antenna structure (an array of waveguide, the grill) and a load i.e. the plasma. It can be shown that the reflection coefficient (ratio of the reflected and direct power) is a convex function of the density in front of the grill and then of the plasma-grill distance. The optimization of the coupling level reduces the reflected power thus maximizing the RF effects on the plasma and at the same time avoiding temporary power shutdown to protect the system from the excess of reflection. It is shown in the paper that the proposed extremum seeking technique achieves a satisfactory level of robustness with respect to measurement errors and well performs both in simulation and in experimental tests, thus leading to an improved effectiveness of the RF heating system.

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Extremum seeking applied to the plasma control system of the Frascati Tokamak Upgrade

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 ...

Chapter 11: The heating and current drive systems of the FTU

Plasma heating of the Frascati Tokamak Upgrade, which will be done by means of a lower-hybrid, electron mode, additional heating system rated for pulsed (1-s pulse length) RF power up to 8 MW at a frequency of 8 GHz, is updated. Specially developed gyrotrons will be used as RF-power generators; two different prototypes are undergoing tests. A low-attenuation transmission system, using overmoded waveguides, will feed the coupling structures. A design of the RF coupling structures has been recently developed on the basis of the experience of the Frascati RF group with similar structures. Most of the critical parts of the RF system have already been tested, with promising results. >

A. Orsini

Papers

Chapter 11: The heating and current drive systems of the FTU

The 8 MW, 8 GHz FTU LHRH system: R and D and design status