A powerful method for diagnosing small-scale fluctuations or waves in magnetised plasma with moderate parameters is enhanced microwave scattering (ES) (Piliya 1966, Fidone 1973, Novik and Piliya 1994). This diagnostics makes use of the extraordinary wave for probing, and the signal back-scattered off low-frequency electron density fluctuations in the Upper Hybrid Resonance (UHR) region is detected by it. When the incident wave of frequency approaches the resonance point, the wavenumber grows strongly up to values much larger than the vacuum value . Along with the wavenumber, the amplitudes of the incident and back-scattered waves increase, thus resulting in strongly enhanced scattered signals. Furthermore, the spatial resolution is high since this process is localised to the resonance region. A single antenna can be used both to launch the probing wave and to receive the scattered signal. The method also permits to measure the spatial distribution of the fluctuations by varying the probing frequency or the magnetic field strength and, thus, displacing the UHR layer.
Since the incident wavenumber strongly changes near the UHR, the Bragg condition for backscattering can be fulfilled in a wide range of fluctuation scales contributing to the back-scattered signal. Due to this integral feature, less a priori information on the fluctuation properties is generally needed for designing the scattering diagnostics.
The wavenumber resolution is achieved in the ES diagnostics by application of the time-of-flight technique developed recently (Gusakov and Piliya 1992; Arkhipenko et al. 1992; Brusehaber et al. 1994). It makes use of the fact that the time delay of the ES signal depends linearly on the wavenumber of fluctuations. Three versions of the ES time-of-flight technique were proposed (Arkhipenko et al. 1995, Bulyginskiy et.al. 1998):

I. The RADAR technique uses microwave pulses for the probing and employs a stroboscopic technique for detecting the scattered signal. It was applied to investigate parametrically driven ion-acoustic waves (Arkhipenko et al. 1994), lower-hybrid wave propagation and decay instability (Gusakov et al. 1995; Brusehaber and Kramer 1997), and small-scale low frequency turbulence (Gusakov et al. 1995). Recently the UHR RADAR scattering technique was applied for diagnostics of density and magnetic field fluctuations in tokamak plasmas (Gurchenko et al. 1999, Bulyginskiy et.al. 2001).

II. The sweep technique that utilises a frequency-modulated incident wave was applied to measure lower-hybrid wave spectra (Brusehaber et al. 1995).

III. The third technique employs harmonic amplitude modulation of the incident wave. With this technique, the lower-hybrid wave propagation and linear wave conversion in tokamak plasma was studied in FT-1 (Bulyginskiy et. al. 1998, 1999,2001, Gurchenko et.al 2002). Recently this technique was used in lower hybrid current drive experiment at FT-2 tokamak (Altukhov et.al. 2002).

Although the above versions of the time-of-flight ES technique enable us to determine the wavenumber spectra of small-scale plasma fluctuations, they have drawbacks. In the RADAR method, the signal-to-noise ratio is small as compared to the stationary ES technique while the sweep technique permits merely investigation of a narrow frequency band plasma wave spectra and, thus, cannot be applied to study turbulence. The third technique is capable of determining only the average wavenumber of the turbulence in the chosen frequency range.
There are two versions of the ES diagnostic proposed recently that combine the virtues of the stationary method (high sensitivity) and the time-of-flight techniques (wave number resolution). They are based on the dependence of the ES signal on the phase of the fluctuations in the backscattering point, namely the Interferometer and the Correlation ES diagnostics. The Interferometer diagnostics is suitable to measure coherent waves launched into plasma. The first interferometer measurements were carried out by Gusakov et al. (1996, 1998, and 1999), who studied the propagation of lower-hybrid waves. The Correlation ES (CES) scheme can be used to diagnose fluctuations with short coherency length. It was first proposed by Arkhipenko et al. (1993) and applied to investigate parametrically driven ion-sound waves with narrow frequency spectra. In paper by Gusakov et al (2000) the CES diagnostics was further developed to study broadband (strong) turbulence and then applied for study ion acoustic turbulence in the helicon discharge Selenin et al (2002). Recently it was successively used for study of tokamak low frequency small scale turbulence in the FT-1 machine by Gurchenko et al (2000). The possibility of CES diagnostics application for investigation of plasma poloidal rotation in tokamak was shown by Budnikov et al (2002).

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