• dispersion interferometer
• field-reversed configuration
• nuclear fusion
• plasma diagnostics

The Field-Reversed Configuration (FRC) is a plasma magnetic confinement system with very peculiar characteristics, which make it an attractive candidate for thermonuclear fusion and other applications. One of the approaches consists of realizing a fast-pulse operating machine, exploiting some favorable features that FRCs intrinsically exhibit such as movability and compressibility, in order to extract enough fusion power from the FRC within its naturally short lifetime.
The scope of this work is to describe the diagnostics system designed and implemented for the CTFR-POP experiment at TriAlpha Energy (TAE). The experiment will attempt to obtain favorable conditions for thermonuclear fusion through the formation, translation, merging and compression of two FRCs in a pulsed regime, with pulse durations of few tens of μs.
The engineering and design of the machine, consisting of an elongated conical chamber surrounded by magnetic coils, set some important constraints to the diagnostics apparatus, first of all about the possible lines of sight for any optical diagnostics.
The diagnostics for this experiment has been developed, installed and run under Prof. Giammanco's group supervision and consists of:
- a Dispersion Interferometer (DI) for the measurement of Line Integrated Electron Density (LIED);
- High-resolution Doppler Spectroscopy (HRD) for the measurement of ion temperature and collective velocity by atomic line Doppler broadening and shift;
- Low-Resolution Spectroscopy for the detection of impurities;
- Faraday rotation polarimetry for the detection of the line integrated longitudinal magnetic field (still under development).

A Dispersion interferometer exploits the dependance of the refraction index upon the wavelength (dispersion) to generate interference between two second-harmonic beams (frequency doubled from the fundamental): the first SHB is generated before the plasma, while the second is generated past the plasma. The DI is the most innovative diagnostics, working in CW and over a longitudinal 12 m path, equal to the whole machine length; the fundamental beam has λ = 1064 nm and is produced by a CW Nd:YAG laser. The noteworthy long path requires optical adjustment of the beam prior to the second frequency duplication and detection. The performance limit for our device is estimated in N = 10^17 ~ 10^18 cm^-3, with time resolution below the μs.

The HRD consists of analysing a selected impurity line with a monochromator, in order to fit the line broadening (which produces a gaussian line profile) which yelds the ion temperature, and to evaluate line shift which directly gives the plasma bulk velocity. The analysed line is chosen among elements heavier than hydrogen in order to minimize Stark broadening.
Our HRD is composed of three viewpoints, with fiber optics that can be moved along the machine and be directed either radially or longitudinally. The range of values of temperature and bulk velocity that can be measured with the implemented apparatus are T = 0.05 ~ 10 keV and v = 3·10^6 ~ 6·10^7 cm/s.

The Low-Resolution Spectroscopy has the purpose to identify the present impurities by lookup in the NIST atomic lines database; moreover, it could allow to roughly measure the electron tempreature by log-plot of line intensities, and electron density through fit of the bremsstrahlung emission. However the procedure for these measurements is still under investigation.

The Faraday Rotation Polarimetry allows to measure the line integrated internal magnetic field (multiplied by the density). This represents another magnetic measurement in addition to standard external magnetic probes. The device is conveniently coupled to the DI frame using the fundamental DI beam (which is not actually used in the DI), whose polarization rotation is measured by a polarizing filter with high extinction ratio (10^8) followed by a photodiode. Expected rotation angles are quite small (< 0.1 rad): this requires a very accurate detection (precision around 10^4), which is currently under design for implementation.

First, the paper gives an introduction about FRCs and the FRC physics, to better explain the requirements that the diagnostics system has to fulfill. Then, the diagnostics setup is presented with focus on the underlying physics and the hardware setup; the procedures for data analysis are subsequently described. Finally, the results of the first recent measurements are presented together with some conclusions, surveying the diagnostics performance and outlining future improvements.