INTRODUCTION
The low aspect ratio, or spherical tokamak Globus-M [1] has recently been built in a support of international spherical tokamak plasma investigation program. These investigations spread the existing data base for the magnetic confinement devices, produce a new insight to fundamental plasma properties and help to achieve record value of relative plasma pressure due to enhanced stability of plasma column. The ST tokamak diagnostic requirements much influenced by the discharge distinctive features are focused on the enhanced locality and repetition rate response. These are resulted from the pronounced non-circular plasma cross-section with steep gradient zones either internally or at the edge of the plasma column and a variety of transient phenomena take place during powerful auxiliary plasma heating. The survey to determine the work scope in diagnostic design for Globus-M is given in the paper. The survey presents the key diagnostic status in the research program and the target measurement requirements. The distribution of diagnostic systems at equatorial ports is shown in figure 1.
Access to the plasma is dominated by the requirement to minimize the number of penetrations through the chamber wall. Therefore, the diagnostic priorities are established in a tight fit the scheduled physical program.
THE DESIGN GUIDELINES
A full set of diagnostics summarized in Table I is falling in three categories. The first category is to cover the physical start-up of Globus-M ST with an emphasis on a discharge current and magnetic field measurement, plasma position and shape identification and in-vessel inspection. The next one is an advanced diagnostic set intended to supply the data on basic plasma parameters: electron temperature and density profiles, ion temperature, plasma composition, radiation profile, Zeffective. The last category is the diagnostic upgrade. The provisions are made in particular to introduce the heating/diagnostic neutral beam capable of delivering ~0.75 MW of 30keV neutral hydrogen atoms with a footprint of 120cm2 in the plasma. It is the challenging task to make the measurements of q profile by MSE, and the density light of impurities, Ti, plasma rotation by CHERS. Thomson scattering diagnostic based on TV TS principle is considered as well.
DIAGNOSTIC START UP SET
Magnetic Diagnostics.
The magnetic diagnostic have been installed on Globus-M to meet the following main purposes:
· measurement of main plasma parameters (current, voltage, energy content , fluxes, etc.);
· analysis of MHD equilibrium;
· feedback control of plasma current, position and shape.
The different type magnetic sensors are used: magnetic probes, flux loops, voltage loops and Rogowsky coils (see Table I). There are two sets of magnetic probes placed in the different tokamak cross-sections. The base set consists of 32 two-component magnetic probes to measure the distribution of poloidal magnetic field along vacuum vessel and is employed for analyzing the MHD equilibrium and shape controlling. There is an additional reserved set in another cross-section.
The flux loops intended to measure the vertical and horizontal fluxes are settled in the plasma position control system. To control the fast vertical plasma displacement the four flux loops are used for measuring the average horizontal flux through the plasma
Detailed description of the Globus-M magnetic diagnostic is presented in [2].Plasma Interferometry
The Globus-M is equipped by 2mm microwave interferometer to measure the electron density averaged in minor radius direction. Three transmitting and three receiving antennas are located in single poloidal cross-section. Probe axes are located at R=0.24m; 0.42m; 0.5m look vertically to equatorial plane to watch the phase shift associated with electron density and the displacement of the column along the major radius. The probe axis placed in equatorial plane is considered as well to measure the electron density averaged along major radius direction. A step by step upgrade from 2 to 1 and down to 0.337 mm wavelength should be started in a short future to reduce the refraction losses.
Plasma Imaging.
High speed plasma view is regarded be a valuable aid in the study of transient phenomena of the discharge development as well as straightforward method of plasma visual boundary monitoring. The prime objective of the design is to expand the detection dynamic range by means of exposure fitting to plasma luminance. Unlike the majority of modern devises based on the automatic exposure control this should be done immediately after the triggering together with frame processing on a basis of very fast feedback. The feedback is obtained with processing a reference plasma image recorded in parallel channel by an auxiliary diode array of a reduced to 4x4 number of units (see Fig. 2). A fast imaging optics meets a requirement of plasma view within a field angle about 90°.
ADVANCED DIAGNOSTICS SYSTEMS
The advanced diagnostic systems to be used in Globus-M have been evaluated for their ability to meet the physical program measurement requirements. The design of individual diagnostics and the integration to the tokamak are progressing through feasibility study and the development of key diagnostic components.
Visible Spectroscopy
It is the prime objective of visible spectroscopy to obtain spatially resolved measurements from the edges or the plasma core to determine the general behavior of working gas and impurity recycling fluxes. Using multi-chord viewing of the plasma cross-section, recycling rates, erosion and redeposition of the elements on different surfaces can be followed. The prospect of analyzing the Balmer series emission is acknowledged as well with plasma density maintained both by the gas puffing and solid pellet fueling. The transitions between Rydberg states of plasma ions can be observed in the visible by charge transfer collisions with beam of injected neutral atoms. Diagnostic applications of each of the above features of the optical radiation from Globus-M is considered together with Doppler broadening and plasma rotation measurements. It stands to reason that a progressive upgrading of the diagnostic instruments is actually important.
A set of the lower order blaze high dispersion grating spectrometer has recently been devised [3] to accommodate the visible spectroscopy applications. The spectrometer design is based on the use of uncustomary large diffraction angles approaching 80°. The large diffraction angles are advantageous as to increase the illuminated grating surface, for a given incident beam cross section defined by imaging optics. In a condition of enhanced angular dispersion specific to the large diffraction angles, the instrument resolving power is not so much limited by aberration image blur. This results in use of a short focal length imaging optics permitting simplicity and compactness of the design.Soft X-ray plasma imaging
The primary function of SXR diagnostics in Globus-M is to understand spatial and temporal behavior of MHD plasma instabilities resulted in the fast variation of SXR signal. Spatially and spectrally resolved measurements of SXR flux gives opportunity to specify electron temperature of plasma. Globus-M SXR pinhole camera design gives possibility to combine these features in one diagnostic set. The view access to the plasma permits two fan arrays with horizontal and vertical poloidal views respectively as is shown in Fig. 3.
A combination of two multichannel poloidally viewing systems facilitates the 2D X-ray plasma imaging to obtain the spatially and temporally resolved profiles of X-ray emission integrated along 2x16 viewing chords.
Each of the fan waists is arranged using a 2x10mm2 slots representing a pinhole camera geometry. The 16-element Si photodiode linear arrays supplied with 16-channel preamplifier hybrid circuit are mounted to the pinhole camera inside the vacuum port in three parallel rows of ~50mm length. The time response of each individual registration channel does not exceed 0.3msec, which make possible to study very fast plasma fluctuations (few MHz frequency range). In addition, each of the row is equipped by the replaceable Be foil absorbers to emphasize the different ranges of spectrum (according to figure 4) for the electron temperature monitoring. On account of the statistical meaningful continuum spectra recording in ~10keV range the provisions are made to a submilisecond temporal resolution.Plasma Bolometry
The recent breakthroughs in Si detector technology (known as AXUV photodiodes) makes it possible to compose the SXR and bolometric systems in a same pinhole camera provided an absolute radiometry applications with a wavelength coverage from SXR to UV and up to visible range. The due regards are given to the uncertainties arising from photodiodes spectral responsivity degradation [4] at the prolonged exposure to UV/VUV plasma radiation and necessity of their absolute calibration. The radiometric study of edge and core plasma dynamics in a submicrosecond range with spatial resolution of ~ 2 cm gives an useful insight into the nature of MHD activity and transport processes studies. Routine set of 4 pyro-electric detectors, one wide view and 3 collimated, is under preparation for radiation power losses measurement.
Plasma Reflectometery
The scanning pulse RADAR reflectometer is to be devised to measure electron density profiles. The system is distinguished by combining the properties of most commonly used frequency swept phase reflectometer and the pulse RADAR instrument [5]. It means that the reflection positions of short microwave pulses from the plasma cut-of-layer could be varied during a frequency scan. The number of probe pulses fired during a frequency sweep interval of *10 *sec is equivalent to number of frequency channels in the fixed frequency channel instrument. Using one channel with swept frequency the plasma outside density profile in 10 variable spatial locations may be obtained during a frequency scan with temporal resolution 1 *sec
It seems reasonable to equip the individual frequency swept channel scanning from 26 to 40 GHz (Fig. 5) and complement it with 3 fixed frequency channels 20, 50 and 60Hz. The choice of probe frequency range for Globus-M fits ~0.4 to 4.5*1019 m-3 cut-off density variations..
A matter of special concern is the density profile reconstruction accuracy. This depends primarily on how accurate the plasma boundary position is determined. A low frequency channel (~20 GHz) is nominated mostly for the improvement of plasma boundary position measurements.Thomson Scattering
There are two Thomson scattering systems under development now on Globus-M tokamak. The first one is developed in the frames of ISTC advanced diagnostics project #1126 and is based on periodically switched Nd glass laser. According to Globus-M engineering design the tangential view plasma probe is favored to enable the recording Te, ne distributions over the major radius. At least two viewing fans are arranged according to figure 6 to observe the inner and outer plasma regions. The outer edge viewing fan takes the advantage of a back-scattered light collection leading to spectrum profile broadening by a factor of about 1.4 and the collection efficiency improvement. A number of design points on spatial profile is taken 20. The diagnostics commissioning is starting from only a few spatial locations in a space from outer to inner wall region. The target requirement is to accommodate a possibly low detection limit starting from a few times 1017m-3 electron density. A multiple spectrum recording during 15-20 laser pulses in the single plasma discharge is supposed. The provisions are made to vary the pulse repetition rate down to 0.5 msec separation time between the pulses. The designing of dedicated probe laser system should be carried out to meet the target requirements. The two candidate laser options are under consideration. The first one is based on a running wave mode master oscillator together with the unstable telescope amplification cavity. The amplifier unit of phosphate Nd:glass Æ30x320mm in telescopic cavity is to emerge the hollow profiled probe beam. The trade off laser option is based on the slab (10x28x300mm) Nd:glass active element comprising the master oscillator and amplifier sections in a same cavity with either single or double pass amplification. The both candidate laser specifications are compared in Table II. The slab arrangement looks preferable regarding the lower pumping energy. The gainful conversion efficiency guarantees the system long-term reliable operation.
The imaging lens optics transmits the light to a four spectral channel polychromator set optimized for either core or edge temperature measurements in the range 0.05-2KeV and down to 20-200eV respectively. The Te dynamic range optimization is maintained by fitting the band pass distribution at interference filter array [6]. The new developed grating devices [3] of a variable dispersion permitting the both high and low temperature capability are regarded as well. The novel grating spectrometer developments are to accommodate the enhanced out-of-band stray light rejection ranging to 10-5-10-6. The detectors are Si avalanche photodiodes used in electronic circuit that is optimized regarding the voltage and current noise ratings for the probe pulse duration ~50 nsec.
The second Thomson scattering system is based on TV TS principle [7] and single pulse powerful laser module l=1.055mm with radiation conversion to the second harmonic. This system is additional to the multipulse one and to be commissioned in 2-2.5 year scale. The enhanced high spatial resolution facilitates investigations of the important features like internal barriers and the magnetic islands. The Te, Ne spatial profile measurements on the ~ 0.5 m viewing chord are feasible provided the satisfactory spectra recording in the density range of Ne ~ 1018-1019 m-3. Collected light is dispersed with an optically fast f/3 stigmatic grating spectrometer. The spectrometer design [3] is based on use of large diffraction angle. The research and design study has given insight into spectrometer performance specifications. Under the condition of wavelength to groove spacing ratio approaching unity and the grating ruled area taken 80x80 mm2 the temperature dynamic range covers 30 - 800 eV in a tight fit with the admitted diffraction angle range from 80° to 60°. The provisions are made to improve the lower temperature limit using the custom camera objective lens with larger format pupil matched for nearly twice as large grating dimensions. The dispersed light illuminates image intensifier coupled to multichannel CCD detector providing radiation recording from nearly one hundred spatial points.
Conclusion
A diagnostic facility for newly commissioned ST Globus-M is under development. This includes the novel concept diagnostic components and approaches. The start-up set for machine operation and plasma control have already been installed on the tokamak. The next coming advanced diagnostics for basic plasma parameter measurements are in the closing stage. The diagnostics upgrade is progressing through feasibility study and the development in a tight fit the Globus-M physical researches.
Last updated Wednesday, 3 Octomber, 2012
