Latest DBS and Pulse reflectometer developments in the TCV tokamak are presente

Coda, Stefano

Molina Cabrera, Pedro Andres

Porte, Laurie

Infoscience - École polytechnique fédérale de Lausanne

Pulse Reflectometer and Doppler back-scattering diagnostics in the TCVTokamakP. Molina Cabrera, S. Coda, L. Porte, and the TCV team [1]Swiss Plasma Center, EPFL, CH-1015 Lausanne, Switzerland.Dedicated short pulse reflectometer (SPR) and Doppler back-scattering (DBS) diagnosticshave recently been developed for the Tokamak a` Configuration Variable (TCV).The DBS diagnostic uses a fast arbitrary waveform generator (AWG) as the main oscillatorand commercial vector network analyzer (VNA) extension modules as the main mm-wave hard-ware. It allows sweepable single or multi-frequency operation. A flexible quasi-optical launcherantenna (inherited from electron-cyclotron resonance heating launchers) allows 3D poloidal(10-58o) and toroidal (-180 - 180o) steering of the beam with 0.2o accuracy. A pair of fast HE11miter-bend polarizers allow flexible coupling to either O or X mode and programmable polar-ization changes during the shot. 2D 3-point ray-tracing simulations reveal an available k⊥ rangebetween 3 and 16 cm−1 with a resolution of 2-4 cm−1. Perpendicular rotation velocity estimatescompare well against ExB plasma poloidal rotation estimates from CXRS. A contributed articlehas been recently accepted for publication in the Review of Scientific Instruments journal [3].The multi-frequency mode proposes a new approach for the production and detection ofmultiple simultaneous frequencies. It consists of creating a double frequency spectrum withthe AWG, feeding this signal into the varactor multipliers inside the VNA extension modules,and directly sampling the entire output spectrum with a fast oscilloscope. It is known that theresponse of frequency multipliers implies the generation of frequency products of the formn f1+mf2 to a double frequency input. Therefore, if a two-tone input composed of f1 andf2 is fed into the x6 varactor multiplers, their output would consist of 6f1, 6f2, and 5 otherintermodulation products. Seven frequencies can thus be produced. The distance between thefirst two tones may be changed and is only limited by the maximum IF bandwidth of the re-ceiver (10GHz). A fast 13GHz analog bandwidth oscilloscope is used to directly sample theIF output of the receiver mixer. Beyond obviating the need for comb-frequency generators andtheir fixed filters in the receiver, the current set-up may change arbitrarily the frequency spanand spacing of the seven output frequencies to best fit specific plasma conditions. Data fromplasma discharges has validated the multi-channel technique agreeing both with regular I/QDBS and with ExB estimates from CXRS. A serious disadvantage of the current approach isthat the plasma could be sampled only for as long as ∼2ms given the limited scope memory(32Mb). This maybe overcome in the future by commercial data-acquisition solutions whichfeature memories of on the 100s of gigabytes (i.e.: Guzik ADP7000).Figure 1: DBS signal power evolution overchanging polarization (α) angle.Another unique capability of the new DBS sys-tem is the demonstrated possiblity of changing thepolarization of the outgoing beam during the dis-charge. A pair of fast polarizer miter bends havebeen used to change the inclination angle of the po-larization ellipse α while keeping a constant ellip-tical polarization angle β in search of an indepen-dent measurement of the magnetic field-line pitch.The best coupling to either X or O mode can be cal-culated by matching the launched beam wavevectorand polarization to the relevant mode in the plasma’s LCFS based on the magnetic field re-construction. The coupling of the launched wave to either X or O mode at the plasma LCFScould be made to change by varying α while keeping β constant. If both the effective α andthe power of the DBS signal during the shot are examined, a measurement of the magnetic fieldpitch angle at the edge of the plasma is possible. Figure 1 shows time traces of both α and βangles during shot 59679. The encoder output of both linear and elliptical miter bend polarizerangles is mapped onto α and β maps over the shot time. Figure 1 shows that the effective αchanges between +20 and -40 degrees while β can be made to remain between -5 and -6 de-grees , aiming for an ideal -5.36 degrees for X-mode best coupling at a chosen toroidal angle of0 degrees. The best coupling to the X-mode wave into the main plasma should occur when theprobing beam’s α is perpendicular to the LCFS B-field line. The DBS signal power is estimatedby integrating the area under the fits to the smooth DBS PSD spectra. A parabola is fit to thepoints around the peak power to determine the time of the peak DBS signal power. This peakis found at time 1.02±0.01 where alpha was 84.5±3. The best X-mode coupling α estimateagrees within uncertainty with the LIUQE suggestion of 81.7±0.4o.A novel approach to short pulse reflectometry is being actively developed for electron den-sity profile and fluctuation measurements in the pedestal of TCV. Taking advantage of fastAWGs, the proposed design allows flexibility regarding pulse frequency, duration, and rep-etition rate. Such flexibility has the potential of overcoming traditional spatial sampling andrange resolution limitations associated with previous SRP implementations. In microwave re-flectometry, the electron plasma density can be inferred from the round-trip group delay of EMwaves reflected from a plasma cut-off layer. Pulse reflectometry consists of sending broadbandmicrowave pulses with a defined carrier and measuring the roundtrip group delay (or time-of-flight (TOF)) using precise timing systems. A useful SPR diagnostic must resolve the densityprofile of the edge pedestal of H-mode discharges with at least 20 points, leading to a rangeprecision of 2.5mm which requires a hardware precision of timing circuits under 17ps. In orderto remain competitive with modern frequency modulation reflectometers, the pulse repetitionrate must exceed 16MHz. It is expected that a Gaussian pulse envelope of 700ps consistent overall frequencies should not experience any significant dispersion [2]. Lastly, the receiver timingcircuits should accomodate power variations in the order of 23dB given that plasma reflectedpower, even if properly aligned, may change during the shot.Figure 2: Block diagram of the SPR setup.Figure 2 shows a diagram of the SPR setup.Instead of using fixed oscillators and a fast PINdiodes to create the pulses, this SPR uses an AWGto generate pulses inside 8-12GHz and uses non-linear varactor multipliers to bring these to the V-band. The AWG’s unique flexibility allows cleanGaussians to be produced. A fast V-band zero-biasSchottky detector has been used to ensure the out-going the pulse envelope of the varactor multiplier remains Gaussian-like inside 650-750psFWHM. This was achieved by carefully adjusting the input pulse width, amplitude, and phasewith the AWG. The heterodyne receiver stage mixes the returned power with a local oscillator(LO) signal. This signal also comes from the AWG, but it is continuous-wave and has a fre-quency difference with respect to the outgoing pulse RF carrier that results in an intermediate-frequency (IF) output pulse centred at 8.5GHz. The IF pulse output is firstly amplified by abroadband, low noise, amplifier. The pulse is then band-pass filtered and fed into a fast Schot-tky detector, which extracts the pulse envelope function. Lastly, this envelope is amplified by a3GHz BW pulse amplifier.Two main approaches have been pursued to accurately quantify the TOF. The first, traditional,approach is to use a combination of a constant fraction discriminator (CFD), a time-to-amplitudeconverter (TAC), and an analog to digital converter (ADC). All these elements have been foundintegrated off-the-shelf in the commercial SPC-150Nx timing module by Becker and HicklGmBH. This module can measure the time between two events with electrical resolution downto 1.48ps. It may be operated at a saturated count rate of up to 10MHz, slightly under therequirement. Also, it may acquire up to 2 full seconds of events at 10MHz. In-situ performancetests of the SPC-150Nx using pulses reflected from a static mirror placed before the launcherantenna show a normal-distributed TOF measured with a 11ps standard-deviation. Artificiallychanging the losses in the system to mimic plasma S11 variations result in a CFD walk of 13ps.The second approach pursued to quantifying the TOF is to directly sample the pulse envelopewith a fast ADC. A 45GSa/s is used to sample the pulse envelope and a fixed synchronizationpulse in separate channels. This approach has the advantage of storing the returned pulse forflexible interpretation in the future. Currently, the analysis consists of using least-squares fittingof the plasma pulse with Gaussian functions. Given the pulse distortion caused by IF filtershape, the fitting ignores the righ-hand-side of the returned pulse. Post-processing may accessnot only the TOF but also peak amplitude and pulse width. Tests against a mirror reveal animproved response of 8ps standard deviation timing jitter when compared with 11ps from theSPC-150Nx. The fits should experience theoretically no walk errors due to the input amplitude,yet more tests are required to confirm this assertion. The disadvantage of this approach is thememory of the scope, which allows only ∼2ms of pulses to be recorded.Data from over 80 plasma discharges has been acquired with the SPC-150NX system. Itquickly revealed an order of magnitude increase in the normal distribution of TOFs from 11psto∼ 100ps in positive triangularity L-mode plasmas. The shape of the raw TOF histograms alsoshowed skewness, best fit by a beta distribution function. Pulse time vectors have also revealedthat about 50% of the pulses are lost, likely because half of the returned power is too low to beaccepted as an event by the module. In L-mode discharges when the cut-off is found around ρψ0.75, a sawtooth signal is evident in the raw data. Spectral analysis reveals a sawtooth frequencyof ∼ 300 Hz in agreement with interferometry data at the same radial location. SPR TOF dataaveraged for 1ms at 1.03s during shot 61337 was used to create the first 7-point density profileof the diagnostic, which shows good agreement with the TS density profiles. Analysis is activelyunderway to extract fast density profile changes in the near future.Acknowledgments: This work is partially supported by Requip grant (No. 206021-150707) of the Swiss Na-tional Science Foundation and by the Helmholtz Virtual Institute on Plasma Dynamical Processes and TurbulenceStudies using Advanced Microwave Diagnostics. This work has been carried out within the framework of the EU-ROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of theEuropean Commission.References[1] S. Coda et al. Nuclear Fusion 57, 102011 (2017)[2] Heijnen, S. H. Pulsed Radar Reflectometry - A new approach to measure electron densities in thermonuclearplasmas. Utrecht University. PhD dissertation. (1996)[3] P. Molina Cabrera, et.al. V-band Doppler backscattering diagnostic in the TCV tokamak. Review of ScientificInstruments. Accepted June 14th 2018.

Pulse Reflectometer and Doppler back-scattering diagnostics in the TCV Tokamak

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Pulse Reflectometer and Doppler back-scattering diagnostics in the TCV Tokamak

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