Ultrashort-pulse Reflectometry Research Articles

Development of ultra-short pulse reflectometry on the Experimental Advanced Superconducting Tokamak (EAST).

Microwave reflectometry is an invaluable diagnostic tool for measuring electron density profiles in large fusion devices. Density fluctuations near the plasma cutoff layer, particularly those that are time-varying on the timescale of the reflectometry measurement, can result in distortions in phase and/or amplitude of the reflected waveform, which present challenges to the accuracy of the reconstructed profile. The ultra-short pulse reflectometry (USPR) technique eliminates the time-varying issue in that reflectometry data are collected on a nanosecond timescale, essentially freezing the fluctuations in place. An X-mode dedicated 32-channel USPR system has been developed and installed on the EAST, covering the operation frequency range from 52 to 92GHz. This system enables high-resolution density profile measurements in the plasma pedestal and scrape-off layer, with resolutions reaching 5mm and 1 μs, respectively. Laboratory testing of the system performance has been conducted, demonstrating the potential of the USPR technique to provide accurate and high-temporal-resolution density profiles in challenging plasma environments.

Ultrashort Pulse Reflectometry (USPR) diagnostic for EAST

Ultrashort Pulse Reflectometry (USPR) is a plasma diagnostic technique involving the propagation of ultrashort duration (∼few nsec) chirps which contain frequency components spanning large portions of the plasma density profile. Upon reflection, each frequency component reflects from a distinct density layer. The reflected wave packet is down-converted and passed through a multi-channel filter bank, with time-of-flight (TOF) measurements made on each of the filtered wave packets which are then used to reconstruct the electron density profile. Applied previously to the Sustained Spheromak Physics Experiment, UC Davis is updating this technique with higher power (>10X) sources, enhanced channel counts, and high speed customized TOF electronics. A 32 channel USPR system is being developed for the Experimental Advanced Superconducting Tokamak (EAST) device with frequencies spanning 52 to 92 GHz, reflecting from the right-hand plasma cut-off layer. The system will employ three monostatic transmit/receive horns each covering a different waveguide band. Each of the two higher frequency bands will have 8 distinct frequency channels, with the lowest frequency band, spanning 52 to 65 GHz, to have 16 receiver channels to better resolve the EAST scrape-off layer. With a pulse repetition rate of 1 MHz and simultaneous acquisition of all 32 TOF signals, electron density profiles may be obtained with time resolutions of as short as 3–6 µsec.

A next generation ultra short pulse reflectometry (USPR) diagnostic.

Ultrashort Pulse Reflectometry (USPR) is a plasma diagnostic technique involving the propagation and reflection of ultrashort duration (∼few ns) chirps. The reflected packets pass through a multichannel filter with time-of-flight measurements performed on each of the filtered packets. A next generation USPR system is under development, spanning 28-75GHz, for use on compact, short duration, magnetically confined fusion devices. This system presents a dramatic increase in performance compared with an earlier USPR system employed on the LLNL Sustained Spheromak Physics Experiment device more than a decade ago. The new system replaces upconverting mixers with higher power active multiplier chains to generate mm-wave transmitter chirps, with custom time-of-flight electronics reducing the time per measurement by a factor of 3X. Finally, the system is equipped with a field programmable gate array for data acquisition and analysis.

Improvement of Power Spectrum in Ultrashort Pulse Reflectometry Signals Using Three Chirp Configuration

The flat power spectrum of the transmitter output signal for the desired frequency range is ideal to achieve the best performance of ultrashort pulse reflectometry. However, the power spectrum of a typical pulse generator decreases significantly as frequency increases. A configuration of three chirped waveforms was employed to improve the power spectrum of the transmitter signal at higher frequencies. To determine the amplification gain required for higher frequency components, three chirped waveforms were theoretically generated and their power spectra were measured using numerical band-pass filters. Based on the results of numerical computations, the three chirp configuration was successfully applied to the design of the transmitter for a broadband system..

Reconstruction of the Electron Density Profile in O-mode Ultrashort Pulse Reflectometry using a Two-dimensional Finite Difference Time Domain

The two-dimensional finite difference time domain algorithm is used to numerically reconstruct the electron density profile in O-mode ultrashort pulse reflectometry. A Gaussian pulse is employed as the source of a probing electromagnetic wave. The Gaussian pulse duration is chosen in such a manner as to have its frequency spectrum cover the whole range of the plasma frequency. By using a number of numerical band-pass filters, it is possible to compute the time delays of the frequency components of the reflected signal from the plasma. The electron density profile is reconstructed by substituting the time delays into the Abel integral equation. As a result of simulation, the reconstructed electron density profile agrees well with the assumed profile.

Numerical Study on Frequency Up-conversion in USPR using MATLAB

In this paper, the O-mode ultrashort-pulse reflectometry (USPR) millimeter-wave signals that propagate into the plasma and cover a frequency bandwidth of 33-158 GHz are examined numerically using MATLAB. Two important processes are involved in the computation: the propagation of the USPR impulse signal through a waveguide and the frequency up-conversion using millimeter-wave mixers. These mixers are limited to intermediate frequency signals that are less than 500 mV; thus, it is necessary to disperse the impulse signal into a chirped waveform using the waveguide. The stationary phase method is utilized to derive a closed-form formula for a chirped waveform under the assumption that the USPR impulse is Gaussian. In the process of frequency up-conversion, the chirped waveform is mixed with the mixer LO signal, and the lower frequency components of the RF signal are removed using high pass filters.

Simulation of Electron Density Profile Reconstruction on Ultrashort-Pulse Reflectometry

For the reconstruction of electron density profiles in the Sustained Spheromak Physics Experiment plasma, ultrashort-pulse reflectometry signals are numerically generated solving a one-dimensional wave equation by means of the central difference scheme. The signals cover an O-mode cutoff frequency range of 33–93 GHz. The lower frequencies (6–18 GHz) of a chirped waveform are up-converted to higher frequencies using four mixers. The reflected signals are down-converted to the lower frequency signals, thereby permitting band-pass filters to compute the time delays of different frequency components. A variety of density profiles are assumed to examine the performance of density profile reconstruction.

Reconstruction method of X-mode ultrashort-pulse reflectometry in LHD

Reflectometry is considered to be one of the key diagnostics to measure density profiles and density fluctuations of fusion oriented plasmas. When an electromagnetic wave is launched into a plasma, the wave is reflected at the corresponding cutoff layer of the ordinary (O) mode or the extraordinary (X) mode. Reflectometry measures the time of flight (TOF) or group delay of the reflected wave. We have applied ultrashort-pulse reflectometry (USPR) to Large Helical Device (LHD) at National Institute for Fusion Science (NIFS). The highspatial analysis method called signal record analysis (SRA) is utilized to reconstruct the density profiles from the TOF signal. Also, it is noted that the remote control system using super science information network (super-SINET) has been introduced to the present USPR system. This remote system is exclusive, and it seems to be quite effective for collaborating experiment of large devices such as ITER.

Study on the Chirped Waveform of the USPR Pulse using the Impulse Response of a Waveguide

In ultrashort-pulse reflectometry (USPR), a chirped waveform transformed from the USPR source impulse signal via waveguide makes it possible to employ millimeter-wave mixers for the frequency up-conversion process. Consequently, the frequency bandwidth of the USPR system is sufficiently wide to cover a large portion of the electron density profile of the plasma. Some physical aspects of the chirped waveform, such as maximum amplitude and length, are critical factors to determine the performance of the system. In this paper, the propagation of the USPR impulse signal through a rectangular waveguide is numerically studied to derive the chirped waveform using the impulse response of the waveguide. The results of numerical computation show that the chirped waveform significantly depends on the waveguide cutoff frequency as well as the waveguide length.

Reconstruction of edge density profiles on Large Helical Device using ultrashort-pulse reflectometry

Reflectometry has been expected to be one of the key diagnostics to measure density profiles. We have applied an ultrashort-pulse reflectometry (USPR) system to Large Helical Device in the National Institute for Fusion Science. Wide frequency band system is required to obtain wide density profile since an incident wave is reflected at the density layer corresponding to its cutoff frequency. The reflectometry utilizes an impulse with less than 30 ps pulse width as a source. Since the bandwidth of an impulse has an inverse relation to the pulse width, we can cover the frequency range of micro- to millimeter waves (18-40 GHz) with a single source. The density profiles can be reconstructed by collecting time-of-flight (TOF) signals for each frequency component of an impulse reflected from the corresponding cutoff layer. We utilize the signal record analysis (SRA) method to reconstruct the density profiles from the TOF signal. The effectiveness of the SRA method for the profile reconstruction is confirmed by a simulation study of the USPR using a finite-difference time domain method.

Simulation Studies on Advanced Microwave Diagnostics and Related Technology

The full-wave Maxwell simulations on electromagnetic wave propagation based on a finite difference time domain method are performed to study the shift in cutoff density due to relativistic electron mass modification. The simulations on one-dimensional ultrashort-pulse reflectometry are also performed and the density-profile reconstruction is shown to have an underestimated density profile by the relativistic effect, and an idea of electron temperature estimation by means of reflectometry is discussed. Its related technology such as metallic wire-made discrete waveguide and beam former is also briefly discussed.

Remote experiment of ultrashort-pulse reflectometry for large helical device plasmas

An ultrashort-pulse reflectometer (USPR) has been applied to the large helical device plasmas in National Institute for Fusion Science for edge density profile measurement. Remote control system using super science information network has been introduced to the USPR system. The remote console at Kyushu University having graphical user interface is prepared to control the instruments of the USPR via the general-purpose interface bus. The operations such as the adjustment of supply voltage fed to amplifiers and the frequency doubler, timing control of the impulse, data acquisition, and monitoring can be performed from the remote site. The position of transmitter and receiver antennas can also be controlled remotely in order to observe the cutoff layer depending on various plasma conditions. The directly recorded signal by a sampling scope is analyzed and reconstructed by means of the signal record analysis method.

Ultrashort-pulse reflectometer performance on Sustained Spheromak Physics Experiment

Time-resolved density profiles on the Sustained Spheromak Physics Experiment device are now generated automatically from data collected by an ultrashort-pulse reflectometry (USPR) diagnostic comprised of 24 frequency channels spanning 34–75 GHz. Upgrades in USPR timing electronics and reconstruction software, coupled with new low noise amplifiers and reduced bandwidth filters, have significantly decreased amplitude and timing jitter enabling simultaneous density profile and multilayer density fluctuation operation. Hardware and software components of the upgraded USPR system are described, and sample profile and fluctuation data are presented. A new system layout is proposed to increase the USPR dynamic range by 8–10 dB.

Full-wave simulations on ultrashort-pulse reflectometry for helical plasmas

The full-wave simulations on ultrashort-pulse reflectometry for helical plasmas are studied based on the FDTD method in two dimensions. The propagation of an ultrashort-pulse electromagnetic wave is computed in helical plasmas modeled for the Large Helical Device magnetic field configuration. The density-profile reconstruction is performed by the Abel inversion method with the time delay data for the reflected waves from plasma, and it is shown that the reconstructed density profile coincides well with the original profile.

Ultrashort pulse reflectometry for density profile and fluctuation measurements on SSPX

A broadband, multichannel ultrashort pulse reflectometry (USPR) diagnostic on the Sustained Spheromak Physics eXperiment device has recently undergone a number of system upgrades, which has resulted in significant improvements in the signal-to-noise ratio of the USPR signals and a dramatic reduction in the number of “lost signals” in which the amplitude of the reflected wave form drops below detection threshold. This has greatly enhanced the ability of USPR to study relatively fast density profile modifications, and allows the simultaneous monitoring of multiple density layers with as short as a 3 μs pulse repetition rate. This article provides details of the upgraded USPR system together with density profiles and fluctuation data.

Ultrashort-pulse reflectometry for steady-state plasmas

An ultrashort-pulse reflectometry (USRM) has been applied to the inductively coupled steady-state plasma (ICP) for density profile measurement. A reflected wave from ICP is directly recorded into a digitizing sampling scope and is analyzed by the signal record analysis (SRA) method. From repetition of the measurement, we have obtained the time evolution of the density profile. In order to apply the USRM and SRA method to high density plasmas such as core region of the Large Helical Device plasma, generation of the pulse with higher frequency up to 90 GHz is essential. We have utilized a frequency upconverter and an active doubler for this purpose. We confirmed the generation of the upconverted pulse in the range of 60–90 GHz and the second-harmonic frequency in the range of 26–40 GHz. Time of flight measurements have been preliminarily performed with a metal target.

Ultrashort-Pulse Reflectometry in the Gamma 10 Device

An electron density profile is observed by using an ultrashort-pulse reflectometry in the central cell of the GAMMA 10 device. The pulse having 65 ps FWHM is launched into the plasma in the O-mode and reflected at the cut off layer. The frequency range of the receiving system is 6 to 11 GHz. Time of flight of the received signal is measured via a time to amplitude converter and processed by a computer. Here, electron density profile lower than 1.5 × 1018 m−3 is reconstructed within one-shot data. The time variation of the electron density profile is acquired. Reflected wave has information of fluctuation, simultaneously. Frequency spectrum of the fluctuation is also observed.

Analytical simulation of microwave reflectometry of a plasma cylinder

An analytical solution of a time-dependent two-dimensional (2D) full-wave equation is obtained for the case of microwave propagation in a plasma with axial symmetry. The 2D structure of the electromagnetic wave in a nonmagnetized plasma (or an ordinary wave in a magnetized plasma) is studied for the general case of arbitrary time dependence of the incident wave emitted by the horn. The solutions cover the cases of conventional microwave reflectometry as well as the ultrashort pulse reflectometry of fusion and processing plasma. Analytical expressions can be further applied to study plasma density fluctuations as well as effects of plasma curvature and multidimensionality. The 2D plasma cylinder model is deemed by the authors to be more realistic as compared to the one-dimensional plasma slab model previously employed in all the analytical and most numerical treatments since the plasma in the fusion toroidal devices, mirror machines, and plasma processing chambers can be considered axially symmetric on the scale relevant to microwave reflectometry.

Analytical study of ultra-short pulsereflectometry

The results of an analytical treatment of the time-dependent 2D full-waveequation are presented here for the case of ultra-short pulse (USP)reflectometry. We consider several models of the plasma geometry, namelylinear and nonlinear slab models, as well as a 2D plasma density profile withcylindrical symmetry. The latter model is more realistic when compared to the1D stratified plasma models previously employed in all the analytical, andmost numerical, treatments, since the plasma in fusion toroidal devices,mirror machines and plasma processing chambers can often be considered axiallysymmetric on the scale relevant to microwave reflectometry.Based on the results of analytical modelling, a signal record analysismethod of profile reconstruction is proposed. The method has the advantageof using raw signal records instead of poorly localizedfrequency modes, which makes it robustfor the profile measurements using USP reflectometry.

Ultrashort pulse reflectometry for electron density profile measurements on SSPX

A broadband ultrashort pulse reflectometry (USPR) diagnostic has been developed for measuring electron density profiles of the sustained spheromak physics experiment (SSPX) device. In USPR, an extremely short pulse or chirped wave form is propagated which contains a broad range of frequency components spanning the desired plasma density profile (or a significant fraction thereof). Upon reflection, each frequency component in the incident waveform reflects from a different spatial location (density layer) in the plasma, thus spreading out the reflected wave packet in time. By simultaneously collecting double-pass time delay data at many distinct frequencies, the time delay data may then be inverted to generate plasma density profiles using a single source and a single set of measurements. On SSPX, wideband mixers are utilized to up- and downconvert 6–18 GHz chirp signals to millimeter-wave frequencies (33–158 GHz) to form a 48 channel O-mode reflectometer system. In this article we describe details of the new USPR system installed on the SSPX device and provide preliminary time-of-flight results.