In a conventional moderate pulse reflectometry system, a wavepacket with a well defined frequency (i.e. narrow spectral width) and moderate time duration (~ 1 nsec) propagates into the plasma from a transmitting horn, reflects, and finally returns to be collected by a receiving horn.By sequentially switching a number of distinct frequency microwave or millimeter-wave sources and performing double-pass time-of-flight measurements of each of the reflected wavepackets, the distance to each reflecting layer is obtained allowing the electron density profile to be computed.The high cost associated with implementing a multichannel MPR system with sufficient frequency resolution (i.e. number of channels) to obtain accurate density profiles has, however, limited the implementation to a 4 channel system on the RTP tokamak which has been recently upgraded to 10 channels for use on the TEXTOR tokamak.
Ultrashort-pulse reflectometry (USPR) significantly reduces the hardware complexity of the MPR system by replacing the series of sequentially switched sources with a single ultrashort-pulse source or chirped waveform which contains frequency components spanning the desired plasma density profile (or a significant fraction thereof). Each frequency component of the incident wavepacket reflects from a different spatial location (density) in the plasma. By separating different frequency components of the reflected wavepacket and obtaining time-of-flight measurements for each component, the density profile can be determined with just one source and a single set of measurements.
As stated previously, USPR involves the propagation of an ultrashort pulse or chirped waveform which contains frequency components spanning the desired plasma density profile. Upon reflection, each component in the incident wave packet reflects from a different spatial location (density layer) in the plasma, thus spreading out the reflected wave packet in time. The reflected wave packet is amplified and passed through a multichannel filter bank. Ultrafast Schottky diode detectors convert each of the filtered microwave wave packets into RF pulses whose double-pass time delay is then measured. In the original USPR concept, extremely short impulse sources were envisaged. Significant advantages have been realized through the replacement of the impulse source with a chirped waveform whose frequency monotonically increases or decreases in time and whose duration is sufficiently short that the frequency components of each USPR channel (occupying a bandwidth df) occur over a period of time t << 1/df. By dispersing the energy contained in a conventional ultrashort pulse into a chirped waveform, its peak power can be significantly lowered without reducing the total energy contained within the pulse. Low cost microwave amplifiers can then be used to significantly increase the energy of the transmitted pulse by 20 dB or more. For an extremely broadband USPR system, wideband balanced mixers may be utilized to upconvert the low frequency chirp to millimeter-wave frequencies. Using highpass waveguide filters to ensure single sideband operation, each mixer is then capable of covering a portion of the IF frequency space. By coupling this chirp signal to a series of millimeter-wave mixers, and utilizing a multithrow IF switch to successively connect the reflected waveform to an IF system, extremely broad frequency coverage may be obtained at relatively low cost. After downconversion by the mixer, the reflected wavepacket is amplified and passed through a multichannel filter bank. Ultrafast Schottky diode detectors convert each of the filtered microwave wave packets into 1-3 nsec duration pulses. Time delay measurements on the post-detection pulses, each representing a different probing reflectometry frequency, are performed in a manner similar to that in moderate pulse reflectometry. Numerous methods exist to measure the time interval between a start and stop pulse. First, the variable amplitude detector pulse must be converted into a constant amplitude pulse using a timing discriminator such as a leading-edge disriminator or a constant fraction discriminator (CFD). CFDs are preferred in this application where the input signals have a wide range of amplitudes but a relatively constant pulse shape, since they trigger at a constant fraction of the pulse amplitude. This makes CFDs relatively insensitive to amplitude fluctuations. The CFDs are followed by time-to-amplitude converters (TACs), which convert double-pass USPR time delay data into analog voltages which are subsequently digitized.
A more elegant, although technologically more challenging, approach to extending USPR to higher frequencies lies in the direct generation of high power ultrashort pulses or millimeter-wave chirp signals. Nonlinear transmission lines (NLTLs) have great promise as impulse and shock line generators. NLTLs have already been reported in the literature which produce ~5 psec FWHM pulses with peak output powers of ~1 W, which would be sufficient for use on devices such as the DIII-D tokamak. In addition, other approaches are under investigation for the generation of ultrashort pulses of even larger amplitude.
The SSPX USPR system is capable of routinely generating reconstructed density profiles as a function of shot number and discharge time. When raw USPR data are decoded and reliable time delay data are obtained for each channel, electron density profiles are ready to be reconstructed by an IDL routine. Generally, the USPR profiles show good agreement with the Thomson scattering profiles in the outer plasma regions, i.e. up to near the magnetic axis, as long as the Thomson scattering data shows monotonically increasing profiles to that point.It should be stressed here that the USPR density profiles are a direct result of Abel inversion of the raw time delay data with no fitting or smoothing technique other than a simple averaging over a 57 ms time window.