ULTRA SHORT PULSE REFLECTOMETRY
SYSTEM OVERVIEW

INTRODUCTION

Time-of-flight radar systems function by reflecting short pulses of electromagnetic radiation from a target medium. The double-pass time delay from transmitter to receiver is then used to compute the distance to the reflecting medium. Plasmas are dispersive media whose refractive index is a function of plasma density; higher frequency radiation reflects from higher density plasma layers. In the case of plasmas, whose refractive index is a function of electron density, collecting double-pass time delay data at many distinct frequencies (thereby reflecting off many distinct density layers) permits the time delay data to be inverted to generate electron density profiles.

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.

PRINCIPLES OF OPERATION

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.

USPR Transmitter Design

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.

USPR Receiver Design

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.

EXPERIMENTAL SYSTEM

The first multichannel USPR system was designed and fabricated by the UC Davis Plasma Diagnostics Group, and successfully tested on the CCT tokamak at UCLA in 1994. CCT Experimental results show good agreement with both theory and with other CCT diagnostics. This first 7 channel system, however, was limited to a single profile measurement per plasma discharge. An improved 8 channel USPR system has since been developed, which is capable of acquiring high resolution (25 psec corresponding to < 4 mm spatial resolution) multichannel time delay data at high repetition rates (> 100 kHz).

This new system will undergo extensive testing on the GAMMA-10 plasma device located in Japan, operated by the Plasma Research Center at the University of Tsukuba. The GAMMA-10 tandem mirror consists of five mirror cells: a central cell, two anchor cells, and two plug/barrier cells as shown to the right. The UC Davis USPR system will be installed on the central cell, where the magnetic field is roughly constant at 0.4 T. Here, a 0.36 m diameter plasma is formed with a nominal electron density on axis of 2x10¹⁸m^-3.

Electron density profiles obtained by the UC Davis USPR system will be compared with profiles obtained by existing reflectometer systems operated by Prof. A. Mase's research group at the University of Tsukuba.

EXTENSIONS TO HIGHER DENSITY, HIGHER FIELD DEVICES

Utilizing wideband double ridged waveguide dispersive lines, it is a relatively easy task to form a 6-18 GHz chirp with ~nsec sweep times. Wideband balanced mixers may then be utilized to upconvert the low frequency chirp to millimeter-wave frequencies, with a second mixer utilized to downconvert the reflected waveform. Using highpass waveguide filters to ensure single sideband operation, each mixer is then cable of covering a 12 GHz portion of the 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 a 6-18 GHz IF system, extremely broad frequency coverage may be obtained at relatively low cost. Such a system is under development for use on the Sustained Spheromak Physics Experiment (SSPX), which is under development at LLNL to be used in further exploring the physics of the spheromak fusion concept. Here, a set of five mixers will be utilized in the SSPX USPR system to cover a millimeter-wave frequencie range of 38-158 GHz. This system will probe the SSPX plasma that will exist within a chamber that is about 1 meter in diameter by 0.5 meter high (located near the bottom of the figure shown to the right). Plans call for the first SSPX plasma by early 1999.

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.