Proceedings of the 40th European Microwave Conference
Imaging of Pulsed Watt-scale Millimeter Waves Using Visible Continuum from a Cs-Xe Discharge M. S. Gitlin#1, A. I. Tsvetkov#2 #
Plasma Physics Department, Institute of Applied Physics, Russian Academy of Sciences Ulyanov Str., 46, Nizhny Novgorod, Russia 1
[email protected] 2
[email protected]
Abstract— We present a high-sensitivity technique for timeresolved imaging of millimeter waves using the visible continuum (VC) from a uniform slab of the positive column (PC) of Cs-Xe DC discharge. Using the discharge technique, field patterns of pulsed watt-scale Ka-band millimeter-waves were imaged at the output of conical horn antenna and in the quasioptical beam. We also demonstrated video-rate millimeter-wave (MMW) shadowgraphy using a slab of the PC of Cs-Xe DC discharge as a 2-D real-time MMW sensor. Near-field shadow projection MMW images of amplitude and phase objects have been obtained using 35.4 GHz radiation for object illumination.
I. INTRODUCTION Determination of the millimeter wave (MMW) intensity spatial distribution can be of great importance for design and manufacture of MMW sources, transmission lines, and antennas, for MMW imaging, nondestructive testing and evaluation, MMW plasma diagnostics, etc. To our knowledge, there is no technique, which would make it possible to measure in real time field patterns of moderate-power millimeter waves for submillisecond pulse duration. The thermographic methods cannot be used to measure the beam profile of subkilowatt millimeter waves with millisecond and shorter pulse duration because of their low temporal resolution, which can be 10 ms at best, and low energy flux sensitivity, which is no better than 1 mJ/cm2 [1]. Time-resolved millimeter-wave field images obtained using 2D detector arrays are of rather poor quality, because of the element-toelement gain and sensitivity variations, the large element-toelement spacing, and the small number of pixels [2]. They can hardly be used for near-field imaging. The drawbacks of the thermographic and 2D detector array MMW imaging techniques require both their further improvement and seeking alternative techniques for time-resolved imaging and measurement of the MMW field patterns. We present a high-sensitivity technique for time-resolved imaging of MMWs using the visible continuum (VC) from a slab of the positive column (PC) of a medium-pressure Cs-Xe DC discharge [3]. The concept of this technique is based on the effect of increasing intensity of the e-Xe bremsstrahlung continuum in the visible region, when the electrons in the positive column of Cs-Xe DC discharge are heated by millimeter waves [3]. The intensity of the visible continuum from the PC of a Cs-Xe discharge increases due to microwave effect almost locally and proportionally to the MMW intensity, and this allows measuring the intensity profile of the MMWs
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incident on the plasma slab. By means of this technique, MMW images are converted into visible images, thus allowing conventional TV cameras to acquire them. This imaging technique has high energy flux sensitivity (about ten µJ/cm2 in the Ka-band), microsecond temporal resolution, and a spatial resolution of 2–3 mm. The technique is broadband and can be used to image radiation intensity patterns in the range from centimeter to submillimeter waves [3, 4]. The CsXe discharge sensor, in contrast to detector arrays, reflects and scatters the MMWs only weakly and can therefore be used for imaging of the transmitted MMW beam and imaging of the near-field patterns of MWW sources and antennas. Another advantage of this technique is that the discharge sensor rapidly restores its properties if a microwave breakdown occurs. II. EXPERIMENTAL In our experiments on millimeter wave imaging a sealed discharge tube (DT) described in detail in [3] was used for generation of the slab of positive column of the Cs-Xe DC discharge (see Figs. 1, 3 and 5). A hollow rectangle parallelepiped glued from plane-parallel fused quartz plates was located in the middle of the tube. Two square plane quartz windows with their apertures 10 x 10 cm2 were set at the distance of 2 cm between their internal surfaces. These windows allowed injecting the microwave beam into the tube with small reflection and without distortions. Two plane anodes and two heated cathodes were placed in glass cylinders 10 cm in diameter. The glass cylinders were glued to the quartz cell. Each pair of the electrodes was connected to a separate current source. The discharge tube was filled with xenon at the pressure of 45 Torr. This relatively high gas pressure is necessary to provide a locality of the electron heating under microwave effect. The discharge tube had sidearms, in which drops of cesium metal were placed. To
Fig. 1 Schematic of the experimental setup (top view) for imaging of the field patterns at the output of conical horn antennas excited by the TEO01 mode
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28-30 September 2010, Paris, France
obtain the required density of cesium vapor, the discharge tube was heated in an oven (see Fig. 1). The oven had a quartz window (OW) 20 cm in diameter. MMW imaging experiments were performed for a discharge current of 1.5 A and a tube temperature of 90O C. Under these conditions, the PC of the Cs-Xe discharge had the shape of a spatially uniform slab which filled the entire tube aperture 10 x 8 cm2. The images of the plasma slab were captured by a black and white charged coupled device (CCD) camera. A set of optical filters (OFS) rejecting atomic emission lines and transmitting the continuum in the visible region was placed in front of the camera lens (CL). The data from the CCD camera were processed with a computer. To evaluate the VC intensity variation ΔI (x, y) under the MMW effect, we subtracted the VC background intensity I0 (x, y), which was obtained using the frame preceding the MMW pulse, from the VC intensity I (x, y), which was obtained using the frame simultaneous with the MMW pulse; i.e. ΔI (x, y) = I (x, y) - I0 (x, y). The variation of VC intensity ΔI (x, y) is almost directly proportional to the intensity of MMWs W (x, y) (with an accuracy of about 10 %) for the MMW intensity range from 0 to Wbr/5 [3]. Here Wbr is the threshold of the microwave-induced breakdown of the plasma slab, which is 5 W/cm2 in Ka-band. For a MMW intensity of Wbr/5, the VC brightness increased by about two times. As the MMW source we used a 35.4 GHz magnetron (see Figs. 1, 3 and 5), which supplied coherent radiation with an output power of up to 20 W in the long-pulse mode. The MMW pulse length was from 10 ms to 100 ms, and the pulse leading edge was less than 0.1 µs. A. Imaging of Near-field Patterns of Conical Horn Antennas Excited by TEO01 Mode To demonstrate capabilities of the technique, we performed experiments on imaging of the field patterns at output of antennas and of quasioptical beams in the Ka-band. The experimental setup for imaging of the field patterns at the output of conical horn antennas excited by the TEO01 mode is shown in Fig. 1. A Marier transducer (MT) was used to transform the TE10 mode of a rectangular waveguide (RW) into the TE01 mode of a circular waveguide. To increase the transverse size of the millimeter wave beam we used conical horn antennas (CHA) with a taper angle of 6o and output aperture radius Rh = 28 mm. The discharge tube window was attached directly to the output of the horn antenna, and the spatial distribution of the MMW intensity was measured in the near-field region. Figure 2(a) shows a two-dimensional distribution of the VC intensity variation ΔI (x, y) under the millimeter-wave effect at the output of the conical horn. The MMW intensity at the field maximum was Wm = 1 W/cm2. The CCD camera exposure time was 100 μs, and the exposure delay with respect to the leading edge of the MMW pulse was 10 μs. The obtained pattern of the VC glow corresponds to the TEO01 mode with a small admixture of other modes (see Fig. 2 (a)). The pattern rotated by an angle of π/2 when a section of the rectangular waveguide, which was twisted by the angle π/2,
Fig. 2 Spatial distribution of the VC intensity variation at the output of a conical horn antenna excited by the TEO01 mode. (a) Two-dimensional distribution of the VC intensity variation. Measured (black lines) and calculated (gray lines) dependences of the VC intensity variation vs. the coordinates (b) y and (c) x
was installed before the Marier transducer. Figures 2(b) and 2(c) show the dependence of the VC intensity variation under the MMW effect vs. the coordinates y and x. For the TEO01 mode, the dependence of the microwave electric field Eϕ (r) 2 2 1/2 on the radius r = (x + y ) is described by a first-order Bessel function [5]: Eϕ (r) ~ J1 (3.832r / R), where R is the radius of a circular waveguide or a conical horn. The power absorbed in the discharge is proportional to the square of the MMW 2 electric field W (x, y) ~ [J1 (3.832r / Rh)] . The VC intensity variation profiles along the x and y coordinates, ΔI (x, 0) and ΔI (0, y) coincide well with the squared first-order Bessel function (see Figs. 2(b), and 2(c)). For the CCD camera exposure time of 100 μs, the MMW energy flux at the field maximum was about 100 μJ/cm2, and a single-shot signal to noise ratio (SNR) was about 10 (see Fig. 2). B. Imaging of Quasioptical Millimeter Wave Beam Figure 3 shows the experimental setup for imaging of a quasioptical millimeter wave beam. The MMW beam comes out of a pyramidal horn antenna (PHA) with a length of 50 cm and an output aperture of 8 x 6 cm2. A plane-convex Teflon lens (PTL) of 20 cm in diameter with a focal length of about 72 cm collimated the millimeter wave beam. The beam is linearly polarized in the y axis, as defined in Fig. 3. A doubleconvex Teflon lens (DTL) of 20 cm in diameter and a focal length of 32 cm focused millimeter waves at the center of the plasma slab. Figure 4 shows a spatial distribution of the VC
Fig. 3 Schematic of the experimental setup (top view) for imaging of a quasioptical millimeter-wave beam
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Fig. 4 Spatial distribution of the VC intensity variation measured in the focal plane of a double-convex Teflon lens. (a) Two-dimensional distribution of the VC intensity variation. (b) Dependences of the VC intensity variation (faint line) and the MMW intensity (heavy line) vs. the coordinate x
Fig. 5 Schematic of the experimental setup (top view). Inset, front view of the test object placed before the tube window
The MMW beam was radiated by a pyramidal horn antenna with a length of 50 cm and output an aperture of 6 x 8 cm2. A plane-convex Teflon lens (TL) 20 cm in diameter with a focal length of about 72 cm was used to collimate the millimeter wave beam. The MMW electric field polarization was directed along the x axis (see Fig. 5). The MMW pulse length was about 10 ms. The repetition rate of MMW pulses was 12.5 Hz, i.e. half of the frame rate of the CCD camera. The CCD camera exposure time was 1 ms, with a delay of 10 µs with respect to the leading edge of the MMW pulse. The test object was placed close to the window of the discharge tube. The width (FWHM) of the quasi-Gaussian beam in the object plane was about 8 cm. The MMW intensity in the center of the beam was of up to 0.3 W/cm2. Time-averaged MMW intensity in the center of the beam was about 30 mW/cm2. We note that this value can be reduced by an order of magnitude by making the MMW pulse length as long as the camera C. Real-time Millimeter-wave Shadowgraphy exposure time. Amplitude and phase objects were used as test objects to Imaging, sensing, and nondestructive evaluation (NDE) evaluate the imaging system. At first, static objects were with millimeter waves is of considerable interest for scientific, imaged. The letters I, A, P, and L from acronyms for Institute industrial, security, and biomedical applications [2, 6], even of Applied Physics (IAP) and Applied Plasma Physics though the spatial resolution of millimeter-wave (MMW) Laboratory (APPL) 50 mm high were cut out of aluminium systems is much worse than that of X-ray, optical, IR, and THz systems. MMW radiation is non-ionizing, and so poses foil sheets. The foil sheets with the cuts were pasted onto fewer hazards to biological objects than X-rays. Millimeter sheets of cardboard and used as amplitude objects. The width waves can penetrate through many opaque dielectric materials, of the rectangular strips transparent for MMWs was h = 12 such as paper, plastic, wood, ceramic, etc., giving MMW mm. The images of the VC intensity variation caused by the systems some advantage over optical and infrared systems for effect of the MMWs transmitted through the slits in the foils imaging, testing and evaluation. MMW imaging also have are shown in Fig. 6 (a–d). The single-shot signal-to-noise ratio was about 20:1 for the MMW intensity some advantages over THz imaging: first, absorption and (SNR) of the image 2 about 0.3 W/cm . The noise level of the images was scattering is lower in the millimeter-wave range, and second, many types of conventional MMW sources and components determined by noise performance of the CCD camera. The are widely available. In active-mode MMW imaging and NDE images obtained were distorted by diffraction, because the slab was located in the Fresnel region of systems a transmitter is used for object illumination. For real- plasma 2 2 time recording of MMW images we used a slab of the positive h / 2λ < ΔzR,L < 2h / λ, where λ is MMW wavelength, and ΔzL, column of Cs-Xe DC discharge [7]. The discharge technique ΔzR are the distances from the object to left and right boundary was applied for active near-field MMW imaging using the of the plasma slab respectively. In this region, quasi-plane shadow projection method. Advantages of the near-field waves transmitted through the slits progressively become shadow projection system as compared with the widely used cylindrical waves [8]. Nevertheless, all the letters can be camera-mode active MMW imaging system are speckle easily recognized. Figure 6(e) shows the image of the letter E reduction and lack of image degradation owing to objective cut out of aluminium foil and pasted onto sheets of cardboard. The height of the letter was 50 mm, and its width was 45 mm. lens or mirror diffraction and aberrations. The experimental setup (top view) is shown schematically The width of the foil strips was 10 mm. The image of this opaque object was sufficiently clear. In Figs. 6(f) and (g), in Fig. 5. intensity variation measured in the focal plane of a doubleconvex Teflon lens. The MMW intensity at the beam center W (0, 0) was equal to 2 W/cm2. The CCD camera exposure time was equal to 200 μs, and the exposure delay with respect to the leading edge of the MMW pulse was 10 μs. Figure 4(a) shows a 2D distribution of the VC intensity variation ΔI (x, y). The faint line in Fig. 4(b) shows the dependence of the relative VC intensity variation under the MMW effect on the coordinate x. We compared the distribution of the VC intensity variations ΔI (x, 0) / ΔI (0, 0) in the focal plane of lenses with the MMW intensity profile W (x, 0) / W (0, 0), which was measured with a mechanically scanned calibrated MWW detector (heavy line in Fig. 4(b)). The patterns measured by the two techniques coincided to within the experimental error.
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Fig. 7 [(a)-(e)] Real-time MMW transmission images of pendulum motion obtained during one half of the oscillation period. [(f)-(j)] MMW transmission movie of the emptying of a glass tube filled with water Fig. 6 [(a)-(d)] MMW transmission images of the cuts in aluminum foils in the shape of letters I, A, P, and L. (e) MMW transmission image of letter E cut from aluminum foil. (f) MMW transmission image of three Septogal pellets. (g) MMW transmission image of a fish-shaped pumice-stone
MMW transmission images of phase objects are shown. The phase objects were made of dielectric materials with low MMW absorption. Fig. 6(f) shows the MMW transmission image of three Septogal pellets (cough-lozenges made by JADRAN, Croatia). The pellets were disks about 3.5 mm thick and 16 mm in diameter. They were arranged in a trefoil and placed inside an envelope. The pellets were clearly visible in the MMW image (see Fig. 6 (f)), although their diameter exceeded the wavelength by less than two times. In Fig. 6(g), a MMW transmission image of a pumice-stone in the shape of a fish is shown. The height, width, and thickness of the pumice fish were 95, 58, and 18 mm respectively. The MMW transmission image of the phase object shows clear image contouring (see Figs. 6(f) and (g)). The minimum of the intensity occurred on the boundary of the object images, between the maxima. This edge enhancement phenomenon originated from MMW edge-diffraction by a transparent object [9] if the wave passing through it had an additional phase shift different from 2πk, where k is an integer. We have also demonstrated that this technique can provide real-time imaging of moving objects. In particular, we imaged oscillation of a pendulum. In Figs. 7(a)-7(e), the continuity of MMW transmission images of the pendulum motion obtained during half of the oscillation period is shown. The pendulum was a Teflon ring suspended by thread about 15 cm long. The outer and inner ring diameters were 25 mm and 13 mm respectively, and its thickness was 8 mm. The ring affected a MMW beam as a three-zone phase filter which focused the MMW beam in the near-field in the focal spot about one wavelength in diameter (see Figs. 7(a)-7(e)). The pendulum oscillation period was about 0.8 s. We also imaged the emptying of water from a glass tube (see Figs. 7(f)-7(j)), whose inner and outer diameters were 6 and 8 mm respectively. The tube filled with water was opaque for
MMWs (see Fig. 7(f)). When we turned on a discharge cock located at the end of the tube (outside the aperture), the water began to flow out of the tube. Figures 7(g)–7(i) show how the glass tube became transparent to MMWs as it was emptied. The water poured out of the tube for about 1 s. In Fig. 7(j), the MMW transmission image of the empty tube is shown. The boundaries of the tube are well-defined in Fig. 7(j) because of the considerable difference in the refractive indexes of glass and air. ACKNOWLEDGMENT This work was partially supported by the Russian Foundation for Basic Research (Project No. 09-08-00728-а). REFERENCES [1]
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