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P-L-12 BROADBAND MM RADIATION from BEAM DRIVEN STRONG TURBULENCE M CZ9726737 P-l-12 BROADBAND MM RADIATION FROM BEAM DRIVEN STRONG TURBULENCE M. Masuzaki, H. Yoshida, R. Ando, K. Kamada, A. lkcda, C. Y. Lee and M. Kawada Department of Physics, Faculty of Science, Kanazawa University, Kanazawa 920-11, Japan ABSTRACT In order to measure broadband mm radiation emitted from rclativistic beam electrons interacting with localized high frequency electrostatic fields in a strong Langmuir turbulent plasma, three types of spectrometer were prepared, a 6-channel filter-bank type covering K, Ka and U bands (18 - 60 GHz), a heterodyne type covering E band (60 - 90 GHz) and a full band detector covering F band (90 - 140 GHz). The spectrum of the mm radiation was measured radially, and also its angular distribution was measured at the end of the chamber. When the plasma frequency at the measuring position was about 13 GHz, the power level was high in K, Ka and U band range, but it decreased steeply in E and F band range. The angular distribution showed the radiation was intense in the axis. This implies the dipolc moment of the caviton electric fields were not aligned to the beam direction. INTRODUCTION When an intense rclativistic electron beam (IREB) is injected into an unmagnetized or weakly magnetized plasma and the beam density is near the plasma density, high-power broad- band electromagnetic radiation above plasma frequency is emitted [1-5]. On the other hand, in a similar situation several experiments show that localized strong oscillating electrostatic fields distribute in the plasma |6 - 8]. This phenomenon is interpreted as that the plasma becomes strong Langmuir turbulence state in which creation, collapse and burnout of cavitons are re- peated [9]. Here a caviton is a density cavity in which large amplitude electrostatic waves are trapped. It has been shown experimentally that when the radiation is emitted the plasma is in a strong Langmuir turbulence state [10]. G. Benford and J. C. Wcathcrall proposed the collective Compton boosting model for the radiation [11]. Measurement of detail of the spectrum shape is necessary to compare the experimental results with the model. Our old observation window for the radiation was only K and Ka bands (18-40 GHz). However, as there was evidence of emission of radiation with higher frequencies, we widened the observation window up to F band, using a heterodyne spectrometer for E band (60 - 90 GHz) and a full band detector for F band (90- 140 GHz). EXPERIMENTAL APPARATUS The experimental conditions were the same as Vacuum those in our previous experiments [2 - 4, 8, 10]. Fig- Pulserad Modified Drift Chamber (1 Gauge ure 1 shows a schematic diagram of the experimental 110A apparatus. A modified 110A produced by Physics In- ternational, which can generate accelerating voltage up to 1.5 McV and diode current up to 27 kA with a pulse duration of 30 ns, was used to generate an Titanium Foi IREB. The diode consisted of a carbon cathode of 3.6 Anode cm in diameter and a titanium foil anode of 20 iim thick. The anode-cathode distance was 3 cm. The drift chamber of stainless-steel was 16 cm in inner diameter and 16 cm long. It had two observation port at 17.5 cm downstream from the anode. The base Plasma Vacuum 5 pressure was kept below 4 x 10 Torr. A plasma was chamber chamber produced using a pair of rail-type plasma guns set op- Fig. 1. The experimental setup. -339- 18-60 GHz KtoK|. Kband I I Horn antenna I to I "" , oscilloscope to JS 10" oscilloscope a I 0 10 20 30 40 50 60 70 60 90 100 E band Horn antennal 60 - 90 GHz iO-40GHz Fig. 2. The electron density vs. t. Shield case ao positc to each other, which were in- Downconverter// stalled at 10 cm downstream from the Coaxial to Waveguide anode foil. The discharge current Adaptor reached its peak about 8 us after the gun firing and decayed with the time to Drr constant of about 20 us. The plasma osciIloscoppe ^C^J^ I - n^PF^[ T^j"waveguide density, /; , at 17.5 cm downstream Band pass filter line up from the anode was measured by a BPF1 BPF2 Langmuir probe and a microwave in- 18.0-22 2 26 5-31.0 terferometer. Figure 2 shows the rela- Concrete shield 22 2-26 5 30 7-35 7 tion between n and the delay time, x, 35 5-40 0 after the gun firing. An IREB of about 1.4 McV and about 10 kA was in- Fig. ?. The si-tup of the E band heterodyne spectrometer jected into the plasma at a preset x, which was taken as a parameter dur- ing the experiment. E band Horn 60 - 90 GHz The mm radiation was mea- antenna sured radially and at the end of the Kband chamber. Spectrometers used were; a Horn Downconverter filter-bank type one for K, Ka and U antenna bands (18 - 60 GHz), a heterodyne type one for E band (60 - 90 GHz) and a full band detector for F band (90 - Coaxial 140 GHz). The filter-bank type used 18-60 GHz to was an improved one of the previously Waveguide used 5-channel spectrometer. The het- Ka band Adaptor erodyne type spectrometer and the full Horn antenna band detector were newly constructed. *to n Figure 3 shows a schematic diagram of oscilloscope the heterodyne type spectrometer and K waveguide its setup for the radial measurement. It Pit consisted of a downconverter unit, an Concrete shield IF unit and a power unit. The downconvcrter consisted of a 90- 140 GHz to bandpass filter, a variable attenuator, a oscilloscope mixer and a local oscillator with fre- quency of 50 GHz. The IF unit con- to sisted of bandpass filters, Schottky di- oscilloscope odes and pulse amplifiers. It covered 10 - 40 GHz in 5 channels. Filters and detectors used were diverted ones from Fig. 4. The setup of the F band detector the filter-bank type spectrometer. Calibration was made at 60 GHz. In this setup full band K, Ka and U band detectors were -340- 10° < 10'' .i 10,-6 - -• to ,-7 ro 10" 50 100 200 10 100 1000 10000 Observed frequency [GHz] Freauencv fGHzl Fig. 5. An experimentally obtained spectrum Fig. 6. An example of calculated spectrum prepared, and simultaneous measurements were carried out. Figure 4 shows a schematic dia- gram of the F band detector and its setup for the radial measurements. It consisted of a bandpass filter, an attenuator, a Schottky diode, and a pulse amplifier. In this setup full band Ka, U and E band detectors were also prepared and simultaneous measurements were done. EXPERIMENTAL RESULTS AND DISCUSSIONS Figure 5 shows a typical radiation spectrum observed at x - 30 jis, at which the plasma frequency was about 13 GHz tit the measuring position. Results from two experimental runs arc compiled into one. In one of them the setup shown in Fig. 3 was used, and in the other the setup shown in Fig. 4 was used. It was ascertained that radiation was emitted to at least about 140 GHz. The power level was high in K to U bands, but it decreased steeply in E and F bands. We calculated radiation spectra according to the collective Compton boosting model [11]. In this model the broadband intense ra- diation is due to the interaction of the density modulated beam electrons with Vacuum intense high frequency electric fields in Gauge cavitons, their characteristic charge den- sity oscillation being assumed to be di- ,0=11 deg polar. Several spectral density function for the beam density modulation are as- Ka horn ladaptor sumed. Our calculations were done for each spectral density function and for coaxial cable each direction of the dipolc moment. (50cm) Figure 6 is an example for which Gaussian spectral density function and dipole moment perpendicular to the di- K adaptor rection of the beam propagation arc as- sumed. The experimental and calculated spectra resemble each other in shape qualitatively, although quantitatively they are very different. We could not oscilloscope find the calculated spectra which coin- cided with the experimental spectrum yet. It is interesting to measure the an- gular distribution of the radiation. When Shield case oscilloscope the dipolc moment of the caviton is par- allel to the beam direction, intensity in Fig. 7. The set up for measurement at Hie end of the chamber -341 - the axis will be null. On the other hand, when the dipole moment is perpendicular to the beam direction, the intensity in the axis will be maximum. We measured the radiation in Ka band at the end of the chamber varying the horn position. Figure 7 shows the experi- mental arrangement. Figure 8 shows the measured radiation power as a function of angle. The radiation power observed was al- most the same when the horn was rotated 90° around its axis. Now let all the dipole mo- ment be aligned to the beam direction, since the electrostatic fields arc produced by the beam-plasma instability and it may be natu- 10 20 30 40 50 ral to think that their direction is parallel to Observed angle [deg] the direction of the beam propagation. In this case the radiation intensity in the axis would Fig. 8. Angular distribution of the radiation be very low level. However the observed power level was rather high in the axis. This result suggests that not all of the dipole moment were aligned to the beam direction. SUMMARY AND CONCLUDING REMARKS In order to widen the observation window for measurement of radiation emitted when an IREB is injected into an unmagnctized plasma, we prepared a heterodyne receiver covering E band in 5 channels and a full band detector covering F band in addition to the 6-channel spec- trometer covering K, Ka and U band.
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