J. Mathew

Production of large-area plasmas by electron beams

An analysis is presented for the production of weakly ionized plasmas by electron beams, with an emphasis on the production of broad, planar plasmas capable of reflecting X-band microwaves. Considered first in the analysis is the ability of weakly ionized plasmas to absorb, emit and reflect electromagnetic radiation. Following that is a determination of the electron beam parameters needed to produce plasmas, based on considerations of beam ionization, range, and stability. The results of the analysis are then compared with a series of experiments performed using a sheet electron beam to produce plasmas up to 0.6 m square by 2 cm thick. The electron beam in the experiments was generated using a long hollow-cathode discharge operating in an enhanced-glow mode. That mode has only recently been recognized, and a brief analysis of it is given for completeness. The conclusion of the study is that electron beams can produce large-area, planar plasmas with high efficiency, minimal gas heating, low electron temper...

Experimental investigations of the formation of a plasma mirror for high‐frequency microwave beam steering

The Naval Research Laboratory (NRL) has been studying the use of a magnetically confined plasma sheet as a reflector for high‐frequency (X‐band) microwaves for broadband radar applications [IEEE Trans. Plasma Sci. PS‐19, 1228 (1991)]. A planar sheet plasma (50 cm×60 cm×1 cm) is produced using a 2–10 kV fast rise time square wave voltage source and a linear hollow cathode. Reproducible plasma distributions with density ≥1.2×1012 cm−3 have been formed in a low‐pressure (100–500 mTorr of air) chamber located inside of a 100–300 G uniform magnetic field. One to ten pulse bursts of 20–1000 μs duration plasma sheets have been produced with pulse repetition frequencies of up to 10 kHz. Turn on and off times of the plasma are less than 10 μs each. The far‐field antenna pattern of microwaves reflected off the plasma sheet is similar to that from a metal plate at the same location [IEEE Trans. Plasma Sci PS‐20, 1036 (1992)]. Interferometer measurements show the critical surface to remain nearly stationary during th...

Electronically steerable plasma mirror for radar applications

An alternative to using a phased array to steer a high frequency microwave beam is to electronically control the orientation of an inertialess broadband microwave reflector. Such a system could steer one or more high power beams for search or tracking radars, and the system could possess wide instantaneous bandwidth. Experiments have demonstrated that a planar plasma mirror can be formed with electron densities high enough to reflect X-band microwaves. Estimates of microwave beam power handling capabilities of such a plasma Sheet exceed 3 kW/cm/sup 2/ for microsecond duration pulses at kilohertz repetition rates. The plasma minor is formed by driving a glow discharge between a hollow cathode and a flat plate anode which are immersed in an axial magnetic field of 150-300 Gauss. The plasma sheet could in principle be steered in one plane using the magnetic field, and in another by designation of the cathode. A 50 cm wide, 1 cm thick, 60 cm long plasma sheet is formed between the cathode and the anode 10 /spl mu/s after the voltage is applied. The discharge also extinguishes within 10 /spl mu/s of the voltage being turned off. For a 4 kV voltage pulse, plasma densities /spl les/10/sup 13/ cm/sup -3/ are achieved within the plasma sheet. The discharge gas is air at 130 mTorr. Single mirrors have been formed with pulse widths of 20-1000 /spl mu/s, and burst mode operation at 10 kHz has been achieved with 30-70 /spl mu/s long pulses. Measurements of the phase using a heterodyned microwave interferometer show that the critical surface is stationary during most of the pulse. A 30 cm diameter Cutler-feed antenna is being used to illuminate the mirror at 10 GHz. The H-plane radiation pattern (for a 90/spl deg/ reflection) measured 4.3 m from the plasma mirror, is very similar to that obtained from a metal mirror of comparable dimensions.

The Naval Research Laboratory Modified Betatron Accelerator and assessment of its results

In this paper the experimental results of the Naval Research Laboratory (NRL) Modified Betatron Accelerator (MBA) [Phys. Rev. Lett. 64, 2374 (1990)] are briefly summarized and assessed. These results show that the addition of the strong focusing (SF) field made a substantial improvement in the confining properties of the device, at least in the intermediate time scale, and that the SF field together with the finite resistivity of the vacuum chamber wall is responsible for the trapping of the beam. There is extensive experimental evidence, suggesting that the slow beam loss during acceleration is caused by the cyclotron resonances. Reduction of several field errors together with operation at higher toroidal and SF field led to beam energies in excess of 20 MeV, while the trapped current was above 1 kA. Preliminary beam extraction experiments with 12 resonant coils have shown that a ∼80 G field disturbance is required to extract the beam from the MBA.

Compact, high‐current accelerators and their prospective applications

This paper briefly surveys the three compact, high‐current accelerators that are presently under development in the United States in support of a national program. In addition, it reports recent experimental results from the Naval Research Laboratory (NRL) modified betatron [Phys. Rev. Lett. 64, 2374 (1990)] with emphasis on the electron‐cyclotron resonance that presently limits the energy of the beam to approximately 18 MeV. Finally, it briefly addresses selective existing and prospective applications of accelerators.

Electronically steerable plasma mirror for surveillance radar applications

An alternative to using a phased array to steer a radar beam is to electronically control the orientation of an inertialess broadband microwave reflector. Recent experiments have demonstrated that a planar plasma mirror immersed in a magnetic field, can be formed with electron densities high enough to reflect X-band microwaves. The plasma sheet can be steered in elevation using the magnetic field, and steering in azimuth may be accomplished by designating cathode initiation sites. Retargeting could be achieved in 20 /spl mu/s or less, and a plasma mirror based radar system could be quite affordable. The measured radiation pattern for a 90/spl deg/ reflection from a 50 cm/spl times/60 cm plasma mirror is very similar to that obtained from a metal mirror of comparable dimensions. As a reflector, the plasma mirror exhibits extremely low loss, the reflectivity is very nearly 100%, and the sidelobes are down by >20 dB. A conceptual design for a plasma mirror based radar configuration shows that a -5/spl deg/ to +60/spl deg/ elevation coverage and a 180/spl deg/ azimuthal coverage is obtainable without blockage using a single 1.5 m diameter X-band illuminating dish. Coverage may be extended to +90/spl deg/ elevation angles with minimal blockage, and 360/spl deg/ azimuthal coverage may be obtained with two dishes.

Retarding field energy analyzer for the characterization of negative glow sheet plasmas in a magnetic field

A retarding field energy analyzer has been developed for diagnosing 300 μs duration, 60 cm×60 cm negative glow, sheet plasmas immersed in a 150–250 G axial magnetic field. The electron density in these 4.5 kV, 13 A, 120 mTorr discharges in air and other gases, is high enough to reflect X‐band microwaves. The presence of the magnetic field makes the suppression of secondary electrons from the Faraday collector surface more difficult. The approach taken here is to bias the entire collection circuit and the amplifiers 90 V positive with respect to the data acquisition room. The differentially pumped analyzer is designed to accept electrons with a large range of perpendicular velocities, and it measures the parallel velocity distribution function of the discharge electrons entering a 0.64‐mm‐diam hole in the anode plate. It gives valuable information about the energy spectrum of the energetic beam electrons emitted from the cathode, and the effect of energy loss and scattering processes on this propagating be...

Recent results from the naval research laboratory experimental FEL program

Abstract The Naval Research Laboratory has been very active in the development of high-power, millimeter-centimeter wavelength FELs. These devices have been operated as amplifiers, superradiant amplifiers, and as an oscillator. The amplifier experiments demonstrated power gains of 50 dB with > 3% efficiency (with no enhancement), and superradiant experiments produced 75 MW pulses at λ = 4 mm with 6% efficiency. These experiments were performed using a 1 2 MeV pulse line accelerator having a 50 ns pulse duration. Most recently, an induction linac having a 2 μs pulse duration has been employed for oscillator experiments. This work has demonstrated steady-state operation for 1 μs at power levels up to 10 MW. The initial power gain per pass is as large as 10 4 , so that turn-on occurs rapidly. This device has operated with various mirror configurations, including a Bragg resonator. The experimental results agree well with recent theoretical work, which predicts efficiencies > 20% in a tapered wiggler oscillator. These results are also directly applicable to the operation of a two-stage FEL experiment.

Recent developments on the NRL Modified Betatron Accelerator

1407_47Since the previously reported successful demonstration of acceleration of electrons to

Beam trapping in a modified betatron accelerator with a localized bipolar electric‐field pulse

OM 12 MeV, higher trapped beam current and longer beam lifetime have been obtained. The improved confinement has led to a higher peak energy