6P15 6P16 Electron Emission from Ferroelectric Thin Film Cathodes Radiography F.Liu and C.B. Fleddermann, University of New Mexico, Albuquerque, NM 87131, USA

Penetration of a Plasma by a Beam in Multipulse

A.G. Sgro and T.J.T. Dwan, Los Alamos National Laboratory, Los Alamos, NM 87545, USA

Electron Emission From Ferroelectric Thin Film Cathodes

F. Liu and C. B. Fleddermann Department of Electrical and Computer Engineering, University of New Mexico, Albuquerque, NM87131

Thin film ferroelectric cathodes have been proposed for several applications, such as flat panel displays. For this application, electron emission currents of 10s of mA/cmz will be required. Strong pulsed electron emission fkom bulk ferroelectric cathodes has been widely reported (100s of A/cm*), but reports on thin film ferroelectric cathodes have shown that only relatively small currents ( 4 . 1 pA/cm2) are obtainable when the thickness is reduced to a few microns. Because of the attractive advantages of thin film cathodes in flexibility and low switching voltage, we are systematically studying ways to improve the emission performance.

Three kinds of ferroelectric thin film cathodes have been fabricated: PLZT 2/95/5 (0.15pm thick), PNZT 4/20/80 (OXpm), and PLZT 25/10/90 (0.15pm). Standard microelectronic techniques were used to make the devices. The top electrodes were patterned into strips that have equal width and spacing between strips. For comparison, three patterns of different electrode strip width, 1.5 pm, 3.0 pm, and 10 pm, were made for each material. The electrode strip widths used here are much smaller than those reported by others (200-300 pm) and are a close match to the thickness of the film. This has been suggested in the literature to produce higher emission.

The cathode top electrode was grounded and negative pulses were applied to the cathode bottom electrode with a typical rise time of 50 ns. A pre-bias voltage up to 4.5 volts was added across the ferroelectric thin film during pulsed switching. The anode was biased to positive 2KV and was used both for electron extraction and emission measurement. The sensitivity of the emission current measurement was about 10 pA.

Emission currents higher than 10mA/cm2 have been measured for applied voltage between 20 and 30 volts. However, the emission reproducibility was not good. Also, the ferroelectric cathodes normally did not survive when pulsed to voltages between 35 and 45 volts. Reasons for the inconsistent emission and potential failure mechanisms will be discussed.

This work is supported through a High Energy Microwave Devices Consortium funded by an AFOSR/DOD MuRI grant and administrated through Texas Tech University.

Penetration of a Plasma by a Beam in Multipulse Radiography

A. G. Sgro and T. J. T. Kwan, Los Alamos National Laboatory

In multipulse radiography, electron beam pulses will encounter a background plasma consisting of low atomic weight elements such as water vapor and hydrocarbons, and also vaporized target material, generated by previous pulses. This is important because subsequent pulses penetrating the plasma plumes may suffer spot size increase and misalignment with the radiographic axis if the beam propaqgation is unstable. The region of this material that is farthest from the target will have a low density and will be collisionless. The beam will collisionlessly penetrate this intervening plasma, rather than pushing a hole through it as would happen in the high density fluid regime. The particle in cell code MERLIN is used to simulate this interaction.

When the beam density is much higher that the background density, the space charge field of the impinging beam rapidly blows away the low density electron background. The ions, being much heavier, take longer to move. In the heavy (i.e., immobile) ion limit, a steady state develops in which the beam undergoes a betaton oscillation along its axial length. When the background density is much higher than the beam density, the background electrons are pushed axially as well as radially. The space charge of the background ions prevents these electrons from being blown too far away. In the heavy ion limit, the system settles into an approximate equilibrium qualitatively similar to the previously mentioned one.

When the beam impinges on a background with a density ramp, the low density region of the background is blown away just as in the first case, while the high density region of the background behaves as in the second case for the same reasons.

In the heavy ion limit, the background plasma does not appear to disrupt the beam. A light ion background i s more complicated and can negatively affect the beam propagation in some cases.

2 72