INSTABILITIES IN LIQUID METAL ION EMITTERS

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INSTABILITIES IN LIQUID METAL ION EMITTERS

G. Mair

To cite this version:

G. Mair. INSTABILITIES IN LIQUID METAL ION EMITTERS. Journal de Physique Colloques,

1989, 50 (C8), pp.C8-171-C8-174. �10.1051/jphyscol:1989830�. �jpa-00229928�

COLLOQUE DE PHYSIQUE

Colloque C8, suppl6ment au no 11, Tome 50, novembre 1989

INSTABILITIES IN LIQUID METAL ION EMITTERS

G.L.R. MAIR

University of Athens, Department of Physics, Division of Mechanics, Panepistimiopolis, GR-157 83

-

Zografos, Athens, Greece

Abstract

-

As the voltage in a liquid metal ion source is raised above onset, a level of current is reached where the liquid cone-shaped anode begins to vibrate under the competing influences of electrostatic and surface tension forces. At this point, ps oscillations appear in the current. In this regime, break-up of the jet-like apex region of the cone, with concomitant droplet emission, is naturally enhanced;

however, the instability growth rates involved in this process are a few orders of magnitude higher than the 1 MHz deduced from current oscillograms and emission frequency spectra.

The emitter in liquid metal ion sources (LMIS) is in the form of a liquid cone, formed and maintained by the competing influences of electric and surface tension forces. The cone has a half-angle of about 50' [l] and its apex region elongates to form a jet. Such a system is naturally prone to instabilities, yet, at sufficiently low emission levels, current oscillograms are virtually noise free. This has been attributed to the stabilising influence of the space-charge in front of the tip [21.

Characteristic us fluctuations appear in the current, typically for currents in excess of 30 FA (or about twice this value if secondary electrons are not suppressed) (Fig la,b,c). Authors sometimes attribute these fluctuations to jet instability and break-up. Theoretical treatments for a cylindrical jet of radius ro indicate "varicose" instability growth rates (a):

for a liquid of density p and surface tension gT [3]. For an average, jet radius of 200 A 141, eq. (1) yields w % 4 x 10 Hz. Even if the whole of the jet is assumed to become ablated, growth rates would still be of the same order of magnitude. A jet of length R and apex radius r would go unstable at a rate [5]

so that for R % 2 x m and r % 50 Fi [ 4 ] , w 1.5 x 10' Hz.

Now, for the liquid cone vibrating under the influence of electrostatic and surface tension forces, a simple analysis shows [6]:

with

x being a small change in the position of the center of mass of the liquid cone-shaped emitter [6]; 8 is 90° minus the base angle of the cone and is taken as 49.3' [I] and rt the needle apex radius. For Ga, eq. (3b) yields a period of oscillation:

with T in US and r in Urn. For typical values of rt in the range of urn, T

is of the order of

4

^{~ s ,} and this agrees with experiment.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989830

It is seen, therefore, that w (or T ) depends on the needle apex radius, which in turn determines the size of the cone.

As already .reported, the % 1 MHz frequency seen in the current oscillograms were also detected in frequency spectra (Fig. 2) and we also detected higher frequencies among thev at % 10, 30, 65 and 100 MHz; similar values have also been reported by Dudnikov and Shabalin [71. Barr [81 also reports a value of s 30 MHz and a value of 2.5 MHz, and he attributes the latter to droplet emission; we think the lower frequency is due to cone vibration. However, it must be noted that the two phenomena do not necessarily contradict each other. It may be. that during each cycle of cone vibration the jet goes unstable and emits a droplet: Since jet instability growth rates are so much faster than the cone vibration frequency, the emission of a proplet would not affect the validity of the analysis leading to eqs. 3a,b.

Furthermore, a droplet emitted each time the cone vibrates would shield the apex. This may explain the fact that the pulses (Fig. 3) are essentially negative at the beam center, gradually becoming positive at the edges. In short, the cause of the fluctuations in current is cone vibration and droplet emission may coincide with this MHz vibration, driving the jet unstable.

Summing up, if the cone starts to vibrate at a given current level

-

^and

there is evidence to that effect from TEM photographs

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this would precipitate or enhance jet instability and break-up. Therefore, it is not surprising that the levels of current at which the cone starts to vibrate, the current shows ws fluctuation and the rate of mass loss from the source shows a sharp rise, all coincide. This value of current is (for Ga) % 60 pA

total current or about half this value for current. Jet instability growth rates probably correspond to frequency spectra peaks from s 100 MHz upwards. Erratic emission of large chunks of the conets apex could well correspond to frequency peaks in the range of tens of MHz; 10 MHz would also correspond to the vibration of cones on sharp (r % 1 wm) needles (see eq.

(4) and ref. 171 1 . t

References

1. G.I. TAYLOR, Proc. Roy. Soc., A280, 383 (1964).

2. R. GOMER, Appl. Phys.,

2,

365-79).

3. A.B. BASSET, Am. J. Math.,

16,

93 (1984).

4. G. BENASSAYAG, P. SUDRAUD and B. JOUFFREY, Ultramicroscopy,

16,

1 11985).

5. G.L.R: ~MAIR, D. Phil. Thesis, Oxford University, 1979.

6. G.L.R. MAIR, J. Phys. D.,

21,

1654 (1988).

7. V.G. DUDNIKOV and A.L. SHABALIN, Sov. Phys. Tech. Phys.,

30,

⁴⁶²

(1985).

8. D.L. BARR, W.L. BROWN and D.J. THOMSON, J. de Physique,

49,

^C6-117

(1988).

Acknowledgements

The author is grateful to Dr P D Prewett of Rutherford Appleton Laboratory for several useful discussions and for presenting this paper in his unavoidable absence.

s c a l e

Fi- .l, Current oscillograms of $a needle ILllS shoxin 7,~ith respect t o zero current l i n e . G t m c e : t o t a l c u r r m t j bottom trace: ion current (collector-released secondary electrons suppressed bjr magnetic f i e 1 6 of %I00 Gauss ag2lied i n a d i r e c t i o n perpendicular t o the beam. (a) vert:1 O ~ A / d i v ,horiz: Sus/Ziv. (b) v e r t : 2 Q B /div, horiz: 5ps/div.

0 ₅ 10 15 20

- ^(MHz)

Fig.2. Freciuenc;r s2ectrum of Gn izeedle MIS obtained v.sinz a pick-up c o i l surrounding the beam. Band widtil: 130 ;CZ [6]

.

Total current: 90 ~ f l .

Fig 3. Current oscillograms, obtained with a collector consisting of con- centric rings, for different portions of the beam. (a): upper trace, be- tween 0 " and 14'; lower trace, between 15" and 3 9 " . (b): upper trace, be- tween 0" and 1 4 " ; lower trace, between 40" and 55". Scale: vert: 5uA/div, horiz: 2 ps/div. Total Current (incl. see. electrons) 84 pA. Current be- tween 0 and l L O , 16.5 u A ; between 15" and 3 9 " , 64 pA; between 40" and 5 5 "

0.7 P A . Negative part of pulses is probably due to beam shielding by the

droplet, whereas positive part might be due to ions being scattered off- axis by the droplet.