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Observation of Elastic Waves Produced by 

Heating From a Modulated Ion Beam 

Howard F . Savage 

Source of Acquisition 

CAS] Acquired 

ICODE) 

(CATEGORY) 

National Aeronautics and Space Administration 

Ames Research Center, Moffett Field, Calif. 

In an investigation of the momentum transfer to metal surfaces from 

a modulated beam of energetic ions, it was discovered that the fo rces 

experienced by the surface did not occur in phase with the pulses of-ions. 

It will be shown here that this dis crepancy is due to elastic waves in 

the target caused by cyclic heating of the surface by the impacting 

particles. 

In a recent publication,l the momentum accommodation coefficients 

of ions with energies between 0. 5 and 4 keY incident on copper and aluminum surfaces were presented. The microbalance2 used for that investigation did not have sufficient sensitivity to measure the variations of 

the momentum transfer with angle of incidence for N; ions with energies 

less than about 1 keY. In an attempt to extend -the measurements to lower 

ion energies and to improve upon the accuracy of the high energy results, 

a new momentum transducer was developed at Ames Research Center whtch , 

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has greater sensitivity, is more rugged, and is more convenient to 

operate . A sketch of the "transducer is shown in Fig. 1. The instrument 

operates as a spring mass system in which the target (shown mounted for 

normal force measurements in Fig. 1) attached to one end of the stem contains most of the mass. On the other end of the stem is one-half of a 

parallel plate capacitor used to produce a known force for comparison 

with the force due to the ion beam striking the target. The stem is 

attached to a pair of piezoelectric beams which serve both as springs 

and as acceleration sensors. This suspension provides essentially a 

single degree of freedom with sensitivity only in a direction along the 

stem. For measuring the component of momentum transfer tangent to the 

target surface, the target is mounted so that the tangent to the target 

is parallel to the stem. To get high sensitivity, advantage was taken 

of the resonance effect of the spring mass system. For this, the ion 

beam was electrically chopped at the resonant freque"ncy (about 140 cps 

for a target mass of 7 g) by means of paraller-"deflector plates " ../ 

(2 -1/2 X 4 em spaced 2-1/2 cm apart) located 40 cm upstream from the target. The transit time f rom the deflectors to the target is then about 

6 ~sec for Ar+ at an energy of 1 keY. 

It was originally intended to make force measurements by applying 

equal forces, 1800 out of phase, on each end of the stem by means of the 

forces due to the ion beam and the capacitor. However , it soon became 

evident that although the capacitor potential and the ion-beam deflectorplate potential were exactly synchronized, being taken from a common 

square wave generator, the forces were not synchronized and a zero output 

could not be obtaine~ . It was also noted'that for supposedly the same 
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test conditions t he variations in magnitude of the output due to the ion 

beam from one test to the next were larger by an order of magnitude than 

the variations expected f rom r andom instrument reading errors. 

A check on t he phase relations (Fig. 2) showed that for Ar+ incident 

on aluminum at 1150 eV, f or example, the normal force due to the ion bombardment lagged about 300 behind the expected phase with indicati ons that 

the phase lag de crea sed w~ th de creasing ion energy . Figure 2 was obtained 

by first photographi ng the os cilloscope trace of the amplified piezoelectric output when the dri ving f orce was the ion beam; a second trace of 

the output was then photographed on the same film when the driving force 

was produced by a square wave potential on t he calibrating capacitor 

adjusted t o give t he same out put amplitude as the ion beam. The oscilloscope was triggered by the common s quare wave generator for both traces. 

It is tempting to attribute this phas e lag t o an actual delay in the 

ejection of target at oms. However, time -of -f light measurements made by 

Stuart, Brower, and Mayer 3 have shown no such -effect. Instead, the 

explanation of the phase l ag and scatter i n the ion beam output presented 

herein is that el astic waves are pr oduced by the cyclic heating of the 

targets. White 4 has also observed elasti c waves produced by cyclic heat -

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ing and has gone i nt o the detai l s of how they are generated and the magnitudes of the st resses involved. The output of t he pi ezoelectri c beams 

is then due to the momentum t r ansf erred through the target by an elasti c 

wave which is the sum of t wo waves; one produced by the ion beam momentum 

pulses and one produced by the cyclic heating due to the ion beam puls es. 

The power, density of t he ion beam was about 0. 03 w/ crnf at an ion energy 

of 1 keY, corr espon~ ng to a computed5 sUrface temperature rise of about 

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10- 3 °C . This temperature increase in turn would induce a peak stress 

which is computed5 to be in the neighborhood of 10- 3 dynes/cmf, as compared to a stress of about 10-2 dynes/cmf due to the force of the ion I 

beam pulses . In order to verify that the phase lag is caused by the cyclic heating of the target, a stroboscopic lamp was used to heat the target at the resonant freQuency of the transducer. The light pulse had a 

duration of 1 ~sec and a power density of about 0.8 W/cm2 , corresponding 

to a temperature rise of about 0.5XIO- 3 °C. With a darkened aluminum 

target, the output of the transducer was of the order of magnitude of the 

output for the ion beam input. When a polished aluminum target was illuminated, the output was only about half that of the darkened target, 

indicating that the detected elastic waves were generated as a result of 

a rise in surface temperature since the temperature rise and resulting 

elastic wave would be greater for the darkened target . If the elastic 

waves had been due to radiation pressure, the pressure would have been 

greater for the polished target. 

Since the peak surface temperature would occur at the end of each 

pulse of ions, the elastic waves caused by the heating would occur later 

in time than those caused by the momentum transfer from the ions to the 

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surface . Thus, the combined peak output would be delaJed and would 

account for the observed phase lag . Also , the variations in the magnitude of the transducer output which occurred for supposedly the same test 

conditions could be caused by changes in the ion beam focusing which 

would change the power density and thus the peak temPerature rise and 

the resulting output due to the thermally produced elastic waves. The 

observed decrease i~ the phase lag with decreasing ion energy would be 

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expected because of the decrease in the ion beam power density, and the 

resulting smaller temperature rise with decreasing energy. 

1 + + H. F . Savage, and M. 'Bader, "M::>mentum Ac commoda tion of N , N2 , and 

A + Incident on Copper and Aluminum From 0 . 5 to 4 keV," NASA TN D-1976 

2V. L. Rogallo, and H. F . Savage, Rev. Sci. Instr. 34, 988 (1963). 

3R• V. Stuart, K. Brower, and W. Mayer, Rev. Sci. Instr. ~, 425 

4R. M. White, J . Appl . Phys. 34, 3559 (1963). 

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Figure Captions 

Fig . 1 . Schematic diagram of the momentum transducer • 

Fig. 2 . Reproduction of an oscilloscope picture showing the phase 

relations between the piezoelectric outputs due to the ion beam force 

+ and capacitor force j Ar incident on Al at 1150 eV. 

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