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Submitted on 1 Jan 1979
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INTERACTION OF AN INTENSE RELATIVISTIC ELECTRON BEAM WITH THE ATMOSPHERE
A. Ali, J. Greig, I. Vitkovitsky, R. Fiorito, R. Fernsler
To cite this version:
A. Ali, J. Greig, I. Vitkovitsky, R. Fiorito, R. Fernsler. INTERACTION OF AN INTENSE RELA-
TIVISTIC ELECTRON BEAM WITH THE ATMOSPHERE. Journal de Physique Colloques, 1979,
40 (C7), pp.C7-773-C7-774. �10.1051/jphyscol:19797373�. �jpa-00219370�
JOURNAL DE PHYSIQUE Colloque C7, supplcfment a u n07, Tome 40, JuiZZet 1979, page C7- 773
INTERACTION OF AN INTENSE RELATIVISTIC ELECTRON BEAM W I M THE ATMOSPHERE
A.W. Ali, J.R.
Greig,I.M.
Vitkovitsky, R.B. ~iorito" and R.F. ~ernsler".Naval Research Laboratory, Washington, D.C. 20375, U.S.A.
X K N a ~ a Z
*
Surface Weapons Center, White Oak, MD 20910, U.S.A.JAYCOR, Alexandria, VA 22300, U.S.A.
Propagation of intense relativistic electron beams (REB) in air at low lr2 and at atmospheric pres- sures, has been studied. Such propagation depends strongly on the interaction of the beam with the atmosphere. We have studied the effect of a high current density (5-10 kA/cm2) pinched REB on the atmosphere. Beam characteristics such as radialand longitudinal beam current distribution and energy loss in the atmosphere, as well as such response of the atmosphere as the formation of ionized and ex- cited species, were measured and compared to cal- culated values.
50 nsec beams with currents S60 -andelectron energies 22.5 MeV were generated using the NRL VEBA pulser. Beams were propagated in the atmosphere
@-ROGOWSKI COIL
DIODE CURRENT / PROBE
FARADAY CUP
1
ELECTRON BEAM
v o A A G E PO$TION P o s l T l o N
PROBE 2
Fig.1: A REB propagating in the atmosphere.
with and without a current return screen. Fig.1 shows the diode configuration in relation to the electron beam propagating without the current return screen. Electrical properties were measured using Rogowski loops, with diameter of 14 cm, and a vari- able-aperture Faraday cup (combined with a calori- meter) at positions shown in Fig.1. Exposures on
thin cellophane films placed in front of the Fara- day cup and an x-ray pinhole camera viewing thebeam side-on (Fig.2) correlate with the radial current distribution measured by the Faraday cup. Radiation output in the visible range was measured with time- integrated photography (Fig. 1) and spectroscopy
(3000 A s A s 6000 1 )
.
Two of the observed lines N+
2A 3371
1
and N2 b9141
were time-resolved and time-correlated with the beam current. No emission from oxygen nor any other species except N2 and N: was observed. The N line intensity, beam current2
and diode voltage are shown in Fig. 3. Beamdeposi-
\
CAMERA RESOLUTION: 0.5 cmFig.2: X-ray pinhole photograph of REB in the air.
*work supported by the Office of Naval Research. tion in the atmosphere was inferred from dE/dx values and confirmed by estimates derived from
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797373
readings of collimated thermo-luminescent devices measuring the x-rays emitted by the atmosphere.
A typical propagated beam has maximum parti- cle energy of 2 MeV and an injection current I b =
30 kA, corresponding to a Budker parameter v/V = 0.3.
at 20 cm from the anode, the beam current density (Ib) half-width is 1.5 cm. Distances between beam foci (pinches) observed in Fig. 1 and Fig. 2 agree with calculated values of the betatron wave length, 1
A B . The hose instability, associated with the u2 2
magnetic diffusion time T = =
a
C8 for beam 2c 8v,displacement1 occurs at a distance of about 30 cm (-5% of the electron range). For the first-25 cm of propagation, the beam is well confined with a peak current density, jb -10 k ~ / c m 2
,
dE/dx-2 kV/cm,and -1 ,T/cm3 is deposited into the air.
DIODE VOLTAGE
(MV) 2.5
CURRENT 30
0 INTENSITY
( 3 3 7 1 i ~ (W/cm
30
Fig.3: Time histories of selected parameters The response of the atmosphere to the beam is malnly through primary ionization processes (beam electrons) followed by thermalization with low energy secondary electrons. To follow these inter- actions, CHMAIR code, an NRZ, sea-levelair chemistry
code, was used. The code indicates that for j b -10 k ~ / c m ~ and realistic current shapes, secondary electron densities of n -1016 cm-3 are achieved.
Also excited state populations (N2 and N)' corres- 2
ponding to excitation temperatures of -5 eV are attained even though the secondary electron temp- erature is o n l y 4 . 3 ev. Thus, time histories of the visible emissions (N2 and N+) follow thecurrent
2
pulse shape (Fig.3) but are substantially modified by absorption at the current peak. (The indicated source intensity of -30 w/cm3 for the N2 2P (0-0) band is the apparent value deduced using the optic- ally thin approximation.) The gas temperature at the end of the current pulse is only 4 0 0 K - t h e de- posited energy being mostly stored in molecular
dissociation. However,as the heated air column thermalizes and expands to pressure equilibrium, the gas temperature should approach-1500 K.
The inductive field, EZ, associated with the beam rise time can set up a counterstreaming cur- rent with density j = neevd. The upper limit of js is -3.5 kA/cm 2 (i.e. about 35% of jb) for E
-2 kV/cm and assuming no gas breakdown. The net current, I = Ib
-
Is, measured by the Rogowski coil at position 1, is within 10-15% of I measured in-b
side the diode, suggesting that I is not signifi- cant. Similar measurement at position 2 (in ~ i g - 1 ) shows agpreximtely 30% lower current (within the 14 cm diameter coil). This decrease appears to be due to beam current loss from scattering, as evi- denced by x-ray emission from the coil surfaces.
Similarly 1 and the identical shapes of currents
B
in the diode, and at positions 1 and 2 suggest that Is is small. Finally, addition of the return
screen did not affect the net current measurements.
1. E.J. Lauer et al, Phys Fluids
