In the electron beam–plasma interaction at an electric double layer the beam density is much higher than in the classical beam–plasma experiments. The wave propagation takes place along the density gradient that is present at the high potential side of the double layer. Such a case is studied experimentally by injecting the electron beam from a plane cathode, without any grids suppressing the gradient, and by particle simulations. The high frequency field concentrates in a sharp “spike” with a half width of the order of one wavelength. The spike is found to be a standing wave surrounded by regions dominated by propagating waves. It forms at a position where its frequency is close to the local plasma frequency. The spike forms also when the electric field is well below the threshold for modulational instability, and long before a density cavity is formed in the simulations. Particle simulations reveal that, at the spike, there is a backward traveling wave that, when it is strongly damped, accelerates electrons back towards the cathode. In a simulation of a homogeneous plasma without the density gradient no spike is seen, and the wave is purely travelling instead of standing.

1.
H.
Gunell
,
N.
Brenning
, and
S.
Torvén
,
J. Phys D
29
,
643
(
1996
).
2.
P. Y.
Cheung
and
A. Y.
Wong
,
Phys. Fluids
28
,
1538
(
1985
).
3.
T. M.
O’Neil
and
J. H.
Malmberg
,
Phys. Fluids
11
,
1754
(
1968
).
4.
S.
Kainer
,
J.
Dawson
,
R.
Shanny
, and
T.
Coffey
,
Phys. Fluids
15
,
493
(
1972
).
5.
H.
Gunell
,
J. P.
Verboncoeur
,
N.
Brenning
, and
S.
Torvén
,
Phys. Rev. Lett.
77
,
5059
(
1996
).
6.
D. R.
Nicholson
and
M. V.
Goldman
,
Phys. Fluids
21
,
1766
(
1978
).
7.
H.
Böhmer
,
E. A.
Jackson
, and
M.
Raether
,
Phys. Fluids.
16
,
1064
(
1973
).
8.
L. D.
Bollinger
,
W.
Carr
,
H.
Liu
, and
M.
Seidl
,
Phys. Fluids.
17
,
2142
(
1974
).
9.
S.
Torvén
,
H.
Gunell
, and
N.
Brenning
,
J. Phys. D
28
,
595
(
1995
).
10.
J. P.
Verboncoeur
,
M. V.
Alves
,
V.
Vahedi
, and
C. K.
Birdsall
,
J. Comput. Phys.
104
,
321
(
1993
).
11.
B. H.
Quon
,
A. Y.
Wong
, and
B. H.
Ripin
,
Phys. Rev. Lett.
32
,
406
(
1974
).
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