data shown in this article $I_b$ is chosen weak enough to ensure that the beam induces no wave growth and the beam electrons can be considered as test electrons. The SWS is long enough to allow nonlinear processes to develop. It consists in a wire helix that is rigidly held together by three threaded alumina rods and is enclosed by a glass vacuum tube. The pressure at the ion pumps on both ends of the device is $2 \times10^{-9} {\,\rm Torr}$. The $4
{\,\rm meter}$ long helix is made of a $0.3 {\,\rm mm}$ diameter Be-Cu wire; its radius is equal to $11.3 {\,\rm mm}$ and its pitch to $0.8 {\,\rm mm}$. A resistive rf termination at each end of the helix reduces reflections. The maximal voltage standing wave ratio is 1.2 due to residual end reflections and irregularities of the helix. The glass vacuum jacket is enclosed by an axially slotted $57.5 {\,\rm mm}$ radius cylinder that defines the rf ground. Inside this cylinder but outside the vacuum jacket are four axially movable antennas which are capacitively coupled to the helix and can excite or detect helix modes in the frequency range from 5 to 95 [MHz]{}. Only the 