(no penetration) at $\alpha \leq \alpha_{\rm cr}$ to $0.17c$ at $\alpha =1$ and then to $0.32 c$ at $\alpha =2$. At $\alpha > \alpha _{\rm cr}$, the magnetic field of the barrier at the moment $t$ roughly is $B(t)\simeq B_0\Theta[x-x_{\rm {pen}}(t)]$ (see Fig. 2). In other words, the front of the beam – barrier interaction is displaced into the barrier with the velocity $v_{\rm pen}$. For $\alpha > \alpha _{\rm cr}$, our consideration of the beam – barrier interaction in the vicinity of the new front, $x\simeq x_{\rm {pen}}$, is [*completely self-consistent*]{}, and no simplifying assumptions besides geometrical ones are exploited.

3\. At the front of the barrier, $x\simeq x_{\rm {pen}}(t)$, the surface current varies in time because of strong nonstationarity of the beam – barrier interaction at $\alpha\sim 1$, and low-frequency electromagnetic waves are generated (Fig. 2). The typical frequency of these waves is about the proton gyrofrequency $\omega_{Bp}=eB_0/m_pc\Gamma_0$ in the field of the barrier $B_0$. The wave amplitude $ B_w$ can reach $\sim 0.2 B_0$.

4\. At $\alpha \sim 1$, strong electric fields are generated in the vicinity of the front of the barrier, $x\simeq x_{\rm {pen}}(t)$, and electrons of the beam are accelerated in these fields up 