extending to near optical wavelengths, complicating the interpretation of such a measure- ment [20]. An even larger difficulty is presented by the destructive nature of the focused beam to conducting surfaces used to create OTR. On the other hand, one can exploit the collective beam fields to indirectly give the beam spot size, by measuring the ionization output of the beam passage through a gas. If several atomic species are used, the beam fields may be deduced, along with an estimation of concomitant σ$_{x}$ . In the end, one does not need to focus more tightly than the spot that produces adequate ionization, − sub-200 nm − in the H case. We note again that this injected beam, mismatched to the ion focusing strength, is initially focused in the plasma, reaching sizes < σ$_{x,eq}$ , and then periodically oscillates with wavelength 2 πβ$_{eq}$ = 0 . 88 mm. At the minimum σ$_{x}$ , its density increases and the assumption of stationary ions can be violated. This effect causes the ions to collapse inside the beam, forming a region of nonuniform, higher ion density n$_{i}$ . This produces in turn nonlinear fields that are potentially damaging to the extremely low ϵ$_{n}$ demanded by linear colliders [21]. This effect is not easily created in existing beams, due to the beam densities needed. In our case, we may enter into a regime of nontrivial ion motion in the current scenario, as illustrated by the OOPIC simulations in Fig. 4. The predicted peak n$_{i}$ is enhanced by a factor of 8 over nominal. GLYPH<cid82>GLYPH<.notdef>GLYPH<cid8>GLYPH<cid77>GLYPH<cid9> GLYPH<cid90>GLYPH<.notdef>GLYPH<cid8>GLYPH<cid77>GLYPH<cid9> GLYPH<cid78>GLYPH<.notdef>GLYPH<cid8>GLYPH<cid77>GLYPH<.notdef>GLYPH<.notdef>GLYPH<.notdef>GLYPH<cid9> GLYPH<cid13> GLYPH<cid19> FIG. 4: Ion density as a function of r and z , with beam located as in same position in window as in Figure 2, showing collapse of distribution (density spiking) behind the beam. However, as the ions do not move until after beam passage, they cannot in this case cause observable growth in ϵ$_{n}$ . This is desirable in terms of demonstrating the maintenance of beam quality in this extremely field scenario. On the other hand, the beam-derived forces 8