! !"# $ $"# % &$# &$! &# ! # !" ! ! "# ! $! ! "# FIG. 2: Beam (green) and plasma (red) electrons in simulation of LCLS 20 pC beam initially focused to $^{σ}$x = 130 nm passage through 3.15 atm H$_{2}$ gas. to the BSI regime, we have used the formalism of Ref. [11] to integrate the ionization rate equations including BSI for hydrogen, assuming a Gaussian-field impulse σ$_{t}$ = 2 fs of 800 GV/m field amplitude. We find ionization occurring again well before ( ∼ 2 σ$_{t}$ ) pulse center. In this scenario, significant ionization rates in H due to BSI ( > fs − $^{1}$) occur when the field exceeds ∼ 40 GV/m. In sum, we thus expect full tunneling ionization of H$_{2}$ gas in the rising edge of the beam, with possible assist from a BSI contribution to rates at higher fields. Of course, there is transverse field variation as well as an induced E$_{z}$ that should be included to fully understand the ionization behavior. This will be done in the simulation code in the future; the experiments then serve to benchmark theoretical and computational models in this newly accessible, extreme regime. In this regard, one sees that the LCLS 2 fs beam may be employed as an unprecedented method for probing the transition from tunneling ionization to BSI in a wide variety of atomic species, by scanning the value of E$_{r,max}$ through change in the focused beam size. This beam represents a tool that thus extends the use of half-cycle-type electromagnetic pulses from the study of ionization in Rydberg atoms [15] to those in the ground state. One may also use these beams in the vicinity of solid material to exploit their properties in exciting condensed matter, for investigating, e.g. magnetic switching [16]. The generation of TV/m plasma wakefields means that one may measure beam energy changes with ease. Other experimental beam parameters present measurement challenges. With such short beams, diagnosing the pulse length requires an extension of experimental methods. The current fastest direct measurement of pulse lengths involves use of RF deflec- 6