The LCLS x-ray FEL occupies the third km of the 3-km SLAC linac, which accelerates electrons in copper cavities pulsed at 120 Hz. For LCLS-2, the first km of linac will be replaced with superconducting cavities driven by continuous RF at 1300 MHz. The normal-conducting photocathode gun will also use continuous RF, at 186 MHz. The laser pulse rate will be variable up to 1 MHz. With a maximum beam power of 250 kW initially, and eventually 1.2 MW, the control of beam loss is critical for machine and personnel safety, especially since losses can continue indefinitely in linacs, and dark current emitted in the gun or cavities can be lost at any time. SLAC protection systems now depend on ionization chambers, both local devices at expected loss sites and long gas-dielectric coaxial cables for distributed coverage. However, their ion collection time is over 1 ms, far slower than the beam repetition rate. We present simulations showing that with persistent losses, the space charge of accumulated ions can null the electric field inside the detector, blinding it to an increase in loss. We also report on tests comparing these detectors to faster alternatives. Show Partial Abstract