Being one of the original problem areas for which the VLA was designed, studies of radio lobes benefit more subtly than other arenas from the proposed VLA enhancements. We note, however, that it now apparent that the lobes of many radio galaxies and quasars are highly filamentary. This increases the need for high-resolution imaging of lobes with good sensitivity. The mechanism for the filamentation, which is very common if not ubiquitous, is unclear. It may be attributable to large-scale (thrashing) instabilities in the jets, to velocity shear in the lobes, or to a variety of thermal and nonthermal cooling instabilities in the lobe plasma. Both (continuum) spectroscopy and polarimetry of the filaments in sources of different powers and in different environments will help to pin down the origin of the filamentation. Contending mechanisms will cause very different effects at high frequencies: if filaments are merely regions of higher field, their spectra will steepen at high frequencies; if they are due to particle acceleration, they should be bright at high frequencies. High-frequency polarimetry with enough sensitivity and resolution to determine the orientations of the magnetic fields within in filaments is also important to understanding the mechanism. To make progress in this arena, sensitive, high-resolution images are needed at both lower, and higher, frequencies than are optimal with the current VLA (4.9 and 8.3 GHz). The A+ configuration is needed at the lower frequencies, and improved sensitivity (increased bandwidth, lower system temperatures) at the higher frequencies.
The depolarization properties of the lobes, which appear to be dominated by a foreground, turbulent magnetized medium around them, offer a probe of the conditions in this medium. The medium is associated with the host galaxy in some cases, with the host cluster in others. The depolarization asymmetries may also help to constrain the inclinations of sources to the line of sight, at least on a statistical basis. Depolarization imaging would be greatly helped by a sensitive 2.4 GHz system (as mentioned elsewhere, the ``gap" between 1.5 GHz and 4.8 GHz in the space that is relevant to polarimetry is unfortunately large) and for some sources the Faraday-thick regime is evidently below 1 GHz, so the 0.6 GHz system would be valuable. Detailed investigation of the depolarizing medium requires resolution of the intervening screen. This has not been achieved in many cases and for sources with low rotation measures may be contingent on high angular resolution at low frequencies (the A+ configuration).
Some objects, all Fanaroff-Riley Class I (plumed) galaxies, show hints of internal depolarization below 1 GHz. Confirmation of this phenomenon, which is important for testing models of jet deceleration by entrainment, needs sensitive, high-resolution, polarimetry at 1.4 GHz and below and would benefit from the high-efficiency prime-focus feeds. More speculatively, might consider what is needed to detect the intrinsic mass flux of the jets of high luminosity sources, in which the Faraday-thick regime is likely to be 300 MHz. The A+ configuration at low frequencies offers the best chance to estimate jet mass fluxes directly by detecting internal depolarization of the lobes.
Studies of radio lobes do not drive the VLA Development Plan particularly hard, but they will benefit from improved resolution (A+ configuration), sensitivity and frequency coverage at the lower frequencies and from improved sensitivity and the E configuration (for scaled-array studies) at high frequencies.