  Since the introduction of the radio-frequency (rf) photoinjector electron
source over thirty years ago, peak performance demands have dictated the use of
high accelerating electric fields. With recent strong advances in obtainable
field values, attendant increases in beam brightness are expected to be
dramatic. In this article, we examine the implementation of very high gradient
acceleration in a high frequency, cryogenic rf photoinjector. We discuss in
detail the effects of introducing, through an optimized rf cavity shape, rich
spatial harmonic content in the accelerating modes in this device. Higher
spatial harmonics give useful, enhanced linear focusing effects, as well as
potentially deleterious nonlinear transverse forces. They also serve to
strongly increase the ratio of average accelerating field to peak surface
field, thus aiding in managing power and dark current-related challenges. We
investigate two scenarios which are aimed at unique exploitation of the
capabilities of this source. First, we investigate the obtaining of extremely
high six-dimensional brightness for advanced free-electron laser applications.
We also examine the use of a magnetized photocathode in the device for
producing unprecedented low asymmetric emittance, high-current electron beams
that reach linear collider-compatible performance. As both of the scenarios
demand an advanced, compact solenoid design, we describe a novel cryogenic
solenoid system. With the high field rf and magnetostatic structures
introduced, we analyze the collective beam dynamics in these systems through
theory and multi-particle simulations, including a particular emphasis on
granularity effects associated with microscopic Coulomb interactions.
