PostgreSQL alpha releases are snapshots of development code. They are intended to preview and test upcoming features and to provide the possibility for early feedback. They should not be used in production installations or active development projects. While the PostgreSQL code is continuously subject to a number of automated and manual tests, alpha releases might have serious bugs. Also features may be changed incompatibly or removed before beta releases.
There are no alpha releases in the PostgreSQL 9.6 version. The developers have chosen to proceed directly to 9.6 beta.