The ionospheric modification experiments are providing the means for understanding mechanisms and physical processes leading to the generation and evolution of ionospheric irregularities. We present experimental results concentrating on the features and evolution, generation conditions and mechanisms of small-scale field-aligned irregularities (FAIs) in the high latitude ionosphere F region induced by the controlled injection of the powerful HF radio waves from the ground into the ionosphere. Experiments have been carried out at the EISCAT HF Heating facility near Tromsø, Norway, at heater frequencies of 4.5 – 8.0 MHz with an effective radiated power of 200 – 750 MW. HF pump wave with ordinary (O-mode) or extraordinary (X-mode) polarization was injected along the magnetic field-aligned direction. Instrument diagnostics included the CUTLASS (Co-operative UK Twin Located Auroral Sounding System) radar, the European Incoherent Scatter (EISCAT) UHF radar at 931 MHz near Tromsø and the EISCAT ionosonde (dynasonde). It was found that the features and physical driving mechanisms of FAIs with the spatial scale across the geomagnetic field of 7.5 – 15 m are significantly different for O- and X-mode HF pumping, presenting challenges for understanding the relevant processes. The main attention is paid to the recently discovered X-mode FAIs. By a contrast to the O-mode FAIs excited by a thermal parametric (resonance) instability at the upper hybrid resonance altitude, the X-mode FAIs are generated via two-step process. In the first step the generation of elongated large-scale irregularities (with the spatial scale across the geomagnetic field of the order of 1 -10 km depending on the background geophysical conditions and the pump frequency) is occurred. As a second step, we suggest that the filamentation instability can be responsible for the generation of small-scale FAIs. As is found from EISCAT UHF radar measurements FAIs greatly impact on the development of strong artificial turbulence such as Langmuir and ion-acoustic plasma waves.