Does practicing masturbation when one's desires are intense and there is a fear of committing fornication, invalidate the fast during the day in Ramadan?

Mere arousal of lust does not invalidate the fast unless the accountable person intentionally invalidates it through masturbation or the like. Thus, the emission of semen by the choice of the accountable person is what invalidates it. As for the emission of madhy (pre-seminal fluid), or the emission of semen merely through thought, it does not invalidate the fast. It is advisable to supplicate, strive against one's desires, and associate with righteous women to get rid of masturbation.