Other acts on the lineup include Twin Atlantic, Creeper, Frank Turner & The Sleeping Souls, Skindred, Saint Agnes and Jamie Lenman.
Guests must take a Covid-19 test before and after the festival, and provide proof of a negative result to gain entry.
Early photos showed fans turning up looking ecstatic despite the poor weather.
The Met Office has issued a yellow warning for thunderstorms in the county until Saturday 19 June, and said to expect spells of heavy rain which may lead to some travel disruption and flooding.
The poor weather is expected to continue across the weekend.
The Download pilot comes after similar events were held in Liverpool in April and May, which the city’s health chief said were “undoubtedly successful”.
They form part of the government’s study to see how Covid-19 transmissions happen in crowds.
“I remember saying halfway through the pandemic, it sort of feels like we’re experiencing what the death of a band would be like, by just not being able to play,” Enter Shikari frontman Rou Reynolds told BBC Radio 1’s Newsbeat.
“I haven’t written any music for like 18 months and the live shows is what I think gives us this fuel as a songwriter.”
“I think there’s going to be a real emotion to the whole thing,” he says.
“This event is effectively helping the return of live music, so it feels like it’s the light at the end of the tunnel.”