I think that the process could be improved by making the sphere permeable only to some compounds or elements.
Maybe the psychic person could heat and energize the air around the sphere and split up the molecules of elements and compounds in the air into separate atoms, and make the hydrogen atoms from water vapor the only atoms that can penetrate the sphere. So there will be a gradual build up of hydrogen inside the sphere, until hydrogen is by far the most common element inside the sphere, and oxygen, nitrogen, carbon dioxide, argon, etc. are only traces.
The free hydrogen that doesn't go into the sphere will react with oxygen and burn, producing heat and water vapor. There should be a very impressive zone of fire and glowing hot air around the sphere, and possibly an impressive zone of fire and devastation on the ground below and around the sphere. The psychic person might get caught in the fire and killed, ending the process.
I suggest that the upper limit of the density within the sphere would soon be reached because the atoms within the sphere would repel any atoms crossing through the permeable sphere. Thus the sphere wouldn't fill up with hydrogen enough to fuse.
Therefore, I suggest a more complicated course of action involving enlarging and shrinking the sphere and changing how permeable it is.
First, create a vast sphere in the air, and make it totally impermeable to molecules, atoms, and ions. Then shrink the sphere to a fraction of its former volume, thus multiplying the density and temperature of the air inside it.Then make the sphere permeable from the inside to the outside. Almost all the super hot air will rush out of the sphere, burning everything around it, and making the interior of the sphere almost a vacuum.
Then make the surface of the sphere impermeable and enlarge it again, making the vacuum inside even thinner. And when the sphere is greatly enlarged, make it permeable only from the outside in, and let only hydrogen in. Then heat up a vast volume of air around the sphere, so that molecules break down into atoms and ions. Only hydrogen atoms can pass through the surface of the sphere into the vacuum inside.
Some of the hydrogen atoms will remain outside the sphere and combine with oxygen. There should be a very impressive zone of fire and glowing hot air around the sphere, and possibly an impressive zone of fire and devastation on the ground below and around the sphere. The psychic person might get caught in the fire and killed, ending the process.
So the almost vacuum within the sphere will fill up with hydrogen atoms until they become so dense that they repeal all hydrogen atoms from entering the sphere. Then rapidly shrink the diameter of the sphere , thus increasing the temperature and pressure, until eventually the temperature and pressure is enough for an instant massive fusion reaction, a hydrogen bomb.
Or maybe stop the shrinking at the exact moment that a slower fusion reaction starts and use it as a power source.
I haven't done any calculations but it would take an immense amount of energy for all the heating up of outside air and all the compression of the size of the sphere.
And I assumed from your question that it would be done using psychic powers, so presumably the psychic person would be magically drawing energy from some unspecified outside source.
Or they could be using a highly advance science and some vast technological machines to do it.