This is the final beam weapon and, other than the corresponding Missile Combo, the final upgrade before facing the boss of Phazon Mines. Aside from Chozo Artifacts, you've gotten nearly everything there is to get outside the mines, and we'll be wrapping up what's left here.
Return to the Geothermal Core, and get to the entrance from South Core Tunnel. Scale the ledges near the door, then locate a Grapple Point under a large circular platform attached to a pillar in the middle of the room. Swing to another circular platform across the room. Note, getting this the first time is fairly easy, but later when the position of the platforms has changed you may find it more difficult. If you have trouble, go two rooms away and come back to return the platforms to their original positions.
There is another platform nearby, but before going there use the Spinner on the platform you're on to raise the upper half. You'll be making the rounds of platforms here and if you miss a Spinner you'll get stuck somewhere further on. When you get to the second platform, activate the Spinner and jump to the upper half of the first platform.
Jump from there to a third platform (the one with the Grapple Point under it), then activate the spinner and follow the Spider Ball track to the upper half. Now jump to the upper half of the second platform. It looks like all that's left to do here is to activate a Morph Ball slot here, but when you do some machines at the tops of the pillars lift the ceiling and reveal a huge Spider Ball maze which circles the room. Drop bombs to clear the Plated Parasites when needed, but other than that you just have to use the skills you've learned up to now and going into detail won't be much help.
When you've circled the room you land on a ledge with a door, so go through.
This floor of this room is mostly lava, so pick your way carefully to the central island and pick up the Plasma Beam. This is the most powerful Beam Weapon in the game, something that won't be entirely obvious because you can already one-shot many enemies. So try it out on something like a Sheegoth to truly appreciate what you've got now.
There is a red door to get out, and the Plasma Beam opens these as well as Blue doors.
Expansions (Phendrana Drifts)
It turns out that the Plasma Beam is the perfect weapon for dealing with ice, and since Phendrana Drifts has plenty of ice it's hiding many treasures that can be collected now. So it's time to take final tour of Phendrana which will cover nearly all of the area. There are a few Chozo Artifacts here which you might as well get as long as you're here, and we'll mention them on the way but details will be in the Chozo Artifacts page. Since you're probably coming from the Geothermal Core, we're going to start from the nearest transport, which takes you to Transport to Magmoor Caverns South. Proceed to Transport Access.
The last time you were here you may have noticed an Energy Tank behind a wall of ice. Now that you have the Plasma Beam just shoot the ice to break through and collect the tank. This is the next to last tank in the game and you can only get the remaining one after the next boss battle.
If you're collecting Artifacts then go to Phendrana's Edge via the Frost Cave to get one near there, otherwise go straight to the Gravity Chamber.
Get to the door from Chamber Access and scan an Ice Stalactite above you to learn that it can be destroyed with heat. So fire the Plasma Beam at it to make it disintegrate and reveal a Grapple Point. Fire the Grapple Beam and swing to the left to a nook with a Missile Expansion.
Continue back to the Frozen Pike. If you're collecting Artifacts now then make a left to the Research Core and continue through the Pirate base to get one in the Control Tower. (The Plasma Beam should make short work of the Pirates you meet here; it almost makes you feel sorry for them.) Otherwise it will be easier to go through the Quarantine cave to reach the Ruined Courtyard. Then Continue to Ice Ruins West.
Ice Ruins West
Starting at the door from the Courtyard Entry way, jump forward to the second ledge and notice an ice covered hole. Fire the Plasma Beam at it to break it open and collect the Power Bomb Expansion inside. If you're careful you may be able to do all this without waking up the Sheegoth who may shake you off the roofs. If you do fall then the Sheegoth is fairly easy to defeat with the Plasma Beam and you can climb back up again.
Go back to Phendrana Shores and return to Ice Ruins East.
Ice Ruins East
Clear the Sheegoth so you won't be bothered, then go under the ledge with the door to the Plaza Walkway and check to the right to see a wall of ice. Shoot it with the Plasma Beam and collect a Missile Expansion.
Return to Phendrana Shorelines again. Check the back of a platform just outside the save station cave to find an ice wall. Break through as usual and collect yet another Missile Expansion.
There are no more expansion in Phedrnana but there is still an artifact in the Chozo Ice Temple.
Expansions (Tallon Overworld)
There is an expansion you can get with the Plasma Beam in Tallon Overworld with the help of the X-Ray Visor, and another close by for which you only need the Grapple Beam and X-Ray Visor.
Return to the Root Cave and get up to the Root Tunnel entrance. There is a Grapple Point you can swing on to reach a platform on the other side, but you might want to clear the Bloodflower and Geemer up here first; they didn't bother you as long as you stayed on the lower levels but they will object now.
Continue platforming your way up to what appears to be a dead end, then switch to X-Ray and jump the cloaked platforms to a ledge with a pillar on one side. Jump to one more cloaked platform and look to the left in X-Ray to see a Missile Expansion in a nook in the wall; leaves block the view in normal vision so it looks like a solid wall.
Go back to the cloaked platform and continue up to a red door.
There is a Missile Expansion waiting for you here.