How would you build this...?

[Foxiekins]

Putting this in the Star Hero section, since it's a Sci-Fi weapon...

It forces plasma through a throat between a paraboloidal chamber and the ellipsoidal focussing chamber, to produce a converging tidal bore pulse of plasma that has a go at punching through targets out to around 50 meters or so, after which the plasma rapidly dissipates... The plasma is a mix of Deuterium and Helium-3, and undergoes a microfusion event in the throat as the weapon fires...

If the plasma bolt doesn't penetrate, you wind up with a plasma splash covering the target and its surroundings... Also, if the weapon overpenetrates, you get a cone-shaped plasma splash beyond the target... A miss would also produce the cone-shaped splash once the bolt passed the 50 meter mark... Plasma is very, very hot, in the thousands of degrees...

Any suggestions for how to do this in Hero System...?

[Curufea]

I'm thinking it'd generate a fair smack of radiation as well - mostly in the chamber of the weapon.

Make it a compound power with the area of effect triggered by a miss, I'd say.

[tgrandjean]

Well, it's not exactly what you want but how about this:

Plasma Shotgun
1.5d6 RKA vs ED, Area of Effect: Cone (+1) 50 active, 10 shots (-1/4), Real Weapon (-1/4), Str Min: 10 (-1/2), 2-H (-1/2), OAF: Plasma Shotgun (-1), Reduced Range: 25" (-1/4) 13 real

Optionally, up the damage to at least 2d6 RKA and give it Penetrating.
Anyway, this version will give a 5" cone shaped splash which as GMs disgression takes place at impact or at max range.

[Edsel]

Here is my take on it. A fairly complex build.

Plasma Gun: (Total: 112 Active Cost, 34 Real Cost)
Plasma Stream: Killing Attack - Ranged 2d6, Armor Piercing (+1/2), Area Of Effect (10" Cone; +1) (75 Active Points); OAF (Plasma Gun; -1), Real Weapon (-1/4), Limited Range (25 hexes; -1/4), Limited Power: Begining Of AE Cone Is Mandated By Circumstance, Either It Occurs At Maximum Range Or As Soon As Beam Hits An Obstacle And If The Obstacle Is Not Penetrated AE Cone Advantage Is Lost (-1/4), 12 Charges (-1/4) (Real Cost: 25) plus
Plasma Splash: RKA 2d6, Semi-Armor Piercing (+1/4) (37 Active Points); OAF (Plasma Gun; -1), Conditional Power: Explosion Attack Only Occurs If Plasma Stream Hits And Fails To Penetrate An Obstacle (-1), Real Weapon (-1/4), Limited Range (25 Hexes; -1/4), Limited Power Explosion Does No Damage To Obstacle That Caused Explosion In The First Place (-1/4), 12 Charges (-1/4) (Real Cost: 9)

Basically a 2d6 AP RKA, If it reaches maximum range or hits and penetrates an obstacle it will cause a 2d6 RKA AP 10" AE Cone. If it hits an obstacle and fails to penetrate it, it will instead cause a 2d6 RKA AP Explosion at that point. I think that this will work as you described it but I may be unintentionally breaking a rule somewhere. If it is unclear I can try to explain it all further.

[Lemurion]

...I'd use a sonic screwdriver.

[AlHazred]

...I'd use a sonic screwdriver.
... and a Hydrospanner.

On the other hand, I think I would build it as an RKA vs. ED, with two naked Advantages associated with it - which one applies is dictated by circumstances, as in your original proposal. You can put the Naked Advantages in a Multipower, Linked to the RKA.

[Foxiekins]

That sounds intriguing, but can you link a Multipower to another power...?