Your blast radius encompasses more than the size of earth, plus you evaporate all rock in said planet:
Firstly lets have a look at the centre of the sun - the stuff you want to pour out on your unsuspecting enemies (and quite possibly everyone else). This link (emphasis by me) says:
It has a density of 150 g/cm³ (150 times the density of liquid water) at the center, and a temperature of 15 million degrees Celsius. The core is made of hot, dense gas in the plasmic state (ions and electrons), at a pressure estimated at 265 billion bar (3.84 trillion psi or 26.5 petapascals (PPa)) at the center. Due to fusion, the composition of the solar plasma drops from 68-70% hydrogen by mass at the outer core, to 33% hydrogen at the core/Sun center.
So lets compare that to your standard day on earth:
* Average air pressure of 1.01325 bars.
* Average temperature $16^{o}C$
* Air density $0.0012gcm^{-3}$.
The main driving force here is the difference in pressure being about 265 billion bar higher so the stuff at the center of the sun will flood through and quickly increase the temperature of the surrounding area to $\approx 15 million ^{o}C$ (not quite ideal conditions to support life). 24
After 6 seconds, at this temperature difference we can expect to have boiled off the ground. The rate at which this will happen is given by:
$$ \frac{dT}{dt} = k (T_{s}-T_{e}) $$ Where $T_{s}$ is the temperature of the sun $T_{e}$ is the temperature of earth and k is some proportionality constant (depending mostly on the make up of gasses). Since this isn't hard science I won't go much further into this than to say that we can assume, at these temperatures and pressures that your particles are travelling around the speed of light so if we look at a basic radius calculation of $r= c t$ where r is the radius, c is the speed of light and t is our 6 seconds, we get a radius of $18\times 10^{8}m$, for a sense of scale Earth's radius is $6.4\times 10^{6}m$. Now perhaps some argument can be made for the boiled off rock delaying this rate of travel but you have to reduce it by a couple of orders of magnitude just to make it that you're only just about evaporating earth...you would need to have it open for much less than six seconds to even have a chance of keeping your planet in tact let alone anyone left alive.
Edit: Okay, you changed it from 6 seconds, however the whole idea is still going to wipe out all life unless you put the portal somewhere other than the center of the sun (the surface is around 6000 degrees rather than the ridiculous 15 million). Whatever you do you'll have to deal with the hydrogen gas which will come through hot enough to ignite and, with some tiny margin where you could possibly survive, you will burn off all the oxygen on your planet.
I know you want it to work though, so picking somewhere hot but much less dense (like the surface) is your best chance of keeping a planet intact. 921