It is important to emphasize that there is nothing significantly new to be gained by further tests at sub-scale sizes (i.e. less than that needed for net power). This is an inherent consequence of the way in which the fusion power output (Pf) and system gain (Qf. ratio of fusion power to drive power) scale with the machine size (R) and electron-confining magnetic field (B). Fusion power scales as the fourth power of the B field and the cube of the size, thus Pf = (k1)B 4R3, while the unavoidable electron injection drive power loss scales as the surface area of the machine, thus is proportional to R

2. Assuming the use of super-conductors for the magnetic field drive coils, the electron losses are the only major system losses. Then, the ratio of these two power parameters is the gain (Qf), which is thus seen to scale as Qf = (k2) B 4R3/R2 = (k2) B4R. Because of this B4R3 scaling of fusion output, which makes fusion power scale as the 7th power of size, and the corollary 5th power scaling of system gain, it is obvious that little can be gained short of building the next system at full-scale.

 We'll know more when the WB-7 is built and tested.  That's happening now; I would imagine results are done in no more than a year, no less than a few months.  Then, I am told, if those results pan out, they will attempt to build a 100MW full-scale device, for about $200M, and then we'll find out for sure about scaling.

Bussard also mentions the gain scaling in his video. http://video.google.com/videoplay?docid=1996321846673788606