Now that you mention this, would an accelerator in space need walls? Could you make it from a series of big circles (maybe solenoids?) at distances of a million

Now that you mention this, would an accelerator in space need walls? Could you make it from a series of big circles (maybe solenoids?) at distances of a million kilometers apart or so? Neptune's orbit is about 15 billion km long, so, you'd need about 15k of such loops. In free fall they would just go around the Sun, so you'd only need to maintain their perfect alignment with some minute adjustments.