High Voltage Coilgun

[FonziDaytona]

Other than the actual coil arcing over, are there any reasons why not to use high voltages for a coil gun? I know that most people use semiconductors for switching, so I’m thinking that’s the reason? I have a Jennings contractor rated for 50kV and 100kA pulses that I could use to switch my 3.2kV 288uF capacitor bank. Assuming the coil can hold off the voltage, is there anything else to consider?

-Matt

[davekni]

Assuming the coil can hold off the voltage, is there anything else to consider?

Safety of course.  If no arcing within coil, high voltage can be better.  Corresponding higher coil inductance makes parasitic wiring inductance (leads to switch etc.) waste less of the capacitor's energy.

[FonziDaytona]

Safety for sure, just figured there were other good reasons that I’m not seeing any coilguns based (essentially) off a can crusher setup. I’ve been crushing cans and figured this might make an interesting add-on for when I run out of cans to crush. :-)

-Matt

[davekni]

I’ve been crushing cans and figured this might make an interesting add-on for when I run out of cans to crush. :-)

Yes, definitely a good use for your can crusher.  I used my coin-shrinking setup to try some aluminum disk launching and impulse forming.  Achieved about 330m/s for small aluminum disks.

[FonziDaytona]

Wow, that’s pretty fast! How many joules does your capacitor bank have?

One thing I thought about was the speed of my vacuum contractor. It’s mechanical so, slow compared to semiconductor. Is that going to be a problem for the “turn off” after firing?

-Matt

[davekni]

Wow, that’s pretty fast! How many joules does your capacitor bank have?

My ancient paper/oil/foil capacitor is 14uF and rated for 20kV, so 2800J.  I sometimes slightly over-charge to get 3000J.  Don't recall if my launching was at 2.8kJ or 3kJ.

For launching small objects at high magnetic field, conductive materials such as aluminum work better than ferromagnetic materials.  Cobalt alloys can achieve 2.4T saturation field.  1.6T to 2T for more common iron/steel.  If external coil field is 10T, can block much of that field due to eddy current, perhaps 8T worth.  That makes stronger repulsion than 10T to 2.4T attraction.  Eddy current launching required very fast pulses.  My coin shrinking discharges are 17us.  I think the slightly larger aluminum launching coil discharged the capacitor in about 30us.  If pulses are long, eddy current decays before sufficient energy is transferred.

BTW, flat disks are efficient shapes for eddy current launching, but terrible projectiles.  I measured velocity by having disk break a trip wire about 100mm above launching position.  Close enough to hopefully not have slowed down too much due to air drag.

One thing I thought about was the speed of my vacuum contractor. It’s mechanical so, slow compared to semiconductor. Is that going to be a problem for the “turn off” after firing?

I've always used capacitor discharge time to control turn-off.  Even my lower power coil guns are that way.  (Launching foam-covered steel pieces for classroom demonstrations.)  Use thyristors for switching those lower-power units, so no turn-off option.  That's likely the only option for you too.  Mechanical contacts are going to be much too slow, unless perhaps a high-speed rotary spark gap.

[FonziDaytona]

Would some beefy diodes help compensate for a very slow turn off? I’m assuming the problem is that the projectile is going to be sucked back into the coil when the field reverses, so some diodes could be used to block this reverse voltage, no? I have some diodes that can handle over 10kA peak (I think they’re actually good up past 20kA).

-Matt

[Karmaslap]

Most people don't have high voltage equipment and can't safely operate it even if they did. That said, induction coilguns very often do use high voltage. Sandia lab's coilgun models/Darpa's mortar used up to 20kV and amateur ones use whatever voltages people can achieve. Induction coilguns don't have to deal with suckback as much, so unless you're really underdamped the projectile has moved away by the time the current has flipped. You could use a diode to eliminate that, but it's probably not always necessary.

For a reluctance coilgun, by far the most popular for hobbyists to make, you limit the voltage because it gets difficult to shut the current off with high voltage and a big inductor. You want the biggest pulse right as the projectile reaches the center of the coil and for it to last only until the center of the projectile reaches the center of the coil. It's harder to tune that with crazy voltages and currents, since the acceleration will be quick and vary based on exact starting conditions. So, if you're willing and able to play heavily with timing, it's feasible. Also, most people like making portable-size coilguns anyway.

You should totally try making an induction coilgun- post a video of course

[FonziDaytona]

Had to lookup what the difference was between an induction and reluctance coil gun is. If I understood, an induction gun has an iron core inside the coil and launches a ring? Similar to a ring launcher with a flat pancake coil?

-Matt

[davekni]

If I understood, an induction gun has an iron core inside the coil and launches a ring? Similar to a ring launcher with a flat pancake coil?

Iron core is optional.  My lower power penny launchers have silicon-iron core (transformer laminations).  My higher-power high-voltage 3kJ experiments (with coin-shrinking setup) used a flat pancake coil and no iron core.  Field is way above iron saturation of 1.6 to 2T.  Iron core would add little, and need to be powdered iron to handle high frequency without excessive eddy current losses.

[Karmaslap]

Inductance coilgun: induces a current in the projectile and interacts with the resulting magnetic field. Any kind of ring launcher counts too.
Reluctance coilgun: Attracts a ferromagnetic projectile to the launcher's coil.

An iron core in the projectile will cause suckback, so I'd avoid it. An iron core in the launcher (for a single stwge ring launcher or the first stage) will help a little. Never made an induction launcher before, I think that's correct.

[FonziDaytona]

Ahh, okay. So then, a “high voltage” launcher using a pancake coil. I built one a long time ago, but used low voltage. It worked pretty well with hard drive plates. I could launch them up high enough that I couldn’t see them. Just heard them come crashing down next to me :-)

-Matt

[Linda aka MethelinaPills]

That reminds me an old epic idea to use rail-gun-like concept to built the HV Coil-gun:
High-voltage multi-stage accelerator: why is it promising?