Monday, April 20, 2009

Bussard Polywell Fusion Funded

The news is that the Bussard polywell fusion device is now attracting a major ( for it ) tranche of funding. This must also be an indicator that every possible fusion energy strategy is now getting a fair hearing and real support. The polywell pioneered by the late Dr Bussard had over a twenty year time span never received more than a pittance and that perhaps two or three times. The sheer weight of time ate up any available capital.

This report tells us that Bussard is sorely missed. It also tells us that scaling is difficult, although I am not sure that having a larger device makes things easier or harder or just with more variation emerging.

We now have several fusion programs modestly funded, including work on cold fusion. We are going to be having news on fusion work streaming out over the next twelve months. This is a radical departure from past practice for the physics that has obviously denigrated small budget attempts.

This is welcome news, as is the sudden burst of interest emerging around cold fusion. The lack of neutrons had killed that approach more surely than any other issue. The polywell is a simple device that needs to be tested over several size configurations in order to perfect the theory itself. That then could lead to an efficiency breakthrough.

In other words it is the type of program that you feed two million plus and fresh talents every year to progressively advance the knowledge. Past work has consisted of perhaps two such rounds stretched out over twenty years. I hate when that happens. I have a drawer full of such projects mostly mundane but needing just that.

I can see the navy been very keen on this technology working. In retrospect they are even the best partner. After all they have a natural heat sink available to dispose of surplus energy.

April 16, 2009

Plasma Fusion (Polywell) Demonstrate fusion plasma confinement system for shore and shipboard applications; Joint OSD/USN project. 2.0 [million]

Introduction to Bussard Fusion

This site has covered IEC (Bussard) Fusion many times. Bottom line is that it is one of the most promising technologies for achieving cheap, clean and non-controversial energy within ten years. Success would alter energy production, the world economy, propulsion of ships and other vehicles and enable inexpensive access to space.

IEC fusion uses magnets to contain an electron cloud in the center. It is a variation on the electron gun and vacuum tube in television technology. Then they inject the fuel (deuterium or lithium, boron) as positive ions. The positive ions get attracted to the high negative charge at a speed sufficient for fusion. Speed and electron volt charge can be converted over to temperature. The electrons hitting the TV screen can be converted from electron volts to 200 million degrees.

The old problem was that if you had a physical grid in the center then you could not get higher than 98% efficiency because ions would collide with the grid. The problem with grids is that the very best you can do is 2% electron losses (the 98% limit). With those kinds of losses net power is impossible. Losses have to get below 1 part in 100,000 or less to get net power. (99.999% efficiency)

Bussard system uses magnets on the outside to contain the electrons and have the electrons go around and around 100,000 times before being lost outside the
magnetic field.

The fuel either comes in as ions from an ion gun or it comes in without a charge and some of it is ionized by collisions with the madly spinning electrons. The fuel is affected by the same forces as the electrons but a little differently because it is going much slower. About 64 times slower in the case of Deuterium fuel (a hydrogen with one neutron). Now these positively charged Deuterium ions are attracted to the virtual electrode (the electron cloud) in the center of the machine. So they come rushing in. If they come rushing in fast enough and hit each other just about dead on they join together and make a He3 nucleus (two protons and a neutron) and give off a high energy neutron.

Ions that miss will go rushing through the center and then head for one of the grids. When the voltage field they traveled through equals the energy they had at the center of the machine the ions have given up their energy to the grids (which repel the ions), they then go heading back to the center of the machine where they have another chance at hitting another ion at high enough speed and close enough to cause a fusion.

Discussion Board Technical Details From IEC Fusion Research Lead Dr Nebel

Some technical comments from Dr Nebel

A few comments on scaling laws….

To a certain extent we are in the same boat as everyone else as far as the previous experiments go since Dr. Bussard’s health was not good when we started this program and he died before we had a chance to discuss the previous work in any detail. Consequently, we have had to use our own judgement as to what we believe from the earlier experiments and what we think may be questionable. Here’s how we look at it: 1. We don’t rely on any scaling results from small devices. The reason for this is that these devices tend to be dominated by surface effects (such as outgassing) and it’s difficult to control the densities in the machines. This is generally true for most plasma devices, not just Polywells.

2. Densities for devices prior to the WB-7 were surmised by measuring the total light output with a PMT and assuming that the maximum occurred when beta= 1. We’re not convinced that this is reliable. Consequently, we have done density interferometry on the WB-7. We chose this diagnostic for the WB-7 because we knew through previous experience that we could get it operational in a few months (unlike Thomson scattering which by our experience takes more than a man-year of effort and requires a
laser which was outside of our budget) and density is always the major issue with electrostatic confinement. This is particularly true for Polywells which should operate in the quasi-neutral limit where Debye lengths are smaller than the device size.

3. As discussed by several people earlier, power output for a constant beta device should scale like B**4*R**3. All fusion machines scale this way at constant beta. Input power scales like the losses. This is easy to derive for the wiffleball, and I’ll leave that as an “exercise to the reader”. This is the benchmark that we compare the data to.

4. As for Mr. Tibbet’s questions relating to alpha ash, these devices are non-ignited (i.e. very little alpha heating) since the alpha particles leave very quickly through the cusps. If you want to determine if the alphas hit the coils, the relevant parameter is roughly the comparison of the alpha Larmor radius to the width of the confining magnetic field layer. I’ll leave that as an “exercise to the reader” as well.

Loss fraction = (summation (pi*rl**2))/(4*pi*R**2) where rl is the electron gyroradius and R is the coil radius. The summation is a summation over each of the point cusps. If you calculate rl from one of the coil faces, then there are "effectively" ~ 10 point cusps (fields are larger in the corners than the faces). The factor that your observed confinement exceeds this model is then lumped together as the cusp recycle factor.

The other model is to look at mirror motion along field lines. For this model you look at loss cones and assume that the electrons effectively scatter every time they pass through the field null region. This model describes the confinement which was observed on the DTI machine in the late 80s.

I don't know how to predict cross-field diffusion on these devices. The gradient scale lengths of the magnetic fields are smaller than the larmor radii and the electrostatic fields should give rise to large shear flows. On top of that, the geometry is 3-D.

The mirror model is a bit of a handwaving model that I believe Nick Krall came up with. The mirror ratio is calculated from the field where the electron Larmor radius is on the order of the device size. Any smaller field than that will not have adiabatic motion. If particles enter the field null region, it is assumed that they effectively scatter. I believe that Dave Anderson at LLNL did a fair amount of particle tracing calculations for FRMs in the late 70s, and not surprisingly saw jumps in the adiabatic invariants when moving through field null regions. I presume similar behavior was observed on FRC simulations. Anyway, it's a ballpark model.

My other comment was related to electrons trapped in the wiffleball. Over most of their orbit there is little or no magnetic field (i.e. Larmor radius bigger than the device size) with the electrons turning when they hit the barrier magnetic field. The electron behavior is stochastic since there are no invariants. We don't have any direct measure of the internal magnetic fields, but we do know the density and have a pretty good idea what the electron energy is. High beta
discharges should expel the magnetic field. The vacuum fields should be in a mirror regime (as was the DTI device) while the wiffleball fields should transition to better confinement. There is about 3 orders of magnitude difference in the predicted confinement times so it's pretty easy to see which regime the device operates in (unless, of course, the cusp recycle is truly enormous).

As you suggest, Bohm diffusion is kind of a catch-all for any kind of confinement you don't understand. We hope we don't end up there, and so far we're OK.

If you are interested in pumps, the specifications for ITER can be found at: . If I am reading this correctly, the pumping power is about 60,000 liters/second. This is ~ 30 times more than the WB-7. It doesn't take a lot of power. Our system takes ~ 500 watts of power. ITER probably requires 10-20 kW.

1 comment:

M. Simon said...

Thanks for the link!