FAQ - Pressure needed in a fusor

If you have a question about this topic, the answer is probably in here!
Post Reply
User avatar
Richard Hull
Site Admin
Posts: 10810
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

FAQ - Pressure needed in a fusor

Post by Richard Hull » Tue Feb 25, 2014 7:41 pm

There is no such thing as a pure vacuum only varying degrees of pressure. We need a special vacuum pumping system for fusor operation as fusion occurs in the fusor at a greatly reduced pressure. That pressure must be one of deuterium gas only! We must remove as much of the atmosphere in the fusor vessel as is possible and back fill to a fusion pressure with deuterium gas only.

Fusion, in a fusor, operates in a delicate and narrow range of pressures where a glow discharge can be established and maintained. This range is between 1 and 20 microns or, 1X10-3 torr and 2X10-2 torr. The best and most successful run pressures are between 5 and 15 microns in a common 6 inch to 8 inch fusor.

There is a critical balance between achieving a pressure where the fusor can actually operate and the values of applied voltage and current demanded to do fusion to advantage.

Too much pressure and the voltage can't be raised into a decent fusion range without drawing too much current and melting the grid. If using too little pressure, almost no voltage that the amateur can supply will operate the fusor in glow mode.

At the bottom end of fusion pressures, in glow mode, (about 4-5 microns), you can get a glow at some very high voltages, but the current might be limited and reduced fusion will be seen, in spite of superb cross section in the process, as the fusor is starved for fuel (deuterium). At the higher, easier end of glow mode, (15 microns or more), the fusor and current must be "conditioned-in" by a skilled operator over a period of time ,(1/2 hour to 1 hour) in order to achieve amazing results where both the voltage and current are allowing the greatest possible fusion to occur.

Only true operation by the amateur can show and refine this process. This is why the demo fusor is a good idea as it trains the person in the dynamics of glow mode fusion via hands-on experience.

related FAQs on the voltage and current needed to do fusion are found at:

viewtopic.php?f=29&t=9171&p=62349#p62349

viewtopic.php?f=29&t=4843

Pressure units - A discussion

I have given my thoughts in the past regarding pressure units solely for use by fusioneers using the fusor. We are not highly technical or the types to write papers for Nature. We are forced into two worlds in our quest for vacuum. One of technical vacuums and the other the scientific vacuum.

For our purposes and for the sake of most reading, we might encounter, the micron and the torr. These are all we need concern ourselves with.

All fusors run solely in a technical vacuum.....Above 1 micron. All fusion pressures can be expressed in microns. Why?.... The cheapest and most used gauges found surplus are TC, (thermocouple gauges). Virtually all of these are in microns. Thus, we work, do fusion and talk mostly in microns in a technical vacuum.

However, most of us strive with diffusion and turbo pumps to achieve some sort of scientific vacuum level in our chambers before introducing the fusion fuel, (deuterium). As this is the case, we might resort to the time honored Torr found in the bulk of classic literature on vacuum. Thus, we might speak of achieving 10e-5 or 10e-6 torr as a base pressure for those advanced enough to speak in scientific notation using torr-speak. This is proper when discussing deeper vacuums than fusor operational pressures.

The beauty of the micron is that when operating a fusor we can use whole numbers. (5, 12, 25 microns). We have an intimate grasp of these pressures due to the base level of most fore-pumps and inexpensive vacuum gauges which we deal with every day. We know that we must be well below 50 microns before starting our diffusion or Turbo pumps and that regardless of where they may take us in the "torr" range, we must add deuterium to at least 5 microns before attempting fusion.

Likewise, there is little sense in using .03 microns. If we are going to use fractional units, use torr as the whole point of using microns was to escape fractional units in fusion operating pressures, preferring whole number units.



Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

User avatar
Werner Engel
Posts: 142
Joined: Sun Aug 18, 2013 3:51 pm
Real name: Werner Engel
Location: Vienna, Austria
Contact:

Re: FAQ - Pressure needed in a fusor

Post by Werner Engel » Sun Apr 27, 2014 2:39 pm

Dear European fusioneers:
As I'm always confused with microns and Torr, here are the european numbers of the pressure range:

It's 1,3x10-2 mbar to 2,6x10-1 mbar.

PS: WolframAlpha does not understand "micron" as a unit for pressure but unfortunatly reads it as distance.

User avatar
Chris Bradley
Posts: 2931
Joined: Fri May 02, 2008 11:05 am
Real name:

Re: FAQ - Pressure needed in a fusor

Post by Chris Bradley » Sun Apr 27, 2014 3:01 pm

When you say 'European' you allude to a misinformation. There are no 'standard European' measurements. Europe largely adopts the International SI system of units. There is no 'mbar' unit in SI. The unit of pressure in SI is the 'newtons per square meter' which is a derivative unit named the 'pascal'.

In fact, the closest to a 'European' measurement of a unit of vacuum pressure is the 'torr', named after Evangelista Torrlicelli, the Italian inventor of the mercury column barometer. The 'torr' came about upon the definition of a millimetre, as one torr is the pressure differential in one millimetre of a mercury column. We use 'torr' here routinely for anything below one micron expressed in a based 10 exponent, as is very common practice in scientific literature (either torr or pascals). You will find no scientific literature referring to 'mbar' as a unit in conjunction with vacuum pressures.

Following from one torr, one micron is the pressure differential in one micrometer of a mercury column, and it is more convenient to write 'micron' than continually '1x10^-3 torr' in the particular range of interest to us 'glow-discharge'-ists.

Once below a micron, then the use of base 10 exponents becomes required, in which case one might as well stick to Torr, which is equally true above 1,000 torr also. Hence; generally we use 1 to 1,000 microns here which is our particular pressure range of interest, and 'torr' for pressures outside that range.

User avatar
Rich Feldman
Posts: 962
Joined: Mon Dec 21, 2009 11:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: FAQ - Pressure needed in a fusor

Post by Rich Feldman » Sun Apr 27, 2014 5:22 pm

The bar is one of those metric units widely popular in spite of a root name different from the SI unit.
It's analogous to the hectare (1e4 m^2) for measuring land areas,
or (historically) the micron and the angstrom unit for measuring short lengths.

Bars are routinely used for tire pressures etc.
Millibars are routinely used in weather recording and forecasting;
perhaps not in scientific vacuum discourse.

A bar is exactly 100 kPa, around 750 torr, barely less than one atm.
So a mbar is exactly 100 pascals (the natural SI root unit),
around 3/4 of a torr or 750 microns of pressure.
Richard Feldman

User avatar
Richard Hull
Site Admin
Posts: 10810
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: FAQ - Pressure needed in a fusor

Post by Richard Hull » Sun Apr 27, 2014 5:46 pm

In many posts I have noted that for we fusioneers, the micron is the unit to use when fusing or demoing. Bottom pumping pressures should use the Torr in scientific notation. If any here are writing or preparing papers for submission to the Royal Society, or Nature, you better use SI units. In this manner you may get the paper approved for publication and send most older vacuumists reading your paper to the conversion tables to figure out what the real pressure is in Torr or microns.

Now, how many here know the mass of your fusors in slugs or the number of poundals needed to push it to a final velocity of 60mph?

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

User avatar
Rich Feldman
Posts: 962
Joined: Mon Dec 21, 2009 11:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: FAQ - Pressure needed in a fusor

Post by Rich Feldman » Sun Apr 27, 2014 9:04 pm

Werner Engel wrote:Dear European fusioneers:
As I'm always confused with microns and Torr, here are the european numbers of the pressure range:
It's 1,3x10-2 mbar to 2,6x10-1 mbar.
Oops, Werner has misplaced a decimal point.
If he's translating Richard's range of 0.001 to 0.020 torr,
the corresponding range of mbar is 1,3x10-3 to 2,6x10-2.

Millibars are almost the same as torrs, the ratio being about 3/4. That's only 1/8 order of magnitude.
Here's one reference: http://www.unitconversion.org/pressure/ ... rsion.html

In the field under discussion, nobody cares about precision of more than 2 significant digits.
But that online calculator gives 10 significant digits.
The "official" conversion factor must implicitly depend on an adopted density of mercury at some temperature,
and an adopted value for Earth gravity at some latitude.
[edit] The torr is now defined as 1/760 atm = 101325/760 pascals.
Richard Feldman

User avatar
Chris Bradley
Posts: 2931
Joined: Fri May 02, 2008 11:05 am
Real name:

Re: FAQ - Pressure needed in a fusor

Post by Chris Bradley » Sun Apr 27, 2014 9:56 pm

Rich Feldman wrote:The bar is one of those metric units widely popular in spite of a root name different from the SI unit.
I beg to differ. It has never been a unit of the SI/metric system.

The bar was a unit introduced by the UK's Meteorological Office in the 20th century, and adopted by the IUPAC who set its value at 100kPa in the 1980's.

The IUPAC is located in Switzerland which is nether a member of the European Union nor even of the European Economic Area.

The 'bar' is, therefore, neither metric, nor European. Its use is merely a habit of particular manufacturers/industries, and does not relate to any widely adopted measurement system.

User avatar
Rich Feldman
Posts: 962
Joined: Mon Dec 21, 2009 11:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: FAQ - Pressure needed in a fusor

Post by Rich Feldman » Sun Apr 27, 2014 11:15 pm

Not so fast, Chris. You are suggesting that "metric" implies SI. To the contrary, the metric system predates SI by more than a century. It was legalized in the USA in 1866. My CRC handbook, 1975 edition, does not even mention SI, nor the pressure unit pascal. It does give conversion factors for bars, torrs (same as mm of Hg at 0 degrees C and, implicitly, standard gravity), atmospheres (1.01325 bar; 760 torr), Baryes (dynes per cm^2), etc.

The liter and gram are obviously metric,
but the SI base unit of volume is the cubic meter
and the SI base unit of mass is the kilogram. Go figure.

I will continue to interpret any unit as metric if its definition is some power of ten
times some unit of SI or its predecessors. That includes liter, barn and are and hectare,
angstrom unit and micron, barye (I think same as decipascal) and bar, etc.

-Rich
Richard Feldman

User avatar
Chris Bradley
Posts: 2931
Joined: Fri May 02, 2008 11:05 am
Real name:

Re: FAQ - Pressure needed in a fusor

Post by Chris Bradley » Mon Apr 28, 2014 7:36 pm

(It is a point of pedantry perhaps, but, hey, this is scientific history which I like!!)

...Still, you have provided no link between 'the metric system' and the 'bar', apart that those words appear in the same book! Again I reiterate that 'the bar' did not exist until it was invented by the Met Office in the early 1900's, so could never have been a part of any metrification before then.

The original 1866 US Act does not include any indication of a 'pressure'; http://lamar.colostate.edu/~hillger/law ... -bill.html . The 'bar' has never been defined in any US Act defining measures. 'Weight per area' is the only way of expressing pressure under the 1866 Act.

The only amendment to that Act of relevance is 2007, 15 USC 205, which states "Sec. 205. Metric system defined: The metric system of measurement shall be defined as the International System of Units as established in 1960, and subsequently maintained, by the General Conference of Weights and Measures, and as interpreted or modified for the United States by the Secretary of Commerce."

In fact, of interest one of the original architects of the metric system, John Wilkins, first secretary to the Royal Society, considered and rejected using atmospheric pressure as a metric measure in the late 1600's. He rejected it when he saw the work of Torricelli, bringing us full circle back to the Torr as the only real claimant to a 'metric' unit of pressure, if ever there was one.

Post Reply