1. Effect of caesium seeding in negative hydrogen ion sources
Marthe Bacal
UPMC, LPP, Ecole Polytechnique, Palaiseau, France
Roy McAdams and Elizabeth Surrey
CCFE/Euratom Fusion Association, Culham Science Center,
Abingdon, Oxfordshire OX14 DBB, UK
Low Temperature Plasma Teleseminar, December 20, 2013
also presented at the 15th Intern. Conf. on Ion Sources,
9-13 Sept. 2013, Japan

2. OUTLINE I. Motivation II. Why are neutral beams required for fusion ?
Negative ion beams – precursors of high energy neutral beams
IV. Effect of plasma electrode work function
V. Effects of caesium seeding
Causes of the H- ion current enhancement by caesium
Effect of gettering the atomic hydrogen by caesium
Conclusion

3. I. Motivation : magnetic controlled fusion
On the route to sustainable power from magnetic confinement fusion, the International Tokamak Experimental Reactor (ITER) is currently under construction at Cadarache in France.
ITER will operate to produce net output of fusion power that exceeds the heating power by a factor of Q=10 and produce a self-sustaining plasma for several hundred seconds.
However ITER is still an experimental device and will not produce any electricity.

4. What will follow beyond ITER ?
Beyond ITER the DEMO machine will produce electricity and demonstrate the requisite technologies to allow commercial production of electrical power.
The route to fusion power leads from today’s tokamaks such as JET, moving through ITER and DEMO to a commercial fusion reactor.

5. Role of DEMO
DEMO will link a fusion source with electricity generation and will be the last machine before a commercial fusion reactor.
The European Fusion Roadmap calls for construction of DEMO to commence in 2030 at the point where ITER has successfully demonstrated the Q=10 performance.
The European programme is currently considering two options as of July 2013 for DEMO: a “pulsed” and a “steady state” option.

6. II. Why are neutral beams required for fusion ?
The neutral beam power is required to heat the plasma to reach the burn stage and sustain the pulse length by current drive.
33 MW are planned for ITER, while 135 MW will be necessary for the steady state option of DEMO.
In ITER a negative ion current of 40 A will be accelerated to 1 MV before negative ions being neutralized. The pulse length will be 3600 s.
The energy of the DEMO beam is also 1MeV at present.

7. The ITER beamline Heating beam parameters AcceleratorD-
Energy 1MeV
Accelerated current 40A
Gas neutraliser
Injected power 17MW
Accelerator
~10MW of power dumped in accelerator
~700kW of electrons exits accelerator
~900kW of backstreaming positive ions
Extensive development programme with test stands operating and being built
ELISE - IPP Garching
one half sized
RFX – Padua
full sized

8. Neutral beam generation from positive versus negative ion beams
A beam of energetic atoms, called also neutral beam, can be generated by a positive ion beam, crossing a gas neutralizer. However the efficiency of neutralization of a positive hydrogen ion declines rapidly as the ion velocity goes up and past 60 keV/amu the neutralization efficiency becomes prohibitively low (see next slide).
The neutralization efficiency of negative hydrogen
ions in a neutralizer cell of optimum thickness remains acceptable at higher velocities, and is nearly independent of beam energy above 100 keV/nucleon.

9. Neutralization efficiency of the ions
From Hemsworth and Inoue, IEEETPS, 33, N° 6, p (2005)

10. III. Negative ion beams – precursors of high energy neutral beams.
Since high energy atomic beams are required in fusion research, the need for their precursors, the negative ion beams, became urgent.
This imposed the development of negative hydrogen ion sources, based on two types of processes :
- In the plasma volume, D- ions are formed by dissociative attachment of electrons to ro-vibrationally excited molecules
- on surfaces limiting the plasma they are formed by the interaction of fast particles with these surfaces.
Volume processes alone cannot produce the required current density for ITER (~30mA cm-2 of D-), therefore efforts concentrated on the surface production.

11. Physics of negative ion surface production
The probability, P, of negative ion production on a metal surface has been calculated by Rasser et al (Surface Science, 118, 698 (1982)) as
F is the surface work function
EA is the electron afinity
v = the particle velocity

12. The volume type negative ion source.
The volume negative ion source consists of the following parts as shown on the following slide  :
a plasma generation region, denoted S (the source region), where a hot plasma is generated by an arc discharge or by RF.
a negative ion production region, from where the negative ions are extracted, denoted E (the extraction region).
a magnetic filter MF, separating these two regions.
The extraction region is limited by a plate denoted PE (plasma electrode) containing the extraction opening and separating the source from the accelerator, which is differentially pumped to allow particle acceleration.

13. Schematic presentation of a volume type negative ion source
This negative ion source is a « volume type » one, operated in pure hydrogen, driven by the electrons emitted by a filament, which are accelerated to several tens of eV.

14. IV. Effect of Plasma Electrode Work Function on extracted H- Ion Current
After seeding a certain amount of Cs into a volume source, it was observed that the negative ion current increased with the plasma grid temperature, up to °C.
Thus a work function of 1.5 eV can be obtained not only in ultrahigh vacuum, but also under plasma conditions.
From Hemsworth and Inoue, IEEE TPS, 33, N° 6, 1700 (2005)

15. Role of the Plasma Electrode in Surface Production
When the source is « caesiated » the plasma electrode surface work function can be minimized
by the choice of its temperature (usually 150 C).
Thus the plasma electrode is the main negative ion producing surface in the source. Its role in the generation of the extracted current can be optimized by applying a bias voltage, Vb, with respect to the plasma potential.

16. Microwave driven source Camembert III
Some of the results to be presented
were obtained in the multicusp
plasma source Camembert III at
Ecole Polytechnique, France.
* The microwave-driven version,
operated with a network of 7 dipolar
sources,1 is shown here.
*In the filamented version2 we performed
the comparison of pure hydrogen and
caesiated operations2.
1Béchu et al, Phys.Plasmas, 20, (2013)
2Courteille et al, Rev. Sci. Instrum., 66, 2533 (1995)

17. Pure hydrogen operation.
Effect of plasma electrode bias on the extracted currents in the microwave driven source Camembert III.
Pure hydrogen operation.
Svarnas et al, IEEETPS, 35, N° 4, 1156 (2007)

18. The maximum H- ion extraction is obtained for plasma grid bias, Vb , slightly higher than the local plasma potential, Vp.
The typical difference Vb – Vp for maximum extraction < 1 V.
When Vb is further increased the volume produced ions are accelerated toward the plasma grid and their density near the plasma grid goes down.
This leads to the decrease of the extracted H- current.

19. V. EFFECTS OF CAESIUM SEEDING
The effect of caesium seeding on the
*extracted H- ion curent and
* co-extracted electron current
dependences on the plasma electrode bias,
observed in two ion sources, will be reported.

20. V.1. Negative ion current extracted from the JAERI 10 Amp ion source
Masanobu Tanaka et al,
Report JAERI-M
(1993)
H2 Pressure in the source : 0.7 Pa

21. V.2. Negative ion current extracted from the source Camembert III (Ecole Polytechnique) from Rev. Sci. Instrum., 69, 932 (1998)
This experiment was effected at two values of the extraction voltage,
in pure hydrogen and caesium seeded operations.
The hydrogen pressure in the source was 0.4 Pa .
These data indicate a clear threshold
of the surface component,
i.e. a sudden  decrease in I(H-) near 0.5 V

22. Effect of Plasma Electrode Bias Voltage on the Coextracted Electron Current, from Camembert III
The use of caesium also reduces the co-extracted electron current, as can be seen on the upper figure.
This reduction of the coextracted electron current is the main reason for the use of caesium in negative ion sources.

23. Several characteristics of caesiated operation
In both JAERI 10 Amp and Camembert III sources
the H- ion current increases due to caesium by
a factor 2.5 at plasma potential.
In both sources H- ions can be extracted also at
bias voltage above plasma potential (Vb > Vp).
In Camembert III the extracted current goes down
by a factor 2 only, when the bias voltage increases
above the plasma potential.

24. VI. Causes of the H- ion current enhancement by caesium
In the whole range of bias voltage: the H- ion current is enhanced due to the gettering of atomic hydrogen by caesium.
At bias voltage below plasma potential: direct production of H- ions by positive ions and atoms incident on the caesiated plasma grid surface.
At bias voltage above plasma potential : the current of volume produced H- ions is enhanced by gettering and flow of negative ions from the bulk plasma.

25. Vp Vp -5 Vp -10 Vp +5 Vp +10 Pure H2 Vp-5 Vp-10 Vp+5 Vp+10
Volume produced ions
Vp-5
Vp-10
Vp+5
Vp+10
Cs seeded source
Volume produced ions eventually enhanced by gettering
IH-
H- ions arriving from the bulk plasma
Bias voltage (V)
Directly produced H- ions due to ions and atoms on Cs surface
*enhanced by the gettering
which reduces H- destruction
*reduced by the gettering
via direct production

26. VII. Effect of gettering the atomic hydrogen by caesium
Gettering reduces the destruction of H- ions by associative and non-associative detachment due to atomic hydrogen.
This enhances the H- ion current extracted in the whole bias range.
Another effect of gettering is to reduce the H- ion
direct (surface) production by atomic hydrogen on caesiated surfaces, i.e. the H- ion current extracted with Vb < Vp.
Thus the consequence of strong gettering is levelling off the
H- current in the whole range of bias voltage variation.
This may explain the weak dependence upon Vb of the H- current in the JAERI 10 Amp source.

27. Recent measurement of the effect of gettering
The reduction of the atomic hydrogen density by gettering by caesium was studied recently by Friedl and Fantz (AIP Conf. Proc. 1514, 255 (2013)
They found that the density of atomic hydrogen is reduced by a factor 2 due to caesium seeding into a plasma produced at hydrogen pressure of 10 Pa.
The change in atomic hydrogen density could be much larger when the hydrogen pressure is as low as in ion source operation, because the same number of absorbed hydrogen atoms represents a higher fraction of their initial density at the lower pressure of Pa than at 10 Pa.

28. Are other getters than caesium enhancing negative ion production ?
If gettering of atomic hydrogen is the principal mechanism enhancing H-, any hydrogen getters, like Ti or Ta should work.
The graph below shows that the H- currents from a source with walls covered with Cs and Ta films are comparable, while a W film leads to reduced H- ion current.

29. Evidence for H- ion flow from the background plasma towards the plasma grid
Experiments in NIFS (Toki, Japan) using CRDS and probes
showed that extracted H- ions at plasma grid bias
higher than the plasma potential originate from
the plasma volume beyond the extraction region.

30. VIII. Conclusion ( I )
I. The enhancement due to caesium seeding of the extracted H- ion current in the plasma grid bias range above the plasma potential is explained by:
gettering of atomic hydrogen by caesium
negative ion flow from bulk plasma.
II. There is a clear threshold of the surface component of the extracted H- current, i.e. its decrease when plasma grid bias exceeds the plasma potential.

31. Conclusion ( II )
III. Since the extracted current is larger when due to the surface component, one could expect that in
some ion sources the operation with negative bias voltage should be optimal.
Strong gettering can level off the extracted
currents in the whole bias variation range. It is suggested that this is the reason for the weak dependence of the extracted ion current on Vb
in the JAERI 10 Amp source.