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R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 1
Some lessons from long pulse operation of negative ion sources and accelerators
R S Hemsworth
(with help from D Boilson, H P L de Esch, L Svensson, A Krylov, C Grand
and P Massmann)
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 2
The KAMABOKO III negative ion source Designed and built by JAERI, Naka, Japan
Model of the ITER reference design of ion source
☺ First tested in Japan: 280 A/m2 of H-
Sent to the DRFC, CEA, Cadarache, France for testing in deuterium.
Short pulse, 5 s operation
☺ Reproduced the 280 A/m2 of H- but:
with deuterium: Ie extracted >10 !! ID-
Increased magnetic filter strength from 450 Gauss.cm to 900 Gauss.cm:
☺ 220 A/m2 of D-, with Ie extracted ≈1 ID-
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 3
-200
0
200
400
600
800
1000
1200
1400
1600
7 8 9 10 11 12 13 14 15 16 17 18 19 20
t (seconde)
I(G2) (electrons, mA)
I(arc) (A)
I(D-), calorimetric, mA equivalent)
KAMABOKO III Negative Ion Source
Arc power = 80 kW, Source filling pressure = 0.35 Pa, Beam Pulse Length = 5 s
Average J(D-) = 21.6 mA/cm2
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 4
Long pulse operation
• operational advantages & difficulties
• reduced negative ion yield
• reduced plasma grid temperature effect
• increased caesium consumption
• caution in the future operations
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 5
Planned changes to the system ☺ New data acquisition system
☺ New timing system
☺ Water cooled arc series resistances
☺ Change cooling water circuit to ensure adequate flows
☺ Remote operation to allow D- operation for long pulses (neutron dose)
☺ New long pulse plasma grid capable of operation at ≈300 °C
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 6
0
50
100
150
200
250
50 100 150 200 250 300
Plasma Grid temperature (°C)
Incr
ease
in n
egtiv
e io
n cu
rren
t (%
)
Effect of plasma grid temperature on the accelerated negative ion current with the KAMABOKO III ion source.Parc ≈ 40 kW, Psrc ≈ 0.3 Pa, Bias ≈ 5 V, with Cs Short (<5 s) pulses, Mo grid
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 7
Cu/Cr/Zr long pulse grid (6 mm thick).A thermal bridge limits the heat flow to the cooling channels along each edge
Long Pulse Grids Provided by JAERI
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 8
Molybdenum long pulse grid (6 mm thick).A set of stainless steel disks act as thermal bridges between the grid and copper cooling tubes traversing the grid.
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 9
0
100
200
300
400
500
600
0 50 100 150 200 250 300
Time (s)
T PG
(°C
)
0
10
20
30
40
50
60
Parc
(kW
)
Tpg
Parc
Frame cooled grid
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 10
Unplanned changes: Remove/change source flanges
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 11
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 12
Extraction grid supports
Originally the extraction grid supported off the plasma grid by small ceramic insulators
These became metallised due to increased number of breakdowns
Replaced by shielded supports mounted on the acceleration grid
Breakdown protection system
Extraction grid feedthrough
……
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 13
shot 5179, pressure variation, 06-2000, Hydrogen with Cs
0
2
4
6
8
10
0 50 100 150 200 250 300 350 400
Time
Sour
ce P
ress
ure
(µb)
,Id
rain
*10
(A)
0
200
400
600
800
1000
Iarc
(A)
press Idrain*10 IG2*50 Iarc
Advantages
☺ Ability to change parameters during a single shot
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 14
0
0.4
0.8
1.2
1.6
2
0 100 200 300 400 500 600 700
Time (s)
IG2
(A)
0
10
20
30
40
50
Parc
(kW
), V
G2
(kV
)
Shot 8978
IG2
VG2
IH-
Parc
Unexpected long time constant effects
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 15
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0 200 400 600 800 1000 1200
Time (s)
IH- &
2*I
G2
(A)
Shot 8994
1
2
3
4
5
6
7
IH-
2*IG2
1. VG2 ~8 kV2. Decreased VG2 by 2 kV to 6 kV3. Increased VG2 by 1 kV to 7 kV4. Decreased VG2 by 1 kV to 6 kV5. Reduced filament voltage6. Reduced filament voltage7. Decreased VG2 by 1 kV to 5 kV
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 16
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
0 200 400 600 800 1000 1200
Time (s)
Idra
in, I
G2,
Ical
(A)
0
50
100
150
200
250
300
350
400
450
Tpg
(°C
), Pa
rc (k
W)
TpgIdrain
5* Parc
Ical
IG2
1000 s extraction of H- beam.
☺ 18 mA/cm2 H- calculated from the Idrain.
Poor transmission: only ~50% accelerated current collected on the calorimeter ≈ 9 mA/cm2
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 17
1000 s extraction of D- beam.Shot 9069, 1000sec D- extraction, 45kW
0
0,5
1
1,5
2
2,5
0 200 400 600 800 1000 1200 1400
Time (sec)
Idra
in, I
G2
(A)
0
50
100
150
200
250
300
350
400
450
Tpg
(°C
), P
arc
(kW
)
Idrain IG2 Ical Parc Tpg (°C)
~60% of accelerated current arrives at the calorimeter ≈ 7 mA/cm2
☺ 11.5 mA/cm2 D- calculated from the Idrain.
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 18
Inertial calorimetry determination of the beam transmission
25
35
45
55
65
75
0 20 40 60 80 100 120 140 160 180 200
Beam on time (s)
Tran
smiss
ion
(%)
Low transmission of accelerated current to the calorimeter ≈55%
Wednesday 11 May 2005 9:25-9:50 D. Boilson: Power transmission from the ITER model negative ion source
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 19
Shot 11074, 20% Temp Effect, 0.3 Pa, 40 kW H2
0
50
100
150
200
250
0 50 100 150 200 250 300 350
Time (s)
Tpg
, Tso
urce
, Par
c
0
0.2
0.4
0.6
0.8
1
Idra
in, I
G2(
A)
Parc Tpg Tsource IG2 Idrain
No significant change in IH- as the PG temperature increased
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 20
0
50
100
150
200
250
50 100 150 200 250 300
Plasma Grid temperature (°C)
Incr
ease
in n
egtiv
e io
n cu
rren
t (%
)
Short pulse
Long pulse
Long pulse grid, cold source walls
Effect of plasma grid temperature on the accelerated negative ion current with the KAMABOKO III ion source.Parc ≈ 40 kW, Psrc ≈ 0.3 Pa, Bias ≈ 5 V, with Cs
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 21
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
8000 9000 10000 11000
shot number
Idra
in/P
arc
FCG ACG
No obvious effect of grid material
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 22
Cs “consumption”
Long pulse operation: 4.6*10-6 g/s/aperture.
Estimate for the ITER ion source 2.8*10-9 g/s/aperture) (Short pulse or low power operation)
The ratio is >1600 Source cleaned with water and the “polluted” water was chemically analysed.
The polluted water contained ≈4.5 g ±20% of Cs & a similar amount of tungsten.
Tuesday 10 May 2005: 9:50-10:15 A. Krylov: Caesium and Tungsten behaviour in filamented arc driven KAMABOKO-III beam source
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 23
Suggestions:
1 Cs consumption: The Cs effect disappears not because the Cs has left the ion source, but because the Cs is somehow “blocked” on the walls and the PG surface in loosely bound tungsten – caesium mixture, the tungsten being evaporated from the filaments during the arc operation.
2 Reduced/no PG temperature effect: The composition of the surface of the PG will change dynamically during a long pulse due to the tungsten evaporation from the filaments. This could mask any effect of the PG temperature on the negative ion yield.
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 24
Other long pulse effects: Dark current This appear when HV is applied on the Cadarache 1 test bed. HV only - no gas
- no filaments operating, - no discharge in the source
Reduced by conditioning, i.e. apply HV in series of shots
wait for dark current to reduce
increase HV & repeat 50 – 100 mA at V >250 kV
Seemingly spread generally over the “anode” and cathode” surfaces (measured in all places that have been checked)
But: We increased the (conditioning) pulse length ability and….
We melted a hole in an electrostatic screen!!!
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 25
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 26
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 27
Other long pulse effects: Future????
With the ITER system we foresaw a possible problem with power deposited on the cryopumps by X-rays created by accelerated electrons (stripping).
The UKAEA have started to investigate this using the MCNP package.
Because they (M Kovari) had to input data on electron back scattering, a potential new problem appeared – 40 kW of electron power to the 80 K cryopump screens.
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 28
This time we have been lucky, the problem has been identified and can be solved.
The future will be interesting.
R S Hemsworth
Euratom TORE SUPRA
IAEA TM on Negative ions Long pulse operation 29
Conclusion
Long pulse operation of the MANTIS test bed has thrown up several important potential problems with the ITER negative ion source.
The longer conditioning pulses on the Cadarache 1 MV test bed have reminded us that small powers can be very important over long periods.
Studies in the EU on the ITER NBI test bed have reminded us that other potential problems are to be expected