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					                                                               Max-Planck-Institut
                                                               für Plasmaphysik




   Long pulse H- beam extraction with an RF driven
           ion source on high power level


                                          W. Kraus

                                     for the NNBI group



                            Max-Planck-Institut für Plasmaphysik
                                   EURATOM Association
                             Boltzmannstr. 2, D-85748 Garching




ICIS 2009, Gatlinburg, USA, September 24, 2009
Outline



      •    Improvements of the source design
      •    High RF power operation
                Long pulses
                Efficiency
      •    Filter field variations




W. Kraus              ICIS 2009, Gatlinburg, USA, September 24, 2009   2
The IPP prototype RF source

Operated on the long
pulse testbed „MANITU“
Goal:
To achieve the ITER
requirements with the small                                           Cs oven
source and extraction area
around 200 cm2 :
Accelerated current density                                                     Driver Ø24.4 cm

20 mA/cm2 (D-)
24 mA/cm2 (H-)
                                                                            Quartz or Al2O3
jel/jion <1, at 0.3 Pa                                                      and internal
                                                                            Faraday shield
for pulse durations up to 3600s
30 mA/cm2 (H-) for the               Uextr < 9kV                     RF coil, 1 MHz,
                                     Uacc =20kV Expansion volume     100 kW
diagnostic injector (IPR)
                                                 31x 59 x19 cm3

W. Kraus                 ICIS 2009, Gatlinburg, USA, September 24, 2009                       3
  Extrapolation to the ITER Source

RF source chosen for the reference design of ITER
Intermediate step: ELISE (Extraction from a Large Ion Source Experiment)


                 4x                                                 2x




   1 Driver
0.59 x 0.31 m2




                                         4 Drivers
                                        1.0 x 0.9 m2                          8 Drivers
                                                                            1.9 x 0.9 m2
  W. Kraus                 ICIS 2009, Gatlinburg, USA, September 24, 2009                  4
Improved insulation of the RF coil


RF coil in an epoxy case filled with
SF6 (1.3 bar)

Reliable operation is now possible at
high power >100 kW without RF
breakdowns at the coil.

No breakdown problems at the ITER
source, driver is in vacuum




W. Kraus             ICIS 2009, Gatlinburg, USA, September 24, 2009   5
Test of a new Faraday shield

Faraday shield with Z-shaped slits




=> No power load on the insulator


Hα radiation in the driver not changed
⇒ No additional power losses by eddy
currents
⇒ Z-shaped slits can be used for
future sources (Elise, ITER)




W. Kraus             ICIS 2009, Gatlinburg, USA, September 24, 2009   6
                                      Limitation of the pulse duration at high power

                                                 Last 34 experimental days                                                                      Example: two successive pulses
                             35                                                                 7

                                                                             jH-    je/jH-                                              9
                             30                                                                 6
                                                                                                                                        8
                                                                                                                                                  Iel 80 kW
H- current density, mA/cm2




                                                                                                                                        7
                             25                                                                 5                                                                           Iel 55 kW




                                                                                                    Electron/ion current
                                                                                                                                        6




                                                                                                                           Current, A
                             20                                                                 4                                       5
                                                                                                                                                              IH-80 kW
                                                                                                                                        4
                             15                                                                 3
                                                                                                                                        3                                    IH- 55 kW

                                                                                                                                        2
                             10                                                                 2
                                                                                                                                        1
                              5                                                                 1                                       0
                                                                                                                                            0        50           100                   150       200
                              0                                                                 0                                                              Time , sec
                                  0        200          400            600         800       1000
                                                         Pulse duration, s
                                                                                                                                        Increase of the electron current is
                                  Long pulses only possible with reduced RF power                                                       steeper at high power
                                  => Lower H- current in long pulses                                                                    => Pulse has to be stopped earlier
                                                                                                                                        to protect the extraction grid




                                      W. Kraus                     ICIS 2009, Gatlinburg, USA, September 24, 2009                                                                             7
260s pulse


                                40

                                35
                                                                               Iel
                                                  75 kW
      Current density, mA/cm2




                                30
                                                                               Iex
                                25

                                20
                                                                        with Ibias control
                                15                60 kW

                                10

                                 5

                                 0
                                     0   50      100        150       200            250     300
                                                          Time, sec


  Electron suppression by biasing the plasma grid can extend the pulse length

    Typical result: 20 - 25 mA/cm2 ,jel/jex< 1, for 100 - 200s

W. Kraus                                  ICIS 2009, Gatlinburg, USA, September 24, 2009           8
          Effect of Caesium conditions

                              je/jH-   HFEff
          3.5                                                 0.35                         2.5


           3                                                  0.3                                                    Ion current
                                                                                            2

          2.5                                                 0.25
                                                                                           1.5




                                                                              Current, A
           2                                                  0.2                                                                  Cs 852, a. u.




                                                                     HF eff
je /jH-




                                                                                                             Electron current
          1.5                                                 0.15                          1

           1                                                  0.1
                                                                                           0.5
          0.5                                                 0.05

           0                                                  0                             0
                0.0   1.0   2.0           3.0    4.0    5.0                                      0     100            200          300         400
                                  Cs/Hb                                                                               Time, sec
 MANITU, 81885-83106


   The source performance (efficiency HFeff=jacc/PRF)                                      As long as the Cs signal rises, the electron

   depends only slightly on the total amount of Cs,                                        current is stable, as soon as the Cs signal flattens

   much more on the distribution and surface properties                                    off or starts to decline, the electron current
                                                                                           increases

Situation improved since most Cu surfaces are Mo coated, last Cu sources have been coated recently.
The generation of homogenous Caesium layers which can withstand long pulse high power operation
is still an open issue (Cs evaporation, distribution, control → posters F2(U.Fantz), N2(R.Gutser)).

          W. Kraus                     ICIS 2009, Gatlinburg, USA, September 24, 2009                                                                9
Power dependence of the H- currents

                                                                                                              Results of 34 days
                                                                                     35

                                                                                     30
Ion currents „saturate“ at high




                                        H- current density, mA/cm2
                                                                                     25
power, no significant increase of                                                    20

the ion current by further raising                                                   15

of the RF power                                                                      10

                                                                                               5
   Efficiency (jacc/PRF) drops
                                                                                               0
                                                                                                   40           60               80                 100              120
                                                                                                                          RF power, kW

Best result                                                                                    35
                                                                                                                 One day, 0.6 Pa, 20s                                      1.4

jion(H-) = 31 mA/cm2                                             H- Current Density, mA/cm 2
                                                                                               30
                                                                                                                                           jex
                                                                                                                                                                           1.2
                                                                                                                                                           jel/jex
22 mA/cm2 on the calorimeter                                                                   25                                                                          1

                                                                                               20                                                                          0.8
(8.7kV, 0.6Pa, 20s)                                                                                                                         jacc
                                                                                               15                                                                          0.6

                                                                                               10                                                                          0.4

                                                                                               5                                                     jacc/PRF              0.2

                                                                                               0                                                                        0
                                                                                                    50   60          70     80        90           100      110      120
                                                                                                                             PRF, kW

W. Kraus               ICIS 2009, Gatlinburg, USA, September 24, 2009                                                                                                            10
Flow of atoms and ions to the plasma grid at high power


H- ions are produced by surface conversion of
                                                                     0.5
                                                                                                                                              ⊥ grid




                                                     Density ratio
atoms, less by ions (but necessary for the                           0.4                                      H/H2
extraction)                                                          0.3
                                                                                                               D/D2                           II grid
H- production at high power affected by:
                                                                     0.2
Flows of atoms and ions flattens off at high                         0.1 H/H2
power
                                                                     0.0
• H/H2 in the driver depends only slightly on the                                                             60        80 100 120 140
power                                                                                                                    Power [kW]
                                                                                                     6
• Ion saturation current measured by Langmuir




                                                                     Ion Saturation Current, a. u.
                                                                                                     5
probes close to the plasma grid saturates
                                                                                                     4

Elise, ITER source                                                                                   3
- Driver diameter 24.4 cm => 28.4 cm                                                                                                    7.3 mT bottom
                                                                                                     2
                                                                                                                                        7.3 mT top
- loss areas in the expansion chamber are
                                                                                                     1
      smaller related to one driver
                                                                                                     0
=> lower losses and higher efficiency                                                                    40        60         80        100         120
                                                                                                                         RF Power, kW


W. Kraus                ICIS 2009, Gatlinburg, USA, September 24, 2009                                                                            11
 Change of the magnetic filter field

 Change of B field close to the plasma grid                   Stronger

 by addition of permanent magnets


driver        Standard




                                        3 additional rows      Weaker



                          plasma grid
         8 rows Co-Sm magnets




 W. Kraus            ICIS 2009, Gatlinburg, USA, September 24, 2009      12
   Asymmetries of the plasma and the beam divergence
                                                                                       6



                                                                                       5
                              2 Langmuir
                              probes




                                                       Ion Saturation Current, a. u.
                                                                                       4

                              2.5 cm from
                              the plasma grid                                          3
                                                                                                                                                     12:2 mT bottom
                                                                                                                                                     12.2 mT top
                                                                                                                                                     9.5 mT bottom
                                                                                       2
                                                                                                                                                     9.5 mT top
                                                                                                                                                     7.3 mT bottom
                                                                                                                                                     7.3 mT top
                                                                                       1




                                                                                       0
                                                                                           40    50      60         70    80     90     100    110          120    130
                                                                                                                         RF Power, kW


                 LOS Cs852
                                                                                       6
                 detection                                                                                                                           12.2
                                                                                       5

Asymmetry of the ion saturation current depends                                                                                                        9.5




                                                    Divergence / (deg)
                                                                                       4

on the filter field (→ poster P. McNeely)                                                                                                            7.3
                                                                                       3

Beam divergence depends on the filter field
                                                                                       2

Weak field => Symmetric currents                                                       1
                                                                                                              top                       bottom

         => constant beam divergence                                                   0

         => homogenous H- profile in the source                                         -250    -200   -150   -100 -50      0      50
                                                                                                                 Vertical Position / (mm)
                                                                                                                                         100   150         200    250




   W. Kraus            ICIS 2009, Gatlinburg, USA, September 24, 2009                                                                                             13
Pressure dependence

                                                                     30
                                                                                60 kW, 40s
Pressure dependence of the                                           25




                                       H- Current Density, mA/cm 2
                                                                                                                        7.3 mT
ion current observed only with the                                   20

weaker filter field                                                  15
                                                                                                             9.5 mT

                                                                     10


                                                                     5


                                                                     0
                                                                          0.3        0.4     0.5       0.6        0.7    0.8     0.9
                                                                                                   Pressure, Pa




W. Kraus              ICIS 2009, Gatlinburg, USA, September 24, 2009                                                             14
Range of the filter field


                        0.014                                                                                   0.014
                                                       Standard filter field                                                                         Standard
                        0.012                          Weaker                                                   0.012
                                                                                                                                                     PG Filter 4 kA




                                                                                        Magbetic Induction, T
                                                       Stronger
Magbetic Induction, T




                         0.01                                                                                    0.01

                        0.008                                                                                   0.008




                                                                                                                                                                      Driver
                                                                               Driver
                        0.006                                                                                   0.006

                        0.004                                                                                   0.004


                        0.002                                                                                   0.002

                                                                                                                   0
                           0
                                                                                                                        0   5         10        15         20             25
                                0   5         10        15           20           25
                                    Distance from the plasma grid, cm                                                       Distance from the plasma grid, cm




                    Different range of the filter fields                                                        The PG filter, planned for the ITER
                                                                                                                source, fills the source and the driver,
Additional magnets change also the B field                                                                      this will have impact on the plasma,
topography in source and in the driver                                                                          further investigations are required


W. Kraus                                           ICIS 2009, Gatlinburg, USA, September 24, 2009                                                                              15
    Conclusion


•     Pulse duration at high power is limited by the increasing electron
      current,
      25 mA/cm2 (extracted) can be achieved with jel/jacc < 1 for 100 - 200s.
•     The efficiency of the H- production drops at high power,
      the ITER source will be more efficient.
•     Magnetic filter field has great impact on the plasma density as well as
      on the H- density profiles.
      The investigation of novel filter field concepts which are applicable for
      large sources is required.




    W. Kraus           ICIS 2009, Gatlinburg, USA, September 24, 2009             16

				
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