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Experimental Study Using RFA Measurements on JET Plasma Acceleration Phenomenon

This study examines results from RFA measurements made on April 5 and April 9, 2002, during the JET C5 campaign to observe particle acceleration in front of the LH launcher. The experiment focused on investigating changes in ion distribution function during LH application. The RFA was strategically placed to detect any acceleration effects. The experiment sought to establish a magnetic connection between the LH launcher and RFA to study the LH wave effect. Noteworthy observations included varying collector current characteristics and the impact of LH power presence on signal behavior. The results indicate potential penetration of fast electrons due to insufficient biasing voltage.

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Experimental Study Using RFA Measurements on JET Plasma Acceleration Phenomenon

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  1. Comments to and conclusions drawn from RFA measurements madeon April 5 (Friday) and April 9 (Tuesday) 2002 on JET, campaign C5 (K. Erents, M. Goniche, J. Mailloux, V. Petrzilka, K. Rantamaki, F. Zacek)

  2. Goal of the experiment: • looking for an experimental proof of acceleration of particles in front of JET LH launcher; • utilization of RFA (Retarding Field Analyser) has been envisaged for observation of changes in ion distribution function during the LH application (located 90o toroidally and nearly 90o poloidally away of the launcher in the ion direction);

  3. Idea of the experiment: • to find the LHW effect if a direct magnetic connection between the LH launcher and RFA is established (as it follows from the magnetic field lines code, q slightly above 3 is needed); • for this reason a ramp-down of plasma current in regime with Bt=2.6T and Ip=2.2MA, starting at t=52s, has been asked and three RFA strokes at t=52, 54 and 56s has been performed;

  4. Experimental set-up: • RFA is mounted in the Plasma Boundary Reciprocating Probe, Octant 1 (i.e. in the poloidal plane 90 degrees toroidally away from LH launcher in the ion direction) at R=3.25m; • vertical position of the probe is changing linearly with the time during the stroke; • the reciprocating probe has two identical RFA (looking from the ion and from the electron side, respectively; the ion side should be magnetically connected with the LH grill); • RFA voltages used (relative to the vacuum chamber): - triangular analysing voltage: zero to +100V (sweeping frequency 50Hz and about 10 swept RFA characteristics are visible during the probe movement) - secondary electron suppression mesh: -180V - collectors: grounded - biasing voltage of the entrance slit: -100V (April 5 ), -150V (April 9) • LH power used for the RFA measurement has been lowered to a value - 0.6MW on April 5 - 0.35MW on April 9

  5. Schematic view of the RFA

  6. ↑6 ↑ 4 ↑ 2

  7. ↑ 4 ↑ 6 ↑ 2

  8. ↕RFA ↑ LHW

  9. no LHW

  10. LHW 0.6MW, q~3.5

  11. LHW 0.6MW, q~4

  12. LHW 0.6MW, q~4.5

  13. General conclusions: • The character of RFA collector current (ion mode operation) dependence on the analysing voltage is substantially different in the first stroke and in those two others, independently on the fact if LHW is switched off (#55310) or switched on (#55312); • such difference between the strokes can be hardly explain by small change of plasma configuration observed between the strokes; is the reason a loss of magnetical connection with the launcher ? • the character of collector current during the first stroke exhibits a certain dependence on the LH power presence: - without this power (#55310) signal is depending on the analysing voltage (i.e. the I-V characteristics are visible), while - with the power (#55312) the signal is on the voltage independent, it is very noisy and moreover, it remains all the measuring period negative; is the reason penetration of the fast electrons, overcoming the insufficient biasing voltage of the entrance slit (100V only) ?

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