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                                                                         MP No. _______

Subject: Measuring the natural vertical instability growth

From: YoungMu Jeon

Group: KSTAR Experimental Division

Date: March    24, 2011

Approved by:                                      Date Approved:

1. Purpose of Experiments
   The purpose of these experiments is to investigate the characteristics of vertical
instability of KSTAR plasmas under current conducting structures. From this experiment,
the natural vertical instability growth rates of KSTAR plasmas will be measured directly
with variations of Ip, elongation, and the relative position to the first wall. As a
consequence, this result will be very important for the next upgrade of vertical control
system and passive stabilizers in KSTAR.

2. Background
   In 2010 campaign of KSTAR, the first attempt to make elongated and D-shaped
plasmas was conducted. In view of the campaign goal, it was successful, so that D-
shaped plasmas were well produced and controlled leading to the first H-mode access.
However, the achievable elongation was limited up to ~1.85 so that when exceeded, the
plasmas became vertically unstable resulting in discharge terminations by VDE.
   There might be several reasons for this limitation. One is due to relatively large time
delay of the first control action against a vertical perturbation. It comes from IVC coil
response and smoothing of vertical position estimator due to the noise issue. Another
reason is, probably, due to underestimated vertical instability growth rate during the
design phase of passive stabilizer. According to the old estimation based on several
reference equilibra at that time, the vertical growth rate was ~60rad/sec (~17msec) in
average. Therefore it was thought that the planned gap resistance on passive stabilizer
was reasonable and the planned control power supply has the sufficient control speed.
However the analysis of vertical instability growth rate of 2010 plasmas showed that the
plasmas were more unstable (~130rad/sec, ~7msec) than the expectation. This means that
even when the first reason is resolved, the vertical instability growth may need to be
reduced more through passive stabilization.

December 13, 2011                                                                       1
  In the current configuration of KSTAR passive stabilizer, the most effective or the only
way to modify the passive stabilization is to change the resistance of gap resistors. For
optimal change of this, we need to know the realistic plasma dynamics under current
conducting structures in KSTAR. In addition, it can provide useful data for EM load
analysis of passive stabilizer as well.

3. Approach
   In principle, the key point to measure the natural vertical instability growth is
removing the effect of IVC on elongated or shaped plasmas. The simplest way might be
to turn IVC off on a specific time. In this proposal, however, we will use a more active
way. That is, to make the plasma vertically unstable, we will apply a very short, relatively
strong vertical perturbation (or kicking) by IVC. After some time duration, the plasma
naturally shows vertically unstable growing in the forced direction. In this way, we can
measure the directional characteristics of VDE as well.
   Based on this principle method, we will investigate several aspects such as Ip,
elongation, triangularity, and the relative vertical location of plasma to the first
conducting wall.

4. Resources

4.1 Machine   and Plasma Parameters

       Toroidal Field: 1.4 – 2.5 T
       Plasma Current: 0.2 to above 1.0MA at flat top
       Working Gas Species: deuterium
       Density: 1-7 x 1019 m-2 (line integrated density)
       Vessel baking: Not necessarily
       Boronization: Not necessarily
       Plasma control: Good shape control
       Equilibrium configuration (if possible, refer to database equilibria): Various
       elongation and triangularity. Also vertical positional offset required
       Special IVC control is required

4.2 Auxiliary Systems

       ECH Power, pulse length : Any
       NBI Power : Any

4.3 Diagnostics
    1. Basic magnetics for plasma current (Rogowski coil), loop voltage (flux loops),
       and plasma position (magnetic probes)
    2. Density measurement (mm-wave interferometer: 10/100 kHz sampling rate)
    3. Electron temperature measurement (ECE)

December 13, 2011                                                                         2
    4. Filterscope /photodiodes (Ha: 10/100 kHz sampling rate)
    5. Radiative power losses (Bolometer)
    6. Impurity measurements (spectrometers)
    7. ASDEX fast neutral pressure gauges
    8. H/D ratio diagnostic or RGA
    9. Locked mode detector

    1. MSE
    2. Charge exchange recombination spectroscopy (Ti)
    3. Thomson scattering (Te and ne)
    4. Reciprocating probe in SOL
    5. Internal MHD activity measurement (soft x-ray)
    6. Infra-red TV
    7. Reflectometer

    1. X-ray crystral spectrometer
    2. Fast neutrals
    3. ECEI
    4. FIR
    5. Neutron detectors

5. Experimental Plan

5.1. Run sequence Plan (total 30 shots)
      1) Produce reference DN shots which is vertically stable with elongation of
         1.4~1.8 and triangularity of 0.4~0.8 in steady state (0.5 day)
         - If proper shots for this purpose are already available, then skipped
      2) Optimize the short pulse plasma kicking by IVC (3 shots)
         - Optimize the pulse duration with maximum voltage for plasma vertical
            kicking as a VDE trigger
         - Of course, the IVC should be programmed to be turned off (zero
            voltage/current output) after this short pulse
      3) Vary the parameters for both upper and lower kicking (14 shots)
         - Scan Ip: 0.2, 0.4, 0.6MA  with kappa 1.6 and delta 0.6
         - Scan elongation: 1.4, 1.6, 1.8  with Ip 0.4MA and triangularity 0.6
         - Scan triangularity: 0.4, 0.6, 0.8  with Ip 0.4MA and elongation 1.6
      4) Measure the effect of plasma location against the first wall (3 shots)
         - Moving up/down the reference plasma by 5cm (10cm if possible) and then
            drive VDE by kicking in that direction

      Total: 1.0 run days (30 shots)

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6. Anticipated Results
The results from these experiments will provide useful data and information for the next
upgrade of vertical control system as well as passive stabilizer in KSTAR. It will be used
for the plasma response modeling, optimal controller design, and EM load/stress analysis
for the major upgrade of KSTAR in near future.

7. References

December 13, 2011                                                                       4

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