+--------------------+----------------------+-------+-----------------+
| **Princeton Plasma |                      |       |                 |
| Physics            |                      |       |                 |
| Laboratory**       |                      |       |                 |
|                    |                      |       |                 |
| **NSTX             |                      |       |                 |
| Experimental       |                      |       |                 |
| Proposal**         |                      |       |                 |
+--------------------+----------------------+-------+-----------------+
| **Title: Transient |                      |       |                 |
| CHI Startup**      |                      |       |                 |
+--------------------+----------------------+-------+-----------------+
| **OP-XP-531**      | > **Revision: 0**    | >     |                 |
|                    |                      |  Effe |                 |
|                    |                      | ctive |                 |
|                    |                      | >     |                 |
|                    |                      | Date: |                 |
|                    |                      | >     |                 |
|                    |                      | **6/2 |                 |
|                    |                      | /05** |                 |
|                    |                      | >     |                 |
|                    |                      | > *   |                 |
|                    |                      | (Ref. |                 |
|                    |                      | >     |                 |
|                    |                      | OP-AD |                 |
|                    |                      | -97)* |                 |
|                    |                      | >     |                 |
|                    |                      | >     |                 |
|                    |                      | Expir |                 |
|                    |                      | ation |                 |
|                    |                      | >     |                 |
|                    |                      | Date: |                 |
|                    |                      | >     |                 |
|                    |                      | > *(2 |                 |
|                    |                      | >     |                 |
|                    |                      |  yrs. |                 |
|                    |                      | > u   |                 |
|                    |                      | nless |                 |
|                    |                      | >     |                 |
|                    |                      |  othe |                 |
|                    |                      | rwise |                 |
|                    |                      | > st  |                 |
|                    |                      | ipula |                 |
|                    |                      | ted)* |                 |
+--------------------+----------------------+-------+-----------------+
| **PROPOSAL         |                      |       |                 |
| APPROVALS**        |                      |       |                 |
+--------------------+----------------------+-------+-----------------+
| **Responsible      |                      |       | > Date          |
| author: R. Raman** |                      |       |                 |
+--------------------+----------------------+-------+-----------------+
| **ATI -- ET Group  |                      |       | > Date          |
| Leader: R. Raman** |                      |       |                 |
+--------------------+----------------------+-------+-----------------+
| **RLM -- Run       |                      |       | > Date          |
| Coordinator: J.    |                      |       |                 |
| Menard**           |                      |       |                 |
+--------------------+----------------------+-------+-----------------+
| **Responsible      |                      |       |                 |
| Division:          |                      |       |                 |
| Experimental       |                      |       |                 |
| Research           |                      |       |                 |
| Operations**       |                      |       |                 |
+--------------------+----------------------+-------+-----------------+
| [**Chit Review     |                      |       |                 |
| Board**            |                      |       |                 |
| (designated by Run |                      |       |                 |
| Coordin            |                      |       |                 |
| ator)]{.underline} |                      |       |                 |
+--------------------+----------------------+-------+-----------------+
| **MINOR            |                      |       |                 |
| MODIFICATIONS**    |                      |       |                 |
| (Approved by       |                      |       |                 |
| Experimental       |                      |       |                 |
| Research           |                      |       |                 |
| Operations)        |                      |       |                 |
+--------------------+----------------------+-------+-----------------+
|                    |                      |       |                 |
+--------------------+----------------------+-------+-----------------+

NSTX EXPERIMENTAL PROPOSAL

Title: Transient CHI Startup OP-XP-531

Authors: R. Raman, M. Bell, T.R. Jarboe, B.A. Nelson, D. Mueller

# 1. Overview of planned experiment 

> We will apply the "Transient CHI" scheme developed in HIT-II to
> demonstrate persistent toroidal current following a pulse of coaxial
> helicity injection (CHI) applied with the newly upgraded CHI capacitor
> bank. This XP has three parts: 1) Demonstrating the persistence of
> CHI-induced toroidal current; 2) handing off the CHI-induced plasma to
> inductive sustainment and ramp-up, and 3) measuring the benefits of
> CHI in saving inductive flux.

# 2. Theoretical/ empirical justification

> Using a new CHI-specific capacitor bank, the method of Transient CHI
> was tested on NSTX during 2004. The capacitor bank produced reliable
> discharge initiation and operation over a wide range of gas pressure
> and injector flux conditions. However, analysis of the results pointed
> to four areas for improvement:

a)  The need for improved pre-ionization so that the amount of injected
    gas can be reduced by a factor of at least four to provide more
    energy per particle from the fixed capacitor bank energy. The
    required capacitor bank energy per particle should preferably be
    about 100eV so that, after losses, the resulting plasma could burn
    through radiation barriers and reach temperatures of about 30eV or
    higher.

b)  Increasing the capacitor bank voltage above the 1kV operational
    limit applied in 2004 should enable breakdown at lower gas pressures
    and increase the available energy by a factor of four over what was
    available in 2004.

c)  Reducing the injector flux footprints is required to produce a
    sufficiently narrow flux foot print for inducing reconnection near
    the injector throat region.

d)  Improved equilibrium control.

# 3. Experimental run plan

> Prior to the start of this experiment, a related Machine Proposal
> XMP-39 will have established the conditions necessary for achieving
> breakdown using the LDGIS (Branch 5) in conjunction with EC
> pre-ionization in the chamber below the lower divertor plates.

[3.1 Demonstrate persistence of toroidal current]{.underline}

> Configure the CHI capacitor bank initially with five capacitors in
> parallel only (C = 25mF). The capacitor bank will be fired at t = 0
> throughout this experiment.
>
> Open line and ground switches in the OH circuit to prevent current
> from flowing in it.
>
> Use 5min HeGDC between shots to maintain constant wall conditions.
>
> Start with the gas timing and plenum pressure developed in XMP-39 for
> breakdown using fundamental EC resonance in the sub-divertor chamber
> (B = 0.64T, I­~TF~ ≈ 45kA), extending the TF flattop to 0.5s to prevent
> possible delayed breakdown as the TF falls.
>
> Start with a capacitor bank charging voltage of 1.0kV.
>
> Program the CHI capacitor bank crowbar ignitron to fire at t = +10ms
> to rapidly reduce the CHI injector current.
>
> Program constant values: PF1B = +4 kA, PF3L = 0, PF1AL = -1kA. (Note:
> all PF coil currents are specified in the physics convention, *i.e.*
> positive current is counter-clockwise from above.) Program PF2L to
> zero at t = 0, -1.5kA at t = 2ms.
>
> Based on the evolution of the discharge, adjust PF currents to obtain
> an equilibrium of the evolving discharge and maximize the toroidal
> plasma current. Use the values listed in Table 1 as a guide. Program
> PF3U and PF1AU to zero at t = 0 and the value listed at t = 2ms.
> Program PF5 to have the value specified for t ≥ 0. Adjust the timing
> of the PF ramps to match the discharge evolution. Note that the values
> (**in bold)** in Table 1 for PF5, PF3U and PF1AU are upper limits
> calculated using the iSolver code (J. Menard) for hypothetical
> steady-state, closed-flux equilibria located at R=0.85m, a=0.6m.
> Calculations of CHI discharge evolution using the TSC code with the
> most up-to-date NSTX vessel geometry and open-field-line currents
> always show lower current requirements for the PF3U and PF5 coils than
> the steady-state calculation. Therefore, initially use a value about
> 25% of that listed in the table and make further adjustments based on
> experimental observations.
>
> Table 1: Reference coil currents (kA) for steady-state equilibrium at
> specified plasma current

  -------------- ---------- ----------- ---------- ----------- ----------- -----------
  **I~p~(kA)**   **PF1B**   **PF1AL**   **PF2L**   **PF5**     **PF3U**    **PF1AU**

  50             4.0        -1.5        -2.0       **-0.21**   **-0.38**   **-0.03**

  75             4.0        -1.5        -2.0       **-0.33**   **-0.44**   **-0.10**

  100            4.0        -1.5        -2.0       **-0.49**   **-0.46**   **-0.05**

  150            4.0        -1.5        -2.0       **-0.79**   **-0.53**   **-0.07**

  150            4.0        -2.0        -2.0       **-0.77**   **-0.55**   **-0.12**

  150            4.0        -2.0        -2.5       **-0.57**   **-0.75**   **-0.60**

  150            4.0        -2.0        -3.0       **-0.51**   **-0.90**   **-0.54**

  50             4.0        -2.0        -3.0       **-0.19**   **-0.64**   **+0.53**

  100            4.0        -3.0        -2.0       **-0.38**   **-0.82**   **+0.14**

  200            6.0        -3.0        -4.5       **-0.67**   **-1.32**   **-0.19**

  150            6.0        -3.0        -4.5       **-0.51**   **-1.22**   **+0.22**

  250            6.0        -3.0        -4.5       **-0.79**   **-1.39**   **-0.90**

  200            6.0        -2.5        -3.75      **-0.67**   **-1.06**   **-0.66**

  150            6.0        -2.5        -3.75      **-0.48**   **-0.96**   **-0.45**
  -------------- ---------- ----------- ---------- ----------- ----------- -----------

> Program the crowbar ignitron to fire at the time of peak toroidal
> current to assess the persistence of the toroidal current after the
> injector current has decayed to zero. Measure the plasma temperature
> and density profiles with MPTS at the time of the peak plasma current
> and during its decay when the injector current is zero.
>
> Once the maximum current has been attained in the five capacitor
> configuration, increase the number of capacitors to 7 then all 10.
>
> Increase the magnitude of closed flux current by raising PF1B to 6kA,
> using guidance from Table 1 for the required currents in other PF
> coils.
>
> If the limit on the capacitor voltage has been raised above 1kV,
> assess the need for repeating the steps above at higher capacitor
> voltage. If the results obtained above at 1kV are satisfactory,
> raising the voltage may be deferred to a later stage of this
> experiment after completing steps 3.2 and 3.3 below.

[3.2 Hand-off to inductive operation using the central
solenoid]{.underline}

a)  Close the OH circuit line switches and enable the OH power supply.
    Starting from the time of peak toroidal current, ramp the OH from
    zero to -10kA in progressively shorter periods to determine the
    response of the plasma to induction, as follows:

> 2V/turn: t~1~ to t~1~+62ms, t~1~ = time of peak CHI-induced current
> when OH ramp is initiated;
>
> 4V/turn: t~1~ to t~1~+41ms;
>
> 6V/turn: t~1~ to t~1~+21ms (Note: this capability will not be
> available if the OH power supply voltage is limited to 4kV during CHI
> operation).

b)  Apply the OH induction for the time needed to ramp the plasma
    current by about 100% of the minimum of the plasma current during
    hand-off or by 100kA, whichever is higher.

    i.  At the time that the inductive ramp is initiated, the PF1B coil
        current should to be ramped to zero. (See the timing diagram in
        the Physics Operation Request.)

    ii. Simultaneously the currents in the upper and lower PF2 and PF3
        coils should become balanced to maintain vertical equilibrium,
        using the ratios I~PF3~/I~p~ = --2.4kA/100kA and I~PF2~/I~p~ =
        +1.2kA/100kA. Program the current in PF5 approximately in the
        ratio I~PF5~/I~p~ = --0.3kA/100kA, to provide radial
        equilibrium.

c)  If induction is not able to sustain the CHI-induced current, either:

    i.  Apply a 40ms, 1 -- 2 MW pulse of HHFW power phased for plasma
        heating (0-π), or

    ii. If the CHI-induced plasma current is above 100kA and the edge
        neutral pressure is below 5E-5 Torr (\\ENGINEERING::GS_IG_TORR)
        during the current decay, apply a 20 ms pulse of NBI at 60 keV,
        initially from one source and adding sources if needed.

d)  After the initial inductive ramp to I~OH~ = -10kA, continue to ramp
    the OH current, if necessary at a lower rate, until the OH coil
    current reaches the maximum of -24kA, to maximize the plasma
    current. Use EFIT reconstructions to adjust the currents in PF3 and
    PF2 coils.

e)  Attempt to introduce normal feedback control of the equilibrium and
    plasma current using the PCS after I~p~ reaches 100kA and after
    imposing a time delay of at least 25ms from start time of current in
    the OH coil.

[3.3 Assessment of Inductive Flux Saving]{.underline}

a)  After reliable, reproducible discharges are obtained, repeat 5 shots
    using CHI start-up assist. If magnetic analysis shows that the
    plasma is reasonably well controlled during the startup phase, open
    shutters of spectroscopic diagnostics, soft-x-ray arrays and MPTS
    and document plasma characteristics. If necessary, use feedback
    controlled current ramp after an initial phase of pre-programmed
    current ramp. Based on the observed current evolution, the value for
    the current setting at which feedback controlled operation commences
    may need to be revised in the plasma control system software.

b)  Develop an inductive startup discharge which reaches the same
    current and configuration in the same time as the CHI-assisted
    discharges in a). This will require bipolar operation of the OH,
    reprogramming the initial PF waveforms and switching to the standard
    LFS gas injector. Note that since the OH power supply voltage will
    be limited to 4kV for CHI experiments, some discharge development
    will be needed. Run 5 shots with inductive-only start-up to measure
    the poloidal flux consumption under identical current ramp-up
    conditions. Determine differences in density profile, current
    profile and temperature profile evolution.

# 4. Required machine, NBI, RF, CHI and diagnostic capabilities

> **NOTE:** Do NOT conduct "hi-pot" tests of any of the NSTX vacuum
> vessel components above 2kV with either the CHI supply capacitors or
> the CHI snubber capacitors connected to NSTX, as these capacitors have
> a rating of only 2kV.
>
> **NOTE**: Ensure LDGIS interlocks are in the "green" state prior to
> conducting any "hi-pot" tests.
>
> **NOTE:** The trained personnel identified in the procedure for
> changing the capacitor bank configuration should be available during
> the initial phase of CHI operations when the capacitor bank size is
> being changed.
>
> **Configuration**

1.  Connect the CHI capacitor to the CHI bus at the machine and connect
    the snubber capacitor and the MOV protection devices.

2.  Reduce the maximum voltage capability on the OH circuit to 4kV (from
    the normal 6kV).

3.  Configure PF1B for 2kV operation to drive current in the
    anti-clockwise direction as seen from above (normal for CHI). Change
    PF1B RIS and software limits to provide 10kA capability (from the
    present 5kA).

4.  Configure PF2L for 2kV operation.

5.  Enable the EC-PI klystron #2 to provide the divertor chamber
    preionization and disable klystron #1 connected to the midplane
    EC-PI launcher. Apply the EC-PI power from t = -3 to +8ms.

6.  Configure HHFW for 0-π phasing during heating, if required.

7.  Increase the gain on the Mirnov coils by connecting the appropriate
    cables and changing the attenuation factor. For Step 3.2 on, return
    the Mirnov coil gains to the standard values used for inductive
    discharges.

8.  Operate the NB sources at 60kV acceleration voltage if NBI is
    needed.

> **LDGIS system checks**

9.  Ensure LDGIS is in the "Puff" mode and has been tested prior to the
    experiment.

10. Ensure LDGIS gas pressure interlocks (PE107, PE104) are functional.

11. Set LDGIS operating line pressure to 100Psig helium.

12. Ensure 2000 Torr D~2~ filling gas pressure in LDGIS system

> **Pre-operational checklist**

13. Ensure MIG and Penning gauges are selected for operation.

14. Remove CHI jumpers.

15. Check TF interlock is set to 10kA.

16. Check for the appearance of ionization light on the fast camera
    during the EC-PI pulse to ensure correct triggering.

# 5. Planned analysis

> The magnetic analysis codes MFIT, EFIT and ESC will be used to analyze
> the plasma equilibrium. It is hoped to be able to use data from the
> MPTS and other kinetic diagnostics to supplement the magnetic data for
> the assessment of flux closure.

# 6. Planned publication of results

These results will be published at meetings and in journal articles.

PHYSICS OPERATIONS REQUEST

Title: Transient CHI Startup OP-XP-531

Machine conditions (specify ranges as appropriate)

> I~TF~ (kA): -**45** Flattop start/stop (s): **-0.02 / 0.5**

I~P~ (MA): **0.5** Flattop start/stop (s): **n/a**

Configuration: **CHI transitioning to CS limited**

> Outer gap (m): **N/A**, Inner gap (m): **N/A**
>
> Elongation κ: **N/A**, Triangularity δ: **N/A**
>
> Z position (m): **0**
>
> Gas Species: **D**, Injector: **LDGIS + LFS midplane**
>
> NBI- **Possibly needed for Phase 3.3 on; 60kV D^0^ injection, 20ms
> pulse**

ICRF- **Up to 2MW in the heating phasing (0-π).**

CHI- **Capacitor bank operation, 2 -- 10 capacitors active**

Shot numbers for setup: **109635.**

![](media/image1.pct){width="5.070138888888889in"
height="4.154166666666667in"}

DIAGNOSTIC CHECKLIST

Transient CHI Startup OP-XP-531

+----------------------+----+-----+-----------------------------------+
| #### Diagnostic      | #  | ##  | #### Instructions                 |
|                      | ## | ##  |                                   |
|                      | #  | Des |                                   |
|                      | Ne | ire |                                   |
|                      | ed |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Bolometer --       | >  |     | > \* Use when reproducible        |
| > tangential array   |   |     | > conditions are achieved         |
+----------------------+----+-----+-----------------------------------+
| > Bolometer array -  |    | >  |                                   |
| > divertor           |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > CHERS              |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Divertor fast      |    | >  |                                   |
| > camera             |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Dust detector      |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > EBW radiometers    |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Edge pressure      | >  |     |                                   |
| > gauges             |   |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Edge rotation      |    | >  | > \*                              |
| > spectroscopy       |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Fast lost ion      |    |     |                                   |
| > probes - IFLIP     |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Fast lost ion      |    |     |                                   |
| > probes - SFLIP     |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Filterscopes       |    | >  |                                   |
+----------------------+----+-----+-----------------------------------+
| > FIReTIP            |    | >  | > \*                              |
+----------------------+----+-----+-----------------------------------+
| > Gas puff imaging   |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > H~~ camera - 1D   |    | >  | > \*                              |
+----------------------+----+-----+-----------------------------------+
| > Infrared cameras   |    | >  |                                   |
+----------------------+----+-----+-----------------------------------+
| > Interferometer - 1 |    |     |                                   |
| > mm                 |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Langmuir probe     |    |     |                                   |
| > array              |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Magnetics --       | >  |     |                                   |
| > Diamagnetism       |   |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Magnetics - Flux   | >  |     |                                   |
| > loops              |   |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Magnetics - Locked |    |     |                                   |
| > modes              |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Magnetics - Pickup | >  |     |                                   |
| > coils              |   |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Magnetics -        | >  |     |                                   |
| > Rogowski coils     |   |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Magnetics - RWM    |    |     |                                   |
| > sensors            |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Mirnov coils --    | >  |     |                                   |
| > high frequency     |   |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Mirnov coils --    | >  |     |                                   |
| > poloidal array     |   |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Mirnov coils --    | >  |     |                                   |
| > toroidal array     |   |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > MSE                |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Neutral particle   |    |     |                                   |
| > analyzer           |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Neutron            |    |     |                                   |
| > measurements       |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Plasma TV          | >  |     |                                   |
|                      |   |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Reciprocating      |    |     |                                   |
| > probe              |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Reflectometer --   |    |     |                                   |
| > core               |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Reflectometer -    |    |     |                                   |
| > SOL                |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > RF antenna camera  |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > RF antenna probe   |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > SPRED              |    | >  |                                   |
+----------------------+----+-----+-----------------------------------+
| > Thomson scattering |    | >  | > \*                              |
+----------------------+----+-----+-----------------------------------+
| > Ultrasoft X-ray    |    | >  | > \*                              |
| > arrays             |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Ultrasoft X-ray    |    | >  | > \*                              |
| > arrays -- bicolor  |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Visible            |    | >  | > \*                              |
| > bremsstrahlung     |    |     |                                   |
| > det.               |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > Visible            |    | >  |                                   |
| > spectrometer       |    |     |                                   |
| > (VIPS)             |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > X-ray crystal      |    |     |                                   |
| > spectrometer - H   |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > X-ray crystal      |    |     |                                   |
| > spectrometer - V   |    |     |                                   |
+----------------------+----+-----+-----------------------------------+
| > X-ray fast pinhole |    | >  | > \*                              |
| > camera             |    |     |                                   |
+----------------------+----+-----+-----------------------------------+